Immediate and Delayed Fire Effects

Giant sequoia seedlings are highly susceptible to fire. Surface temperatures of about 58 °C or greater can kill them [12,251].

The bark of sapling and pole-size trees is thin compared to that of older giant sequoia trees, and their limbs and foliage reach near the ground, making them vulnerable to even low-severity fire [19,91,251]. During the 2021 KNP Complex Fires, 26% of 15-year-old giant sequoia trees in Redwood Mountain Grove were killed. Trees that survived were taller and located in the central portions of the largest canopy gaps (0.2–0.4 ha), where anecdotal evidence suggests that fuels from litterfall from mature overstory trees were less, while trees that were killed were generally shorter and located near canopy gap edges [258].

Young giant sequoias are more fire resistant than young trees of other species because their bark is thicker. At Blodgett Forest Research Station, 12-, 22-, and 32-year-old giant sequoias had lower mortality 1 year after prescribed fires (9%, 25%, and 27%, respectively) than five associated species (Douglas-fir, white fir, ponderosa pine, sugar pine, and incense-cedar), despite moderate levels of crown scorch, a result attributed to giant sequoia’s thicker bark [270].

Typically, mature giant sequoia trees can survive repeated low- or moderate-severity fires in part because of their thick, fire-resistant bark and elevated crowns [18,21,25,251] (see Fire Adaptations). However, even low-severity fires can damage mature giant sequoias. Branches, bark, cones, and litter accumulate at the bases of trees and provide abundant fuel that can burn and smolder for weeks even after low-severity fire, scorching and consuming bark, and often leaving large fire scars [18,21,23,90,91] (fig. 1). Repeated fires over centuries can produce ever-enlarging scars [251], with some extending deep into [201] or up the full length of the trunk [90].

Most ancient trees show evidence of past fires [40,232]. For example, all trees >3 m DBH showed evidence of past fires in Calaveras Big Tree State Park [40]. Many ancient trees have fire scars that encompass a large percentage of the basal circumference [40,90,251]. In the Giant Forest, fire scars comprised as much as 70% of the basal circumference (3.3%–69.5%) and as much as 54% of the cross-sectional area (3.2%–53.7%) on 90 trees [176]. Because tree canopy water supply is disrupted by fire scarring, there is a direct relationship between the size of the basal fire scar in mature giant sequoias (those >3 m DBH) and the likelihood of damage to the trunk, branches, and foliage on the tops of the trees [187]. Reduction of supporting wood from scarring predisposes the tree to falling and provides an opening for fungi responsible for root disease and heart rot [91,251] (see Mortality).

Fire scars are associated with large giant sequoia mortality when subsequent moderate- or high-severity fires result in prolonged internal burning [196]. For example, in moderate- and high-severity burned areas resulting from three wildfires in Giant Sequoia National Monument and Sierra National Forest (the 2015 Rough Fire, the 2017 Pier Fire, and the 2017 Railroad Fire), large (>1.2 m DBH) giant sequoia mortality was 2.4 times higher for trees with a prior fire scar than those without [196]. However, there does not appear to be a relationship between the presence of a fire scar and mortality following relatively low-severity fires [25,129,142].

Fire can enlarge fire scars, possibly increasing the risk of death [251]. In Giant Forest Grove, prescribed fires damaged the callus tissue around existing fire scars 52% of the time, enlarging them 35% of the time. Fires that burn late in the growing season appear more likely to enlarge fire scars than those that burn early in the growing season [176].

Both young and mature trees can survive partial or nearly complete canopy injury as long as foliage is supported by sufficient wood and maintains adequate connection to roots [19,196,201,211]. Trees are more likely to die as the proportion of crown scorch increases [19,196,211]. In Blodgett Forest Research Station, the probability 13- to 14-year-old giant sequoias dying 2 to 3 years after prescribed fires remained at or near 0% until about 40% percent crown volume scorch (PCVS). Mortality then increased with increasing PCVS such that at about 90% PCVS, mortality exceeded 50% [19]. For larger trees (15 to 100 cm DBH), no giant sequoias died with less than 90% PCVS and few died when PCVS ranged from 90% to 95% 5 years after a 1991 wildfire in Sequoia National Park. However, all giant sequoias died with complete PCVS [211]. In Black Mountain Grove, mortality of large (>1.2 m DBH) giant sequoias 3 years after the 2017 Pier Fire was associated with greater than about 85% crown damage (crown scorch and crown torch) for fire-scarred trees and greater than about 90% crown damage for unscarred trees [196].

Most mature giant sequoia trees that are killed by fire are found within high-severity burned areas and die from 1) crown scorch (killed by convective heat of high-intensity surface fires without foliage combustion) or 2) crown fire or crown torch (killed by foliage combustion) [21,23,142,194,204,220,221]. For example, after the 2020 Castle Fire in the Alder Creek Grove in the southern Sierra Nevada, 72% (144 of 200) of dead giant sequoias >1.8 m diameter occurred in areas that burned with high severity, 25% (49) with moderate severity, and 4% (7) with low severity. None occurred in unburned areas. Three years following the 2017 Railroad Fire in the Nelder Grove in Sierra National Forest, 89% (34 of 38) of dead giant sequoias >1.2 m DBH occurred in areas burned with high severity, 5% (2) with moderate severity, 5% (2) with low severity, and none occurred in unburned areas [221]. Similar results were found following other recent wildfires in the central and southern Sierra Nevada, including the 2015 Rough Fire, the 2017 Pier Fire, and the 2021 KNP Complex, SQF Complex, and Windy Fires [142,196,197,204].

Photo Credit

Photo courtesy of the National Park Service.

Figure 6—A firefighter inspects a burned giant sequoia during the 2021 KNP Complex Fire in Sequoia National Park, which killed thousands of giant sequoias.

Mature giant sequoias are better able to survive moderate- and high-severity fire than associated species. After the 1987 Pierce Fire—which burned with mixed to predominately high severity after nearly a century of fire exclusion—foliage scorch occurred well into the canopy of even the largest giant sequoias, with 24 of 148 giant sequoias over 2 m in diameter having canopy scorch. One year later, 14 of these appeared to be dead, indicating a 91% survival rate within the 20-ha burned grove area. All other canopy trees died except for these large giant sequoias [219].

Mortality may be delayed in young giant sequoia trees for 1 to 3 years after fire [19,109,142]. At Blodgett Forest Research Station, over 80% of the 13- to 14-year-old giant sequoias that were dead 2 to 3 years after prescribed fires were still alive 6 months after fire. The researchers speculated that, in addition to fire damage, other factors such as drought or insects may have contributed to the delayed mortality [19]. In moderately burned areas in the 2021 Windy Fire perimeter, mortality during the first postfire year averaged 7%. Continued mortality was projected to reach 17% in succeeding years [142].

Postfire Regeneration Strategy

• Tree with a sprouting root crown
• Crown residual colonizer (on site, initial community)
• Initial off-site colonizer (off site, initial community) [223]

Fire Adaptations

Giant sequoias are adapted to a mixed-severity fire regime characterized by predominately low-severity surface fire with small patches of intense, high-severity surface fire [104,220] (see Fire Regimes). As giant sequoias age, they quickly become fire-tolerant and exhibit the following adaptations to a mixed-severity fire regime [90]:

• they have an aerial seed bank within serotinous cones [13,29,90,93,251] (see Seed Production and Predation and Seed Banking)
• they release large numbers of seeds following locally intense fire [93,118,251] (see Seed Dispersal)
• their seeds germinate well in postfire conditions [104,209,214,220] (see Postfire Germination and Seedling Establishment)
• they have physical characteristics that make them resistant to fire-caused mortality [90,93,145,222] (see below)

Physical characteristics that make giant sequoias resistant to fire include the ability to survive very high levels of crown damage resulting from fire [19,196,201,211] and the ability to reconstruct damaged crowns via epicormic sprouting [90,93,160,251] (see Vegetative Reproduction and Regeneration). Mature trees have thick bark with little to no resins, which protects the cambium from heat, making the trees resistant to surface fire and less flammable than associated conifers [90,93,105,239,251]. While the loose outer scales are fibrous and burn with ease, the thicker parts of the bark do not hold a flame well and are burned through only when accumulations of fuel at the base of trees burn for a long time or when fire is repeated multiple times, producing ever-enlarging fire scars [90] (see Immediate and Delayed Fire Effects). Ancient trees commonly have fire scars that date to when they were about 40 years old [105] (see table 6)—indicating that fire resistance is gained at a relatively young age [90,145].

While young giant sequoias have low-hanging branches making them vulnerable to fire [19], they don’t remain this way for long. Giant sequoias grow fast, raising their canopies off the ground quickly (see Plant Growth). Mature giant sequoias are tall and have high canopies with self-pruning lower branches [90,251]) (see Botanical Characteristics) reducing their risk of initiating crown fires [90,93,93,222]. In addition, subcanopy giant sequoia trees that survive fire are likely to grow fast when nearby trees are killed by fire, which may facilitate their eventual emergence into the canopy [267] (see Postfire Growth and Mortality).

Fire resistance scores based on physical traits (e.g., bark thickness, maximum tree height, and degree of self-pruning) and traits relating to litter flammability (e.g., flame length, percentage consumed, and flame duration), indicate that giant sequoia fire resistance is similar to sugar pine, and both species are less fire resistant than Jeffrey pine and ponderosa pine. Among associated species, giant sequoias are the tallest at maturity and have the thickest bark (table 1) [222].

Plant Response to Fire

Summary

After fire, giant sequoia seed dispersal, germination, seedling establishment, growth, and survival are highest where locally intense, high-severity surface fire has burned small patches because these fires stimulate seed dispersal, expose mineral soil, and create canopy gaps with reduced vegetation competing for light and soil resources [214]. Canopy gaps play a critical role in giant sequoia regeneration and recruitment into the canopy [105], and a patchy fire pattern contributes to their survivability in frequent fire forests [105,115] (see Fuel Loads).

Fire creates conditions that are favorable for giant sequoia regeneration; however, the severity and intensity of fire influences the degree of favorability [90,93,217,251]. Fire:

• stimulates seed dispersal
• removes associated shade-intolerant plant species 
• creates canopy gaps
• increases understory light penetration
• consumes duff and litter and exposures mineral soil
• increases soil wettability
• increases availability of soil nutrients
• reduces pathogenic fungi

Postfire Seed Dispersal and Seed Banks

While giant sequoia lack a soil seed bank, mature trees have a large aerial seed bank within serotinous cones that remain green and closed high in the tree canopy for decades [13,29,90,93,251] (see Seed Production and Predation and Seed Banking). Seeds are dispersed when cones die [90,93]. Hot air produced by locally intense surface fire rising high into the canopy kills, dries, and opens giant sequoia cones, dispersing large numbers of seeds [93,118,251] (see Seed Dispersal). For seeds to be dispersed, the fire must be intense enough to scorch the lower crowns of the largest trees and heat the cones, but not so intense that the fire burns in the giant sequoia crowns. A crown fire is likely to kill the seeds either by incinerating cones while they are still in the canopy or by burning the thin peduncles of the cones (fig. 4) causing them to fall onto the long-smoldering fuels below, where seeds are then killed [93,220]. Uniformly low-intensity fire often lacks the cone-opening heat pulse required to disperse seeds [93,93,141,220,251].

After fire, seeds are typically dispersed near parent trees (maximum dispersal distance is about 500 m [93], so surviving mature trees close to burns are important on-site seed sources [162,204]. A large recruitment pulse can occur following fire as long as mature trees provide a seed source. If high-severity patches are large and seed sources distant from burned areas, giant sequoias may fail to disperse seeds into recently burned areas [142,197,204]. Additionally, the aerial seed bank may fail to disperse seed to burned areas when crown fires consume seed cones, such as in the historically unprecedented high-severity crown fires of 2015, 2017, 2020, and 2021 [142,196,197,204,220,221].

Photo Credit

Photo courtesy of the National Park Service.

Figure 7—Fire creates favorable seed beds for giant sequoia germination and seedling establishment.

Postfire Germination and Seedling Establishment

Summary

In general, giant sequoia germination, establishment, and survival is greater in burned than unburned areas when seed sources are present and soil moisture is adequate (e.g., [2,93,114,115,118,149,181,194,208,214]). Regeneration is greater in canopy gaps burned at high-severity than low-severity (e.g., [92,118,141,149,194,204,214,217]) because small intense, high-severity surface fires release seeds and create favorable postfire conditions. While low-severity fires promote better seedling establishment and survival than unburned areas, recruitment from low-severity surface fires may be insufficient for long-term maintenance of groves [214] (see Ensuring Adequate Regeneration). At the opposite extreme, giant sequoia germination and establishment can be low following crown fires that kill mature giant sequoia trees over large areas due to a lack of nearby seed sources [142,204,220] (see Postfire Seed Banks).

Small patches of intense, high-severity surface fire expose mineral soil and create canopy gaps required of giant sequoia regeneration (see Fire Severity: Canopy Gaps) [93,118,251]. Seeds falling at times other than shortly after a fire face shading and/or thick litter and duff, and either fail to germinate or die soon after [93] (see Germination). Seedlings established after fire may have increased soil nutrients (see Postfire Soil Nutrients) and less potentially pathogenic fungi [93,251].

Fire Severity

Postfire Seed Beds

Photo Credit

Photo courtesy of the USDA Forest Service.

Figure 8—One year after the 2017 Pier Fire, dense patches of giant sequoia seedlings were found in areas that contained burned litter and duff and exposed mineral soil.

Germination of giant sequoia seeds is optimum when seeds are buried shallowly in moist mineral soil [206,244,251] (see Germination). Surface fires that consume litter and duff and expose mineral soil create conditions that promote germination and subsequent seedling survival [93,251] and few to no seedlings become established in the thick duff of long unburned groves [88,90,93,118,206,214]. For example, in Giant Forest and Redwood Mountain Grove, establishment was higher in areas that burned at low-, moderate-, and high-severity than in unburned control sites that had not been burned in at least 50 years. Seventeen percent of mature giant sequoias trees sustaining low-severity fire, and 45% and 86% of trees sustaining moderate- and high-severity fire, respectively, had surrounding giant sequoia seedling establishment that ranged from a few individuals to dense seedlings, while only 4% of trees in unburned plots had surrounding seedling establishment [149].

Surface fires that burn litter and duff, leave a soft, friable, ashy soil surface on which lightweight seeds fall and get buried. In addition, high fire temperatures enhance soil wettability, water penetration, and soil moisture retention that facilitates seedling root penetration and prevents desiccation [18,54,93,118,206,251]. Soil surface friability is short-lived because rain, snow, wind, and other mechanisms compact the soil over time, leaving newly fallen seeds exposed on the surface and thus vulnerable to desiccation [88,90,217].

Places where logs and stumps burned away, leaving a fine layer of ash on the mineral soil surface, are especially good seed beds [217]. Following prescribed fires in Kings Canyon National Park, giant sequoia seedlings were especially abundant in soils severely burned by the combustion of dry logs, and seedlings often established in a linear manner conforming to the position of burned logs [90]. In some places, ancient giant sequoias grow in a row, suggesting that they germinated where a single log had burned (Mastroguiseppe, Crater Lake Institute, personal communication cited in [9]). Low-severity fire is often associated with incomplete log combustion and thus fails to create these types of favorable seed beds [220].

A review concluded that the greatest number of seedlings establish during the first and second years after fire. The first cohort of new seedlings—those germinating the first spring or early summer following fire—is almost always the largest. A second cohort often germinates the second spring or early summer (likely averaging <20% the size of the first cohort). Sometimes a very small third-year cohort germinates, but its survival is usually minimal and probably does not contribute to long-term recruitment. Giant sequoia seedling densities then decline dramatically in the years and decades following [217]. Cohorts of trees that all date from the same fire result in a fine-scale mosaic comprised of canopy gaps with even-aged vegetation in various stages of succession [28,116,116,251] (see Succession).

Photo Credit

Photo courtesy of the National Park Service.

Figure 9—A giant sequoia grove in Sequioa National Park burns in the 2021 KNP Complex Fire.

Postfire seed beds are only suitable if there is enough soil moisture, and studies report finding giant sequoia seedlings in moist sites after fires, such as in drainages or where runoff accumulates [2,212]. For example, following the 2021 SQF Complex and KNP Complex Fires, high giant sequoia seedling densities occurred within watercourse bottlenecks, where seeds were deposited after being caught in runoff [204]. In Whitaker’s Forest Grove, 1,253 giant sequoia seedlings occurred in burned areas 12 months after a June prescribed fire. By 16 months after fire, only 96 (8%) remained. Most the remaining seedlings were found in an area that received spring drainage. In unburned areas, fewer seedlings were found and no seedlings survived to 16 months [2].

Seedling mortality is high the first few years after germination [92,115,118,251]; however, some studies report greater survival in higher than lower severity burned areas [92,93], perhaps because of increased wettability of soils in severely burned areas [93]. For example, in Redwood Mountain Grove, a total of 7,003 first year seedlings were marked from 1966 to 1968-, 1- and 2-years following treatments. Treatments included pile burning, scarification with heavy equipment, scarification plus surface fire, and surface fire alone. In 1974, 8 to 9 years after treatments, only 391 seedlings remained. Survival was about 8 times higher in pile burns (the most severely burned treatment) than in other treatments, suggesting that high-severity fire was important to seedling survival, a result attributed to increased soil wettability in more severely burned areas [93]. By 1986, 20 and 21 years after treatments, seedling survival remained higher in the pile burns, indicating that seedlings that established on severely burned plots had better survival than those on less severely treated plots [92].

Canopy Gaps

Giant sequoia seeds germinate, and seedlings establish best in half full-sunlight in small (hundredths of a hectare to a few hectares) fire-created canopy gaps [18,88,206] (see Regeneration Processes). Small, locally intense, high-severity surface fires (including prescribed fire (e.g., [48,219])) can create canopy gaps in dense forests by killing groups of shade-tolerant trees, sprouting shrubs, and seeds in the soil seed bank. For example, 18 canopy gaps created by prescribed fires in Giant Forest ranged from 0.067 to 1.17 ha and had as many as 2,956 seedlings and saplings/ha [48].

Uniformly low-severity fires will not kill trees, shrubs, or seeds in the soil seed bank, and may instead stimulate shrub resprouting and germination of associated species [251]. Following prescribed fires in 1969 in the Redwood Mountain Grove, more than 99,000 giant sequoia seedlings/ha were found the first postfire year in a severely burned area, declining to about 1,000 seedlings/ha the third postfire year. This area had sparse seedlings of deerbrush—a dominant shrub species—throughout the 3 postfire years. Areas burned with lower severity had denser deerbrush seedlings and generally sparser giant sequoia seedlings during the first 2 postfire years, but similar amounts in postfire year 3. No seedlings of either species occurred in unburned control plots during any year [115,118]. For further information, see the FEIS Research Project Summary of Kilgore's studies, which summarizes the following publications: [113,114,115,118,119,120]. Following the 2021 Windy Fire, high-severity burned plots generally had higher relative proportions of small stems of giant sequoia (regeneration and resprouts) than white fir and incense cedar, while low- and moderate-severity burned plots generally had higher or similar relative proportions of small stems of incense-cedar and white fir than giant sequoia [142].

If canopy gaps are too large—such as those created from large, crown fires— dispersed seeds are more likely to desiccate from the heat of full sunlight before germinating or die shortly after germinating [18,88,206,251]. Large, high-severity burned areas typically retain less snow and melt out earlier compared to small canopy gaps, exacerbating the summer drought experienced by giant sequoia seedlings [220]. In addition, litter develops slowly in large gaps, failing to aid in giant sequoia seedling survival [18,90,214,220,251].

The optimum size of canopy gaps for giant sequoia regeneration likely depends on site characteristics such as topography and soil moisture, but canopy gaps up to about 4 ha are associated with high postfire seedling densities, although most are smaller [48,115,118,132,141,177,214,217] (see Seedling Establishment and Mortality).

Fire Frequency

In giant sequoia-mixed conifer forest in Sequoia and Kings Canyon National Parks, giant sequoia seedlings were more frequent in plots with second-entry burns than in plots with first-entry burns or nearby unburned plots (table 2). While giant sequoia seedlings were not an indicator of second-entry burns, seedlings were almost exclusively found in second-entry burned plots leading the researcher to conclude that repeated burning was critical for regeneration and persistence of this species. The time between first- and second-entry burns spanned 8 to 17 years. Burns occurred between June and November and included low- to moderate-severity prescribed fires and wildfires [253].

Postfire Weather

Giant sequoia seedling establishment and survival are greatest when the first one or two summers following fires are wet, allowing the seedlings to develop a large enough root system to endure future droughts, and corresponding to the period when seedlings are most abundant and vulnerable to drought [93,149,214]. In three groves in the southern Sierra Nevada, giant sequoia seedling establishment was higher following two burns that preceded wet climatic conditions than following two burns that were followed by drought conditions [149].

Along those lines, seedling establishment may be minimal during years with below-average precipitation. At Mountain Home Demonstration State Forest, giant sequoia seedling density was low (0–141.5 seedlings/ha) because of below-average annual and summer precipitation following fire and because of low seed dispersal— apparently because the burning treatments did not sufficiently scorch the crowns to release seeds [209]. Following high-severity crown fires in the last decade, researchers hypothesized that environmental stress due to exceptional drought and high ambient temperatures likely contributed to regeneration failure [197,204,217,217,220].

Postfire Soil Nutrients

Ash substrates may enhance early giant sequoia growth because of increased nutrient availability. Giant sequoia seedlings were planted within, on the edge, and outside of ash substrates following experimental burning and there was an immediate and positive effect of ash substrate proximity on giant sequoia growth that persisted for 10 years. Seedlings planted in the centers of ash substrates consistently outgrew (in both height and basal diameter) seedlings that were not planted within ash substrates. Mortality tended to be greater for seedlings planted outside ash substrates, but differences were not significant. Increased soil nutrients (e.g., available nitrogen) in ash substrates were suggested as a likely mechanism increasing giant sequoia seedling growth [272].

Studies that examined soil nutrients after fire in giant sequoia groves found differing results but mostly conclude that fire increases some soil nutrients beneficial to giant sequoia growth [39,54,81,205], although one study concluded that changes in physical properties (i.e., water-holding capacity) appear more important than soil chemical properties for seedling germination and survival [54]. For more information about nutrients in soils and stream water following fire in giant sequoia forests, see the following: [39,115,205,260,262].

Postfire Growth and Mortality

Giant sequoias that establish after fire grow quickly (e.g., [18]) and can outgrow associated conifer species for at least the first 7 years following canopy-opening disturbances such as fire [266]. Growth and survival of seedlings and saplings is typically greater in burned than unburned areas [92,93,107,214,263,272]. Similarly, fire can increase the number of giant sequoia saplings. In Sequoia and Kings Canyon National Parks, there was a marked increase in recruitment of giant sequoias in the 2.5- to 10-cm diameter class 5 and 10 years after prescribed fire, while no giant sequoia recruitment occurred in the unburned plots during this time. The number of giant sequoias in the larger diameter classes remained stable from before fire to 10-years after fire [107].

Fire severity appears to promote postfire growth, with higher growth rates in more severely burned areas [84,92,93,214]. For example, in the Redwood Mountain Grove, seedlings in the most severely burned areas (burn piles) grew at a faster rate than those in other treated areas on a mesic site, but growth rates were statistically similar among treated areas at a drier site. By posttreatment year 20 and 21, the height growth of giant sequoias on the mesic site remained significantly greater on burn piles than on other treated areas [92]. Six years after the 2017 Railroad Fire in Nelder Grove, maximum height of giant sequoias that established in burned areas was positively correlated with wildfire severity [84]. Lower growth rates in low-severity burned areas were likely a consequence of lower light and moisture availability resulting from competition with surrounding vegetation [220].

Mature giant sequoias that survive locally intense, high-severity surface fires typically increase radial growth rates, due to reduced competition from killed surrounding vegetation (i.e., they show a growth release) (e.g., [30,34,89,219,230,232]). Large radial growth release has been observed in tree rings after historical wildfires (e.g., [30,34,91,149]) and after contemporary prescribed fires (e.g., [148,149,201]). Tree rings of 19 trees from Giant Forest documenting 333 fires from 500 to 1700 indicated slow growth during fire years (likely because these were dry years) and fast growth after fire [30]. In the Mariposa Grove, giant sequoias grew faster immediately after a fire in 1889. Radial growth rate in the first 5 years after fire was nearly double the prefire growth rate and then tapered off gradually over 40 years until it resembled the prefire rate [91]. In the Mountain Home Grove, a large, severe wildfire in 1297 was followed by a growth release of unusually large magnitude and duration, possibly due to reduced competition coupled with increased precipitation. The 1297 fire followed a persistent drought (1292–1296). The postfire growth release was apparent for about 100 years [34].

Generally, radial growth rates increase with fire severity as long as trees are not severely injured [149,174,201]. Low-severity fires often do not reduce competition or increase soil nutrients enough to increase radial growth rate of giant sequoia trees, while moderate- to high-severity fires can release trees that are not severely injured [148,149,201]. In Redwood Mountain Grove, growth release was observed 1 year after a prescribed fire that charred giant sequoias and killed small neighboring white firs but was not observed in unburned trees [201]. Another study in the Redwood Mountain Grove found greater radial growth in areas of moderate-, high-, and very high-severity up to 15 years after a 1977 prescribed fire than in control areas not burned in at least 50 years (unburned areas). However, radial growth was delayed in areas of very severe fire due to extensive foliage injury. In the Grant Forest, trees in a low-severity burned area had postfire radial growth rates similar to trees in an unburned area 10 years after a 1980 prescribed fire, while trees in a moderate-severity burned area had higher postfire growth rates, and trees in a high-severity burned area had even higher growth rates. Trees that experienced very high-severity fire, showed declining growth rates in the first years following the fire. This group, which included some surviving trees with foliage damage, increased in growth during postfire years 2 to 5 then growth rate declined more rapidly than trees in moderate- and high-severity burned areas [149].

Moderate- and high-severity fires also appear important for recruitment into the forest canopy. In one area of the Redwood Mountain Grove, survival of giant sequoias to 34 years old was 26% (80 of 312 marked seedlings) in high-severity pile burns and 3% (51 of 1,561) in other lower-severity treatments, which included low-severity surface fire and/or mechanical scarification. In another area, survival to 35 years old was 13% (50 of 377) in higher severity pile burns and nearly 0% (24 of 4765) in other lower severity treatments [194]).

Giant sequoia growth and survival increases with increased precipitation [36,238,267], so giant sequoia growth may not increase after fire if weather is dry. In Giant Forest, the largest and longest sustained growth increases following prescribed fires in the 1970s and 1980s occurred following a relatively high-severity 1981 fire, which was followed by a wet winter [149].

Fire-Pathogen Interactions

Reviews suggest that fire may reduce populations of some fungi potentially pathogenic to giant sequoias [93,163,174,176,251]. For example, fire may reduce Penicillium spp., which produces necrosis of the hypocotyl in giant sequoia seedlings [93]. Annosum root disease caused by the fungus Heterobasidion annosum may be transmitted to giant sequoia from white fir and California red fir via root contacts with infected firs. Transmission of the fungus is expected to increase as fir densities increase, so transmission is expected to be greater in fire-excluded forests. Conversely, periodic fires that reduce fir in stands are expected to reduce the risk of annosum root disease transmission [163,176].

Fire may damage giant sequoia roots, stems, limbs, and other tree tissues, the wounds of which serve as entrance points to pathogens and insects. For example, heart rot and other decay-causing fungi frequently develop in fire scars [174,176]. In Giant Forest Grove, 77% (69 of 90) of fire scars sampled had 17 different fungi species, many of which can cause decay [176], which is a major contributor to mature giant sequoia toppling [172] (see Mortality). Belowground decay was present on 63% of 90 fire scars and aboveground decay in 42% of fire scars [174,176].

Fire-Insect Interactions

Little information was available about fire-insect interactions in giant sequoia-mixed conifer forests, but one study in the Redwood Mountain Grove suggested that the highest seedling mortality due to insects occurred in the first growing season after fire, then declined, presumably because in the first year after burning, giant sequoia seedlings were one of the few foods available to insects. As giant sequoia seedling numbers declined and other plant species increased, mortality due to insects was less. Insects responsible for the damage were early instars of Pristocauthophilus pacificus, a camel cricket, and larvae of the geometrids Sabulodes caberata and Pero behrensaria [93,208].

Fuel Characteristics

Physical Properties of Fuels

Information on the physical properties of giant sequoia fuels, including average diameter, surface-area-to-volume ratio, and specific gravity in various fuel size classes are available [247], as is information on the depth, weight, and bulk density of woody, litter, and duff fuels of giant sequoia [248]. Compared with 19 other species, giant sequoia woody fuel bed depth, and litter and duff depth combined were the greatest. Average weight of woody fuels was the heaviest and litter and duff fuels combined were the second heaviest. These fuel characteristics influence fire behavior and can be used to predict fire intensity and rate of spread in fuel models [248].

Flammability

Studies that examine flammability characteristics of giant sequoia leaf litter, suggest that it ignites slower, burns longer, and releases less heat than associated species. One study found that giant sequoia had the longest time to ignition, the longest sustainability, the slowest spread rate, the second lowest maximum flame height, and the second highest temperature integration >100 °C (a proxy for heat release) among seven associated species (white fir, California red fir, incense-cedar, Jeffrey pine, sugar pine, ponderosa pine, and California black oak) [46]. Another study that examined burning characteristics—including maximum flame length, flame time, ember time, burn time, percent combusted, and mean rate of weight loss—of giant sequoia needles in a burn chamber in relation to 12 other conifer species found that flame time for giant sequoia lasted longer than any other species tested [68]. In another study of ash and heat content of needle, litter, and duff fuel components of giant sequoias and 18 other conifer species, giant sequoia foliage and litter had the lowest average heat content with ash, and its bark had the lowest average heat content without ash [250].

The configuration of the fuel bed together with the time of year (e.g., summer drought [120]) are important determinants of flammability [3]. Giant sequoia fuel beds are relatively compact compared with that of ponderosa pine and sugar pine and thus have relatively low flame heights and are less likely to burn. For example, in Whitaker’s Forest, summer prescribed fires did not carry or consume small giant sequoia branches and twigs within the fuel bed well, and total fuels were reduced more when branches and twigs were absent than when they were present. Conversely, total fuels of sugar pine and ponderosa pine were reduced more when branches and twigs were present. The researchers concluded that fuels of the pines can effectively be consumed by spring, summer, or fall burning, but giant sequoia fuels require drier summer or fall conditions [3].

Fuel Loads

Accumulation of fuels in giant sequoia-mixed conifer forests can be substantial, especially in fire-excluded forests [168] (table 3). For example, total accumulation of ground fuels (woody material plus litter and duff) in four fire-excluded mixed-conifer forest types in Sequoia and Kings Canyon National Parks ranged from 83,255 kg/ha in plots dominated by ponderosa pine to 239,302 kg/ha in plots dominated by giant sequoia, indicating that plots dominated by giant sequoia had the greatest potential for the most intense fires among the four forest types examined [168]. Other studies found more fuels in mixed-conifer forests with giant sequoia than without [42,110].

A photo series for quantifying fuels and assessing fire risk in giant sequoia groves is available: [255]. Total fuel loading ranged from 15,692 to 161,403 kg/ha in the 18 giant sequoia-mixed conifer stands photographed. Fire behavior can be predicted using the fuels information in the photo series combined with appropriate weather and environmental variables [255].

Highly variable fuel loads, arrangements, types, and moistures promote variable fire patterns, frequencies, sizes, and intensities. Fires may be intense in small areas within mature stands that have heavy fuel loads and may be less intense or not burn at all in young stands with light fuel loads [115,120]. Thus, in combination with biotic and abiotic factors, repeated fires of varying intensities form a mosaic of variously sized canopy gaps with even-aged vegetation in various stages of succession within uneven-aged forests [115] (see Stand Structure). Initially, fuels accumulate slowly in canopy gaps, which lack large trees that shed needles, cones, and branches. As canopy gaps age, the rate of surface fuel accumulation increases, allowing subsequent fire to burn with sufficient intensity to thin trees in its interior [105,115].

Historically, fine fuels—primarily leaf litter and duff but not grasses—contributed to fire spread [59]. However, downed woody fuels and ladder fuels have become more abundant with decades of fire exclusion [115]. Increasing densities of shade-tolerant saplings, (e.g., white fir) has increased ladder fuels and the probability of surface fires moving into the crowns of giant sequoias [119]. In the past, shade-tolerant saplings were readily killed by frequent low-severity surface fires [115] (see Historical Fire Regimes). Therefore, the development of ladder fuels was minimized, such that the risk of large crown fires that killed large giant sequoias was largely absent [115,119]. Fire exclusion since the late 1800s has resulted in the development of ladder fuels and increased potential for crown fire [119] (see Contemporary Fire Regimes).

Since the 1960s, prescribed fire and mechanical treatments have been used successfully to reduce the potential for high-severity surface and crown fires by reducing litter, duff, and the number of white firs in small diameter classes [107,119] (see Fire Management Considerations). For information about fuels reduction following prescribed fires, see the following: [1,3,107,109,110,114,119,131,166].

Fuel Accumulation Rate

Giant sequoias produce abundant litter each year (table 4). For example, in Nelder Grove, giant sequoia produced the greatest amounts of foliage, twigs, and branches annually among 11 conifer species. Foliage deposition rate was about 3,000 kg/ha/year for the three largest diameter classes of giant sequoias [249]. However, annual variability in litterfall in giant sequoia forests can be high [224].

Fuels do not accumulate under giant sequoias at rates that would permit surface fires to carry annually [21] or even biennially [32]. A model predicted that a 5-year fuel accumulation of giant sequoia litter should be enough to carry at least a patchy fire [144]. In recently burned fire-excluded forests, it can take up to 10 years or longer for fuels to accumulate to prefire levels (e.g., [106,107,109,110,166]). In a recently burned fire-excluded (no fire within 30 years) giant sequoia-mixed conifer forest in Yosemite National Park, total fuel load (duff and woody fuels) increased to 72% of prefire levels within 5 years: woody fuels accumulated to 91% of prefire levels, while duff had accumulated to only 15% [131]. In fire-excluded forests (no fire within >60 years) in Redwood Mountain Grove, total fuel load (litter, duff, and woody fuels) 7 years after fire was 53% of the prefire load (100,812 kg/ha versus 190,520 kg/ha). This was considered sufficient to support another prescribed fire. Many young white firs (up to 51 cm DBH) established during the period of fire exclusion. When they were killed by the initial fire, their falling needles, twigs, and branches built up surface fuels rapidly. The researchers predicted that fuel accumulation rates following subsequent fires would become slower as fuels from young trees are thinned in successive burns [166]. In Sequoia and Kings Canyon National Parks, combined duff and woody fuels accumulated to 75% of prefire levels 10 years after prescribed fire in giant sequoia-mixed conifer forests that had not burned in more than 40 years (127,329 kg/ha versus 95,497 kg/ha). Most of the accumulation was from woody fuels (i.e., fire-killed branches and small trees), reaching 103% of prefire levels. Duff accumulated at a slower rate, reaching 28% of prefire levels 10 years after fire (fig. 10) [106,109].

Photo Credit

Data from Keifer (1998) [106].

Figure 10—Consumption of duff and wood before and up to 10 years after prescribed fires in giant sequoia-mixed conifer forests in the southern Sierra Nevada.

While 5 to 7 years may be sufficient to build up enough fuels to support a low-severity, surface fire [166], about 35 years may be required for fuels to accumulate enough to carry a crown fire [194]. For example, in two areas of Redwood Mountain Grove, 34 to 35 years after treatments that included pile burning and/or surface burning, ladder fuels had developed with abundant downed wood, a dense understory (or subcanopy) of shrubs and small trees, and an intermediate subcanopy of white firs and incense-cedars that had the potential to carry fire into the lower branches and crowns of mature giant sequoias in these areas [194]. Historically, fires probably occurred at mean intervals of less than 20 years (see Historical Fire Frequency). Given relatively frequent fires, it is unlikely that fuels would have built up enough to allow large, continuous high-severity fires (including crown fires) to occur, as have been observed in the past decade [179].

After fire, fuels may accumulate slower in mixed-conifer forests dominated by giant sequoia than in other mixed-confer forest types [3,110]. Ten years after prescribed fires in Sequoia and Kings Canyon National Parks, total fuel loads accumulated to a mean of 66% of prefire loads in giant sequoia-mixed conifer forests, 83% in white fir-mixed conifer forests, and 84% in ponderosa pine-mixed conifer forests [110]. A simulation of fine fuel accumulation in the absence of disturbance found ponderosa pine forests had the highest accumulation rate, followed closely by sugar pine forests, while that of giant sequoia forest was much slower. However, the researchers noted that the simulation was based on 100-year-old stand of giant sequoias and the rate of accumulation is likely to be greater in older stands [3].

Decomposition Rates

Giant sequoia heartwood is extremely resistant to decay due to its high tannin content. The heartwood of giant sequoia logs and stumps may remain undecayed for centuries or millennia [90,185,251]. For example, a sample of wood from the outermost portion of a heavily burned giant sequoia stump on the edge of Circle Meadow, Giant Forest, was radiocarbon-dated as approximately 2,100 years old [89]. Without the tannin content of the heartwood, sapwood usually decays within a few years where conditions are moist [90].

Leaf litter decomposition of giant sequoia is slow relative to other associated species. After 3.6 years, leaf litter decomposition of three conifer species in Sequoia National Park was slowest for giant sequoia (28.2% mass loss), followed by sugar pine (34.3%) and white fir (45.1 %) [224]. In Whitaker’s Forest, the average yearly decomposition rate of giant sequoia litter was lowest, and duff highest compared with that of ponderosa pine, sugar pine, and white fir individually [3]. Studies of litter decomposition rates and factors affecting them include the following: [3,224,225,249].

Fire Regimes

The historical fire regime period is defined in this review as starting about 1,000 years before European-American settlement and ending with European-American settlement, which occurred from about 1875 to 1890, depending on location [117,120]) [215]. This period is selected based on pollen records from meadow sediments that suggest that most of the changes in present grove boundaries were completed by about 1,000 years ago [215]. European-American settlement brought about changes in ignitions and fire exclusion that altered the fire regime (see Contemporary Fire Regimes).

Historical Fire Regimes

The giant sequoia-mixed conifer forest corresponds with LANDFIRE’s Mediterranean California Mesic Mixed Conifer Forest and Woodland Biophysical Setting (BpS) (BpS 10280_4_6_7_12) [130]. This BpS covers mixed-conifer forests in the Sierra Nevada with and without giant sequoias. See Stephenson et al. (2024) for a succinct overview of giant sequoia fire ecology including past fire regimes [220].

Fire Ignition and Season

Fires occurred in late summer and early fall and were caused both by lightning and American Indian burning [21,30,35,164,231]. Lightning-caused fires were common in giant sequoia groves and most occurred from July to September [21,164]. American Indian burning is thought to have added to the number of fires that would have occurred by lightning alone [27,117,120,135]. American Indians primarily burned in late summer and early fall, similar to the timing of most lightning ignitions [135]. Annual tree rings of 19 trees from Giant Forest documenting 333 fires for the period of 500 to 1700 indicated that fires were predominately late-season (late summer and fall) fires, occurring after latewood formation had already begun [30].

American Indians burned for various reasons in giant sequoia forests, such as increasing growth of food-producing plants (e.g., oaks) and browse for wildlife [14,21,114,120]. They burned most commonly in the immediate vicinity of larger settlements in the lowlands and more sporadically in adjacent foothills and lower montane vegetation types as well as around frequently used summer camps at higher elevations [164]. American Indian burning contributed to the complex vegetation mosaic in giant sequoia-mixed conifer forests [120]. See the review by Kilgore (1979) for more information about American Indian burning in giant sequoia-mixed conifer forest [120].

Fire Type

Historically, fires were predominately low- and moderate-severity surface fires with scattered small patches of high-severity surface fire and occasional torching of single or small groups of trees, in which most mature giant sequoias survived [104,120,220]. While some small (usually <0.1 ha) patches of passive crown fire occurred, active crown fires that spread over large areas were likely absent as evidenced by ancient giant sequoia trees spread throughout most groves [90,103,116,120,215,220]. For example, in Redwood Mountain Grove, a study of the fire history indicated that the grove had not experienced a crown fire of any meaningful size for at least the preceding 2,000 years as indicated by the distribution of living ancient giant sequoias [120].

Fire Intensity and Severity

Surface fuels were typically sparse with pockets of dense fuels due to historically frequent fire. Thus, fire severities were mostly low to moderate with patches of high-severity [116,120,219,252]. Recurrent fires killed many seedlings and saplings and some subcanopy trees but left the forest canopy largely intact. Fire intensities in pockets of dense fuels were high enough to kill many or most pines, firs, incense cedars, and oaks of all ages, including giant sequoias up to 200 years old, which would torch in “hot spots”. Importantly, large giant sequoias within the canopy gaps usually survived [28,116,120,214,219,220]. While some patchy crown fires (ranging in size from a single tree to groups of trees to perhaps a hectare) were relatively common, crown fires over large areas (greater than about 100 ha) were not sustained by available fuels and the uneven-aged, discontinuous canopies [219,252].

Large (>10 ha), high-severity fires (75%-100% mortality of large trees) were extremely rare if not unprecedented historically [177,220]. LANDFIRE models suggest that 50% of fires were of low severity, 39% were of mixed severity, and only 12% were of “replacement” (high) severity [130]. A fire in 1297, in the Mountain Home Grove, was the most severe fire found in multi-millennial giant sequoia tree-ring records. It followed the most persistent drought (1292–1296) observed in the record of reconstructed precipitation between 1100 and 1500. This fire killed a large proportion of understory trees and possibly some larger giant sequoias. It covered a relatively large area from 16 ha to several square kilometers [34]. However, it showed no evidence of killing 75% to 100% of large trees as evidenced by the fact that many millennial-aged giant sequoias survived and recorded the fire in their rings across all sampled areas of the grove. A large majority (89%) of samples showed either 1) little change in postfire growth, suggesting local low-severity fire (29%), or 2) often substantial increases in postfire growth (60%), suggesting the death of many competing pines, firs, and incense-cedars, but <50% crown scorch to the giant sequoias themselves. The remaining 11% of sampled giant sequoias showed initial postfire growth declines—typically lasting about 20 years and suggesting >50% crown scorch—but even those giant sequoias often grew near other giant sequoias that showed little change in, or increases in, postfire growth, suggesting that the areas of >50% crown scorch may have been localized rather than extensive [34,220]. An unusually large cohort of giant sequoias established within a short period following the 1297 fire, suggesting that this one fire, while unusual, had a disproportionately large effect on the modern structure and dynamics of the Mountain Home Grove [34].

Fire Frequency

The estimated historical mean fire frequency is 17 years for the Mediterranean California Mesic Mixed Conifer Forest and Woodland Biophysical Setting based on LANDFIRE succession modeling, which includes forests with and without giant sequoias [130]. However, fewer fires occurred in the more mesic giant sequoia sites than in the adjacent drier pine sites [35]. In a review of past fire regimes, Stephenson et al. (2024) reports that on average, at least one fire burned somewhere within a given large giant sequoia grove every 2 years, and a fire burned at the base of any individual giant sequoia every 15 to 17 years [220]. Estimates of mean fire intervals in giant sequoia-mixed conifer forests are given in table 5 and ranged from 1 to 32 years over multiple millennia [35,120,170,229,230,231,232]. Mean fire intervals varied in response to site and environmental factors that affected ignition sources, fuel accumulation rates, fuel moisture content, and burning conditions [252].

There may be considerable variation in fire frequency within groves. Fire occurs more frequently at a specific point in a grove, on average, than the amount of time it takes for a giant sequoia to become fire resistant [90,93,105,239,251] (see Fire Adaptations). This implies that regeneration within a grove may be sustained under that pattern because not all young giant sequoias will burn in every fire. In locations most favorable for successful establishment and early growth (i.e., in canopy gaps), fuels tend to be sparser and accumulate slower than in adjacent forested areas. Thus, fire in canopy gaps is less frequent than in the surrounding forest. By the time fuels have accumulated enough to carry fire, the more vigorous seedlings and saplings may be large enough to survive a low-severity fire [251] (see Fuel Characteristics). Based on the assumption that the period from germination to first fire scar reflects the fire-free period in the canopy gap in which the giant sequoia seedling established, trees in recent canopy gaps in Giant Forest and Mariposa Grove burned about 4 to 8 times less frequently than those in more mature forest (table 6) [105].

Fire Pattern and Size

Historically, most fires in giant sequoia-mixed conifer forests were small and irregularly shaped [120]. As with contemporary fires, historical fires patterns and sizes were highly variable due to variations in weather, topography, and fuel abundance, arrangement, type, and moisture content [120] (see Fuel Loads). In general, periods of relatively frequent fire had predominately small fires, while periods of less frequent fire had larger, more widespread fires [230]. The largest fires occurred during extended periods of drought [120,230]. In the Redwood Mountain and Bearskin Creek areas, tree ring analysis indicated that most fires were small (0.001-16 ha) and were generally confined to a single slope or drainage. Spot fires (those recorded by individual trees; ranging from 0.001 to 1 ha) occurred at 1-year intervals. Larger fires (from about 1 ha to more than 2,000 ha) burned somewhere within the 1,800-ha area at 3-year intervals between 1726 and 1881. Large fires that spanned two drainages (16–2,000+ ha) occurred at 26-year intervals. Large fire years were often positively associated with drought conditions [120]. Small surface fires resulted from discontinuous fuel patterns and generally sparse fuels, with localized dense pockets of surface fuel resulting in locally intense fire behavior and creation of canopy gaps [120,258]. Canopy gaps sizes were variable, ranging from a single tree to several hectares [215,217] (see Historical Forest Structure).

An estimated 305 ha burned annually in giant sequoia-mixed conifer forests in Sequoia and Kings Canyon National Parks prior to European-American settlement, with relatively more area burned on southern aspects (205 ha/yr) than northern aspects (100 ha/yr) [33].

Historical Forest Structure

Giant sequoia-mixed conifer forests were typically multi-layered and uneven-aged and had canopy gaps with even-aged vegetation creating a mosaic of vegetation characteristics [28,115,116,201,220] (see Succession). Forest structure and composition likely varied from grove to grove in response to American Indian burning, local environmental conditions (e.g., elevation, slope, aspect, soil characteristics, etc.), and recent disturbance history [215]. In general, characteristics of giant sequoia groves prior to European-American settlement included the following:

1. Fuels: Fuel accumulations were variable, but generally low because frequent surface fires limited fuel accumulation, especially of downed woody fuels. Although scattered, large piles of woody fuels occurred [103,215,220]. Fine fuels that contributed to fire spread were mostly leaf litter and duff from trees and shrubs. Grass was not a substantial component of giant sequoia forest understories and did not contribute substantially to the fine fuels that carried frequent fire [59].
2. Tree Abundance: Small trees were regularly killed by frequent low-severity surface fires, creating a forest understory that was relatively open [21,120,120], although tree clumps that escaped fire for several intervals, and thickets of shade-tolerant species (e.g., white fir and incense cedar), would often develop in these places, creating a mosaic of diverse vegetation characteristics [190,215]. Estimates of historical density and basal area of giant sequoia forests are given in Stohlgren (1992) [227] and Stephens (1998) [210]. 
3. Tree Species Composition: Forests were dominated by shade-intolerant and moderately shade-intolerant species like giant sequoia, sugar pine, ponderosa pine, and Jeffrey pine [103,190]. White fir and California red fir, in particular, were maintained at relatively low densities because their seedlings and saplings are much more sensitive to fire than giant sequoias and pines [103].
4. Shrub Abundance: Dense fire-promoted shrubs (e.g., manzanita, ceanothus, and plum) occurred in small patches of high-severity fire [190,215]. Stand reconstruction in Redwood Creek watershed in Kings Canyon National Park indicated that about 19% of the watershed was covered by shrub aggregations in 1890 [28].
5. Giant Sequoia Regeneration: Giant sequoias established after fires in canopy gaps created by small patches of high-severity surface fire (“hot spots”) [89,214]. Canopy gap sizes were variable, ranging from a single tree to several hectares [215,217], although most likely ranged from 0.04 to 0.8 ha [177]. Rarely, large canopy gaps of more than 10 ha were created [215]. Canopy gaps were created by the local deaths of pines, firs, and incense-cedars but usually not by large giant sequoias, which are more resistant to fire [217].

Photo Credit

National Park Service image.

Figure 11—Historically, small pockets of intense surface fire created canopy gaps where giant sequoia seedlings could establish and recruit into the canopy. Here, young giant sequoias occur in canopy gaps created by fire.

Contemporary Fire Regimes

Fire exclusion began with European-American settlement first by removing America Indian ignitions and second by active fire suppression efforts [115]. Fire exclusion has reduced the frequency of low- and moderate-severity surface fires and led to a departure from historical forest conditions [120,201]. The probability of large-scale, high-severity fires in giant sequoia-mixed conifer forests has substantially increased because fuel density and horizontal and vertical continuity have dramatically increased relative to historical conditions [252].

Fire Ignition and Season

Lightning ignitions may not have changed substantially since European-American settlement, but American Indian ignitions have been severely reduced [120]. European-American settlement in the Sierra Nevada began in the 1860s, with extensive livestock grazing, logging, and mineral exploration [33]. American Indian cultural burning ceased around 1875, depending on location [26]. Initially, ignitions by European-American settlers replaced American Indian ignitions, and in some areas ignitions by settlers may have increased ignitions due to both planned and accidental ignitions [14,120]. A gradual shift occurred starting in the 1890s to a period of reduced human ignitions and increasingly effective fire suppression programs [120,201]. An area was burned in General Grant National Park in 1904 to reduce the fuel hazard, and this became the first “prescribed burn” in the national park system. A policy of fire prevention and exclusion was established after this and remained in effect for the next 60 years [14]. Prescribed fire was introduced in many groves starting in the late 1960s [108]. Today, prescribed fires are integral to managing giant sequoia groves (see Fire Management Considerations).

Fire Type

Excluding frequent low-severity fire has resulted in dense forest conditions with ladder fuels that connect the multiple canopy layers and increase the risk of crown fires [90,93,116,116,168,177,251] (see Contemporary Forest Structure).

Fire Frequency, Severity, and Area Burned

Fire was excluded from most giant sequoia groves after European-American settlement until about the 1960s when prescribed fire was introduced [166,215,244]. Fire records from 1959 to 2009 indicate only three wildfires burned into giant sequoia groves: the 1987 Pierce Fire in Redwood Mountain Grove, the 1987 Case Fire at Case Mountain Grove, and the 2008 Solo II Fire in the Black Mountain Grove [141]. Since 2015, however, there have been many large, severe wildfires including the 2015 Rough Fire, the 2017 Railroad and Pier Fires, the 2020 Castle Fire, and the 2021 KNP Complex, SQF Complex, and Windy Fires [142,196,197,204]. These fires have had substantial effects on giant sequoia populations. Preliminary estimates indicate that the 69,000-ha Castle Fire, which burned in about 20 giant sequoia groves and totaled about 3,900 ha within the groves, killed as many as 10,000 large giant sequoia trees (≥1.2 m in diameter), representing about 10% to 14% of the total population of all large trees across the species’ natural range [180,221]. Twenty-seven giant sequoia groves were fully or partially within the fire perimeters of the KNP Complex Fire and the Windy Fire, burning 2,472 ha of grove area. These fires killed an estimated 2,261 to 3,637 large giant sequoias, which make up an estimated 3% to 5% of all large giant sequoias [197]. Together, these studies estimate that 13% to 19% of large giant sequoias may have been killed during fires in 2020 and 2021 alone, much of it in large, contiguous areas of high-severity fires. From 2015 to the end of the 2021 fire season, roughly 65% of the range of giant sequoia had burned in a wildfire [196]. Departure in fire frequency from historical estimates varies among locations from low to extremely high (fig. 12) [233].

Photo Credit

Map from [233].

Figure 12—Fire-return interval departure (FRID) classifications for giant sequoia groves within Sequoia and Kings Canyon National Parks. FRID value of 1 = extreme departure (5 or more maximum return intervals missed), 2 = high departure (2 – 5 intervals missed), 3 = moderate (0-2 intervals missed), 4 = low (time since last fire < max return interval) [233]. Many areas with low departure have been managed with prescribed fire [66].

Once a large, high-severity fire occurs in a previously fire-excluded stand, a positive feedback loop is possible. After severe fire, abundant woody fuels remain, and shrubs become dense. These areas have a high probability of reburning at high severity, especially during extreme fire weather. If severely reburned areas are too far from surviving tree seed sources, a type conversion to shrub fields with few surviving tree seedlings may occur [220].

The area that burned at low and moderate severity in recent years is less than during the presettlement period, while the area that burned at high severity is greater [137,237,259]. For example, estimates of annual area burned at low-to moderate-severity (AALMS), percentage of area burned at low-to-moderate severity (PLMS), and total annual area burned (AAB) in moist mixed-conifer forests, including those dominated by giant sequoias, in modern times (2010–2020) are below estimates of those for the presettlement period (prior to 1850). Conversely, estimates of annual area burned at high-severity (AAHS) and percentage of area burned at high severity (PHS) in modern times are above estimates of the presettlement period (table 7) [259]. The tendency of modern forest fires that escape initial attack and burn large areas at high severity is driven by 1) unnaturally high fuel loadings and 2) weather conditions that reflect a steadily warming climate [259].

Many studies note a greater proportion of high-severity wildfires in giant sequoia-mixed conifer forests (19%–30%) today [142,197,221] (table 8), compared with only 12% historically, as estimated from LANDFIRE models of mixed-conifer forests with and without giant sequoia [130].

Contemporary Forest Structure

Forest structure and composition have changed since European-American settlement. Changes include the following:

1. Fuels: Abundance, density, and horizontal and vertical continuity of fuels have substantially increased [108,116,215,252].
2. Tree Abundance: Density and frequency of small, shade-tolerant trees have increased, while that of young giant sequoias and other shade-tolerant trees have declined [116,168,215,227].
3. Tree Species Composition: The dominant tree species in giant sequoia groves is the same today as historically, but there are lower proportions of shade-intolerant species like giant sequoia, ponderosa pine, and Jeffrey pine, and higher proportions of shade-tolerant species like white fir and incense-cedar [28,116,177,215,215].
4. Shrub Abundance: In some locations, there may be fewer aggregations of fire-adapted, shade-intolerant shrubs [28,118,215]. Shrub aggregations in Redwood Creek watershed in Kings Canyon National Park declined from about 19% of the watershed in 1890 to only 11% of the watershed in 1977 [28]. However, in other locations, there is risk of type conversion from giant sequoia forest to shrub fields following recent high-severity fires with distant giant sequoia seed sources [220] (see Fire Severity).
5. Giant Sequoia Regeneration: There are fewer canopy gaps with early successional vegetation and more dense stands with closed forest canopies [90,116]. Low giant sequoia regeneration is commonly attributed to dense forest understories and buildup of litter [168,256]. Giant sequoia seedling and sapling abundance is lower than historical levels [168,185,256].

In areas where fires have been excluded for long periods (e.g., FRID = 1, fig. 12), the number of canopy gaps and the diversity of gap sizes have been reduced. This, coupled with the build-up of duff and litter that would have been consumed by frequent fire, has resulted in lower seedling establishment and recruitment. Thus, groves have fewer young giant sequoias than they had historically [233]. Canopy gap size has increased in areas that have experienced high-severity fires over large areas [37].

For additional fire regime information, search FEIS for this species by entering the species name on the Advanced Search page and selecting “Fire Regime” as the publication type.

Fire Management Considerations

The U.S. federal government manages most giant sequoia grove area (Forest Service: 49%, National Park Service: 28%, Bureau of Land Management: <1%). The rest is managed by the University of California Davis (11%) and the Tule River Indian Reservation (4%), or is privately owned (8%) [215]. Sequoia National Park and General Grant National Park (now part of Kings Canyon National Park) were founded in 1890 with the intent of protecting giant sequoia groves from logging [33,244]. For a history of giant sequoia logging and national park establishment, see Vale (1975) [244].

Broad goals of fire management in giant sequoia groves include:

1. Protecting groves by reducing hazardous fuel accumulations to prevent or minimize the risk of severe wildfire.
2. Restoring groves to the range of conditions that existed historically (see Historical Fire Regimes).
3. Sustaining populations of giant sequoias by ensuring adequate giant sequoia regeneration [44,93,151,215].

Prescribed fire alone or with mechanical treatments are recommended to achieve these management goals (e.g., [14,16,84,93,151,215]). For example, the fire and fuels management plan for Sequoia and Kings Canyon National Parks focuses on fuel reduction and seed bed preparation using prescribed fire to promote giant sequoia reproduction [151,243]. In addition, “natural prescribed fire” and “wildland fire use” (i.e., managing lightning-ignited wildfires under specified conditions) are frequently recommended (e.g., [14,16,84,119]). Herbicides may also be used in some cases to reduce competing vegetation [198].

Fire managers have been using prescribed fire to manage giant sequoia groves since 1969 [108] (see Contemporary Fire Regimes). Although resources, policy, and boundary limitations along with planning impediments have limited their implementation in many groves [196]. It is often recommended that prescribed fire characteristics (e.g., frequency, severity, pattern, and size) mimic that of historical fires such that the historical forest structure is maintained (e.g., [9,45,119,215]). This includes enabling fires to burn at low to moderate severity, with some smaller patches of intense, high severity fire that create canopy gaps necessary for giant sequoia regeneration [196,220] (see Mimicking Historical Fire Characteristics). Because fire exclusion has increased fuel loads and altered forest structure, it is not always possible to apply prescribed fire in a way that mimics historical fires without undesirable effects, such as large tree mortality. Under these conditions, mechanical and manual treatments may first be used to reduce fuels [215,243] (see Grove Restoration). Specific restoration treatments might involve reducing surface fuel loads and the density of small trees (e.g., <80 cm diameter), reducing surface fuels and understory around monarch giant sequoias [243] and creating canopy gaps [9,244].

Photo Credit

U.S. National Park Service image.

Figure 13—Prescribed fire in a giant sequoia grove in Kings Canyon National Park.

Whether restoration treatments are needed depends on past grove management and specific restoration goals [215] (see Grove Restoration). In a review of giant sequoia ecology and management, Stephenson et al. (1996) stated that a restoration goal should be to “come as close as is practical to restoring grove structure and composition to the usual range of conditions that existed during the 1,000 years preceding Euro-American settlement”. The term “usual range of conditions” was meant to exclude rare extremes that may have occurred over the last millennium, such as the large expanses of trees that were likely killed by the widespread, severe fire of 1297 in Mountain Home Grove [215] (see Historical Fire Regimes: Fire Intensity and Severity). Once restoration goals are met, the objective shifts from restoration to maintaining the range of historical fires using prescribed fires and/or lightning-ignited fires [9] (see Mimicking Historical Fire Characteristics). When regeneration is insufficient following such treatments, planting giant sequoias is recommended. In heavily visited giant sequoia groves, additional measures might be warranted. For example, ancient giant sequoias are sometimes protected by removing nearby understory trees or raking duff away from their trunks [9] (see Protecting Individual Trees).

Grove Restoration

A 1996 review of giant sequoia ecology and management classified past grove management into four broad categories: 1) areas that have been continuously protected from both fire and logging, 2) areas that have been protected from logging but treated with prescribed fire, 3) areas that were logged before 1980, and 4) areas that were logged after 1980 [215]. About one-fourth (23%) of the area of all naturally occurring giant sequoias was logged between 1880 and 1980, mostly early in that period. Logging included most or all giant sequoias. Logged groves have regenerated as complex mosaics of forest patches of differing structures (i.e., tree diameter, height, and density) and species compositions. Most of these heavily logged groves are now dominated by dense growth of young trees or brush. Fuels have accumulated to the point that many historically logged areas are now at risk of unusually severe wildfires [215]. Grove areas that have been continuously protected from both fire and logging and those logged before 1980 are most at risk of high-severity wildfires and may benefit most from fuel reduction treatments [115,215] (see Contemporary Fire Regimes).

In many areas, fire alone can effectively reduce surface fuels, reduce the density of small shade-intolerant trees, and increase the proportion of small giant sequoias [85,106,107,114,123,183,216,246]. In other areas, fuel accumulations may be so high that prescribed fires either cannot be conducted safely or could kill numerous old giant sequoias and pines. In those areas, mechanical fuel reduction treatments are needed before prescribed burning. Once fuels are at acceptable levels, repeated prescribed fires can maintain desired conditions [115,179,215,216].

First-entry prescribed fires occur under specific weather and fuel conditions, and fire managers use ignition patterns that mediate fire behavior [215]. Fire-excluded groves often accumulate fuel quickly after prescribed fires. This is because dense thickets of small trees are killed and eventually fall, contributing to downed woody fuel. Duff accumulates comparatively slower after fire [166,215]. The second prescribed fire consumes the abundant woody fuels that resulted from the first fire. After the second fire, fuels accumulate slower because fewer understory trees are killed that contribute to surface fuels (see Fuel Characteristics). Therefore, successive prescribed fires may be less frequent [106,110,119,131,215] (see Mimicking Historical Fire Characteristics). One study suggested that two or three fires about 10 years apart would return stand density to historical levels following decades of fire exclusion [109]. Another study suggested that several repeated burns, perhaps at 5- to 8-year intervals, may be required to initially reduce ladder fuels, followed by periodic burns at longer intervals (probably at 8- to 20-year intervals) to prevent the development of dense growth of shade tolerant white fir [119]. A review of fuels treatment practices for mixed conifer forests states that the longevity of prescribed burn effects range from 10 to 14 years in Sierra Nevada mixed conifer forests [57], this suggests a similar interval may be warranted for giant sequoia groves.

The risk of large, high-severity wildfires in groves can be reduced by reducing hazardous fuels with treatments including prescribed fire inside and/or outside groves [108,142,215]. For example, a low- to moderate-severity November prescribed fire in Redwood Mountain Grove reduced the potential for severe fires by reducing surface and subcanopy crown fuels and by increasing the mean height of the base of the crown from 0.9 to 4.9 m. Surface fuels were reduced from 203,500 to 30,100 kg/ha, while live crown fuels were reduced from 18,000 to 7,800 kg/ha. Modeled spread rates (3.8 cm/sec versus 0.05 cm/sec) and intensities (28.8 cal/cm²/sec versus 0.25 cal/cm²/sec) decreased substantially from before to after the prescribed fire, respectively. After the prescribed fire, there was little to no forward spread predicted under extreme conditions [119]. In giant sequoia groves in Sequoia and Kings Canyon National Parks, prescribed fires reduced tree density by 61%, with the greatest reduction in trees ≤80 cm DBH. The resulting forest structure fell within the historical range and included targets of 10 to 75 trees/ha for trees ≥80 cm DBH, and 50 to 250 trees/ha for trees <80 cm DBH, for a total tree density of 60 to 325 trees/ha [108].

A giant sequoia management strategy recommends the following restoration goals based on reference (historical) conditions:

• Giant sequoias should account for approximately 55% to 75% of total grove basal area and ≥10% of the total trees.
• Other trees should account for 25% to 45% of the total basal area, with white fir being the dominant species among these. Incense-cedar, sugar pine, ponderosa pine, and California black oak are also important components of most groves, but even in combination should occupy only 20% of the total basal area. For less common associates, such as Jeffrey pine, Douglas-fir, and California red fir, no specific recommendations were given other than to recognize their importance in groves [177].

Fuel treatments that reduce slash fuels are more effective at reducing fire risk than those that do not [213]. Fire behavior models for giant sequoia-mixed conifer forest indicate that prescribed burning, mechanical thinning, and chipping followed by prescribed burning, and salvage or group selection cuts with slash reduction treatments resulted in lower average fire line intensities, heat per unit area, rate of spread, area burned, and scorch heights than treatments that did not treat slash (e.g., cut and scatter, salvage, or group selection). These treatments resulted in fire behavior that was more extreme than in untreated forest [213]. Wei (2012) created a fuel treatment optimization model in giant sequoia groves to improve the efficiency of suppressing future wildfires by strategically allocating fuel treatments [254].

Fire behavior can be moderated in areas where fuels were reduced by fuel treatments or previous wildfires [9,83,142,197,219,220]. For example, prescribed fire and mechanical thinning treatments within the last 10 to 15 years reduced fire severity during the 2021 KNP Complex Fire and allowed fire crews opportunities to safely fight fire more effectively [197,220]. Similarly, fire effects to large giant sequoias during the 2021 Windy Fire were moderated by the 2016 Meadow Fire, which burned primarily at low severity where previous treatments had reduced tree densities [142].

Low- to moderate-severity wildfires can aid in restoration of groves [42,142]. Following three wildfires that burned inside and outside of groves (the 2021 SQF Complex, 2021 KNP Complex, and 2020 Rattlesnake Fires), short-term restoration goals for surface fuel reduction were met after fire in all severity classes. Burned areas had 80% less fuels than unburned areas, overall. Fine woody debris, litter, and duff declined with severity, while coarse woody debris was more variable. Restoration goals for small tree (<80 cm DBH) density were roughly met after low- and moderate-severity fire, but small tree densities tended to be slightly lower than restoration goals after high-severity fire. Large tree (≥80 cm) densities tended to be lower than restoration goals after fire in all severity classes, possibly due to high levels of recent drought-induced tree mortality. After fire, the large amount of standing fuels, the relative scarcity of large trees, and the potential lack of fuel heterogeneity were still of management concern [42]. In areas of the 2021 Windy Fire that burned at low and moderate severity (38% and 25% of burned grove area, respectively), tree densities, surface fuels, and basal area were reduced to desirable levels and the diameter distribution of live trees shifted to a more even distribution characteristic of reference stands (i.e., relatively low densities of trees <51 cm DBH). Low- and moderate-severity burned areas had low levels of crown damage and mortality of large giant sequoia trees (mean ≤7%, but up to 17% with estimated delayed mortality). In low- and moderate-severity burned areas, the density and basal area of giant sequoias often increased relative to that of shade-tolerant species, such as white fir. In addition, the density and relative abundance of giant sequoia regeneration increased to levels within the natural range of variation [142].

Compared to the beneficial effects of low- and moderate-severity wildfires, high-severity wildfires can hinder attempts of achieving restoration goals through mortality of desirable, large trees. In areas of the 2021 Windy Fire that burned at high severity (33% of burned grove area), 72% of large giant sequoias died (80% with estimated delayed mortality), >99% of other large conifers died, and giant sequoia regeneration densities appeared insufficient (i.e., below reference condition), particularly in areas of very high severity fire. Nearly all high-severity patches were larger than the natural range of variation for mean patch size in Sierra Nevada mixed-conifer forests [142].

Mimicking Historical Fire Characteristics

Once fuels in giant sequoia groves have been restored to historical conditions, researchers recommend using a combination of prescribed fires and/or wildfires that maintain the frequency, severity, pattern, and size of fire historically to the extent possible as a long-term strategy to ensure long-term sustainability of groves [9,169,215].

Frequency

Burning frequently mimics historical fire frequencies and prevents high fuel accumulation. (see Historical Fire Regimes). Fuel accumulation rates suggest that prescribed fires at approximately 10-year intervals will prevent a return to hazardous fuel levels present in fire-excluded forests [106,107,109,110,131] (see Fuel Characteristics). After about 35 years, fuels may accumulate enough to carry a crown fire [194]. Using historical fire frequencies and fuel accumulation rates, a strategy for managing giant sequoia groves suggests burning groves at 5- to 20-year intervals [177].

Intensity and Severity

Historically, fires in giant sequoia-mixed conifer forests were spatially variable but mostly of low and moderate severity with small patches of locally intense, high-severity fire that create canopy gaps required for giant sequoia regeneration (see Plant Response to Fire) [104,220]. Prescribed fires can create canopy gaps that produce abundant giant sequoia regeneration provided they have small patches of high-severity (e.g., [219,220]). However, uniformly low-severity prescribed fires may not create gaps or promote giant sequoia regeneration [141]. Silvicultural treatments create canopy gaps and expose mineral soil, but without the convective heat from pockets of intense fire that stimulate seed dispersal of crown-held seeds, postfire regeneration is likely to be sparse [209,214,215] (see Silviculture).

Slash pile burning can create areas of severe fire where giant sequoia regeneration can occur, especially in fire-excluded forests where fuels have accumulated to levels that preclude broadcast burning without first reducing fuels [88]. Slash piles should be positioned away from giant sequoias 1) to reduce potential damage to the trees and 2) to reduce competition of regenerating seedlings for sunlight and soil resources with these trees. However, they should be close enough to stimulate seed dispersal [92,209].

Modern (2010–2020) estimates of percentage of area burned at low-to-moderate severity are lower than those prior to European-American presettlement, while modern estimates of percentage of area burned at high severity are above those prior to European-American presettlement [259] (see Contemporary Fire Regimes: Severity). To reverse these patterns, it is necessary to substantially increase the area and percentage of forest burned at low to moderate severity, but especially at moderate severity because prescribed fires that burn giant sequoia groves have been mostly of low severity [237,259]. Achieving these goals will likely require increased use of prescribed fire, wildfire managed for resource benefit, and/or other types of intentional fuel treatments [259].

Pattern and Size

Under historical fire regimes, giant sequoia-mixed conifer forests were a mosaic of even-aged cohorts in small canopy gaps within an uneven-aged forest [93] (Stand Structure). This suggests that prescribed fire should be applied in a patchy manner over time [45,93,215]. Researchers recommend varying burns spatially and temporally to create a mosaic of vegetation by 1) using small-scale prescribed burns, 2) periodically altering burn unit boundaries, 3) placing burns of different ages next to each other to create “a variety of burn contrasts”, 4) retaining “pockets” of understory vegetation, and 5) leaving some areas unburned each year [14,44,45]. Alternatively, larger low-severity burns could create a variety of successional stages in a mosaic if they include scattered, higher severity burned patches (“hot spots”) that create small forest canopy gaps [169,256].

To maximize giant sequoia regeneration in various age classes, a review recommends the following attributes of canopy gaps and successional stages (see Seedling Establishment and Mortality):

• Canopy gaps should be small (e.g., from about 0.04 to 0.80 ha).
• Canopy gap boundaries should be diffuse.
• Recently created canopy gaps (<10 years old) should occupy 1% to 10% of grove area, except for sites with unproductive soils.
• Early successional patches (vegetation 10–20 years old) should dominate 30% to 40% of grove area.
• Mid-successional patches (20–150 years old) should dominate on 40% to 50% of grove area.
• Late-successional patches (>150 years old) should occupy 10% to 20% of grove area [177].

Fire Season

Historically, fires occurred in late summer and early fall [21,164] (see Historical Fire Regimes). Prescribed fires in giant sequoia-mixed conifer forests generally occur in late spring and early summer as well as in fall [9,156,157]). Very little information is available about how prescribed fire timing affects giant sequoias. One study suggested that damage to giant sequoias may be greater following late season than early season prescribed fires due to differences in fuel moisture, fire weather, and/or other factors affecting fire behavior [176] (see Immediate and Delayed Fire Effects). This suggests that prescribed fires could be conducted in early season to minimize adverse effects [174]. In Sequoia National Park, researchers concluded that burning areas with high fuel loads in early season (late June) rather than during late season (late September–late October) when fuels are drier may promote heterogeneity in fire effects that more closely approximate the expected patchiness of historical fire [122].

Protecting Individual Trees

Photo Credit

National Park Service image.

Figure 14—In 2021, the General Sherman Tree was wrapped with protective foil around the tree's lower 3 to 4.5 m as the KNP Complex Fire approached Giant Forest. While giant sequoias are fire-dependent and able to withstand the heat of moderate-intensity fires with their thick, insulating bark, more intense fires have damaged or killed many large sequoias in recent years.

Giant sequoia groves are culturally significant and parks and other public lands with accessible giant sequoia groves are heavily visited each year [44]. Preservation of these “cultural objects" or “trees of special interest” is a management goal of the parks [44,151] and special precautions are often taken to prevent fire from injuring them [180,243]. For example, in the fire and fuels management plan for Sequoia and Kings Canyon National Parks an objective is to protect trees of special interest from fire. In the event that fire should spread to a tree of special interest, the objective is to prevent more than 30% of total crown scorch on trees ≥1.2 m in diameter and to prevent excessive bark charring 1) >3 m above the ground, 2) around more than 50% of a tree’s circumference, and 3) on more than 10% of trees ≥1.2 m in diameter [151]. In 2021, firefighters wrapped the General Sherman Tree and other large giant sequoias with protective foil around the tree's base as the 2021 KNP Complex Fire approached Giant Forest [180]. However, compared to other techniques (e.g., thinning and prescribed burning), wrapping tree bases may be the least effective in protecting them [180]. During the 2021 Windy Fire, firefighters reduced fuels at the base of large, fire-scarred trees and extinguished fires burning large giant sequoias, protecting many of these trees [142]. Other techniques designed to minimize bark charring and crown scorching include burning out from the base and using foam or water to protect fire scars from igniting [167].

During the 2021 Windy Fire, efforts to protect giant sequoias included constructing fire lines and removing ladder fuels surrounding groves or groups of giant sequoias and setting up sprinkler systems in advance of the fire. Some giant sequoias had duff and woody debris scraped away from their trunks. After the fire burned through the groves, firefighters extinguished hot spots. Initial observations indicated that giant sequoia trees treated before the fire were more likely to survive than untreated trees. Those with duff and woody debris scraped away, especially near fire scars, were less susceptible in most cases. In the 2020 Starvation Complex Fires, four out of six giant sequoias that had surrounding fuels removed before the fires reached them survived. An estimated 116 trees not accessible before the fires died. Similarly, in the Long Meadow Grove, more than a decade of fuels reduction efforts helped save the giant sequoias along the Trail of 100 Giants [5]. In the 2021 Windy Fire postfire ecological assessment, Meyer et al. (2024) recommended localized fuel reduction treatments 40 to 70 m around the base of large giant sequoias, especially those with large fire scars, which are more vulnerable to postfire mortality (see Immediate and Delayed Fire Effects). They recommended targeting trees in upper topographic positions and in portions of the grove with high postfire tree densities and surface fuel loads [142].

Ensuring Adequate Regeneration

High postfire seedling densities are critical to maintain giant sequoia populations over time. Seedling censuses from mostly prescribed fires were used to construct reference densities that are sufficient to sustain giant sequoia populations. One year after fire, mean seedling density was 172,599 seedlings/ha, with high variability (95% confidence interval ≈ 60,000/ha - 320,000/ha). Such high densities are required to sustain a grove because seedling mortality is high during the first few postfire years. Five years after fire, the reference density averaged only 5% of the mean first-year density [220]. Therefore, even if giant sequoia seedlings appear to be abundant soon after a fire, the cumulative effects of high losses over time could mean they are insufficient to replace millennial-aged giant sequoias killed by fire [217]. An analysis of giant sequoia demography suggests that regeneration following prescribed burning in groves is often adequate to maintain population viability [262].

If natural regeneration is inadequate, such as in very high-severity burned areas where mature, seed-producing trees are killed over large areas, planting nursery-grown seedlings can help regenerate giant sequoia stands [215]. One study found poor germination rates and high seedling mortality from direct seeding in burned and unburned canopy gaps, but higher survival of planted seedlings. The researchers determined that seeding was “impractical as a restoration treatment,” and recommended planting seedlings instead. They noted, however, that the number of seeds sowed was very low relative to the number of seeds potentially dispersed naturally following patches of locally intense, high-severity fire [263]. Similarly, planting seedlings following severe mechanical disturbances can produce densities of small trees that are similar to levels observed after high-severity fires [262]. Giant sequoias exhibit a high degree of fine-scale genetic variability (e.g., [47,51,55,245]) (see Genetic Diversity) and maintenance of the genetic integrity of local sequoia populations requires using only local seed stock [177,215].

Planting success varies, in part, with sunlight and soil moisture availability within small canopy gaps, and thus, the size and configuration of canopy gaps and placement of plants and seeds within them affects planting success [141] (see Seedling Establishment and Mortality). Because seedlings are vulnerable to desiccation [88], sufficient moisture is important in survival of giant sequoias seedlings [93,149,209,214] (see Postfire Weather). Planting designs that incorporate variable densities and spatial arrangements that vary by microsite and topographic position are most likely to be resilient to future wildfires and climate change. More accessible locations (i.e., relatively gentle slopes near trails) where postfire giant sequoia regeneration is relatively low could be prioritized to increase the efficiency of reforestation efforts within severely burned groves lacking natural regeneration [142]. Planting should occur soon after canopy gap creation because shrubs and other competing vegetation could grow rapidly making proposed planting areas inaccessible [204]. Plus, resultant shrublands can facilitate a feedback loop of sequential high-severity fires [204,220] (see Contemporary Fire Regimes).