Pringle Falls Experimental Forest and Research Natural Area

Pringle Falls Experimental Forest is the oldest experimental forest in the Pacific Northwest. It was established in 1931 by Thornton T. Munger, the first director of the PNW Research Station. He envisioned it as a research center for silviculture, forest management, and incidence and effects of insects and disease in ponderosa pine-dominated forests east of the Oregon Cascade Range. Munger selected the 3,042 ha site of the Pringle Butte unit in 1914, and the 1,430 ha Lookout Mountain unit was added in 1936. Within the Pringle Butte unit, the Pringle Falls Research Natural Area was set aside for nondestructive research. The still functional headquarters buildings were constructed between 1932 and 1934.

Photo caption: Administrative headquarters at Pringle Falls Experimental Forest in central Oregon. USDA Forest Service photo by Paul Anderson.

Climate

The climate of this area is strongly influenced by the Cascade Range to the west and the Great Basin Desert to the east. Most precipitation occurs as snowfall and is received by mid-April in the hydrologic year. Annual precipitation averages 610 mm on Pringle Butte and 1,020 mm on Lookout Mountain. Daytime temperature maxima in the summer range from 21°C to 32°C. Summer nights are cool and frosts can occur throughout the growing season.

Climate models predict warming of air temperature in all seasons, with the greatest increases in summer. Annual precipitation is not predicted to vary much from current conditions but changes in timing and magnitude of snowpack and snowmelt are expected.

Historical and projected annual temperature (A) and precipitation (B) under the RCP 8.5 climate change scenario for the South-Central Oregon Adaptation Partnership assessment area. From Halofsky et al. (2019).

Geography and Substrate

Pringle Falls Experimental Forest is in the La Pine Basin in central Oregon. The terrain is flat to gently rolling, interspersed with small volcanic peaks and cinder cones. Pringle Butte, the oldest known geologic formation in the area, is a 5-million-year-old shield volcano rising 300 m above the surrounding basin. More recent deposits are sand and silt sediments, overlain with additional sands and gravels deposited by glacial outwash from the Cascade Range. Lookout Mountain, the highest point in the experimental forest (1,592 m), is a 300,000-year-old shield volcano emerging from La Pine Basin sediments. Overlaying the entire area is a 0.5 to 2m thick layer of pumice dacite and ash resulting from the eruption of Mount Mazama (now Crater Lake) nearly 6,600 years ago. Here, due to their youth, andesitic soils derived from this eruption have only a thin weathered surface layer, with low organic matter content, low nitrogen, sulfur, and phosphorus content, and high porosity with little capacity for water retention in situ.

Vegetation

Dense stands of ponderosa and lodgepole pine with antelope bitterbrush, Idaho fescue, and western needlegrass occur on flats and basin bottoms that are slow to drain in the spring. Because of topography pooling cold air from higher elevations, these stands are prone to frequent frosts that can kill ponderosa pine seedlings. In the mixed-conifer forest over 1,500 m in elevation, stands may contain ponderosa pine, grand fir, Shasta red fir, sugar pine, western white pine, whitebark pine, and mountain hemlock. Fire suppression during the 20th century largely altered the landscape within and surrounding Pringle Falls Experimental Forest. This led to high stand density comprised of young multi-storied forests and heavy fuel loads at the stand to landscape scale. Prior to the advent of modern fire suppression, the historical fire regime included low-intensity burns every 7 to 20 years, coupled with infrequent large and more intense fires.

Today, forest stands within the experimental forest are a mosaic of varying biological diversity, shaped by aspect, elevation, and disturbance events including wildfires 180 and 110 years ago, insects and disease outbreaks, and forest management. Because past forest treatments focused on optimizing ponderosa pine resiliency to insects and disease, and “leave” trees were preferentially ponderosa pine, it is the dominant conifer throughout most of the Pringle Falls Experimental Forest. Shrubs include antelope bitterbrush, ceanothus, and greenleaf manzanita.

Photo caption: Cascade lily (Lilium washingtonianum). USDA Forest Service photo by Lorelle Sherman.

The Pringle Falls Experimental Forest and Research Natural Area are about an hour’s drive south of Bend, Oregon, and are easily accessed from U.S. Highway 97. Overnight facilities within the PFEF are available for small groups involved in on-site research or educational activities by reservation only.

Lat. 43° 42' N, long. 121° 37' W

Research, Historical & Present

Ongoing Historical Studies

Some of the earliest forestry research in central Oregon occurred within the Pringle Falls Experimental Forest. A rating system still in use for determining the susceptibility of ponderosa pine trees to western pine beetle attack was based on west-wide monitoring that included ponderosa pine stands in this experimental forest (Keen 1936). In 1950, stand structure and growth of ponderosa pine seedlings released from a lodgepole pine-dominated stand were investigated (Mowat 1950).

During the 1970s, permanent research plots were established to investigate the response of ponderosa pine to fertilization, and the release and subsequent growth of ponderosa pine, lodgepole pine, and grand fir in differing tree densities. James W. Barrett initiated an ongoing study, known as the levels-of-growing-stock study, designed to investigate how tree and stand growth rates vary with tree spacing, and how the shrub-dominated undergrowth vegetation influence development of thinned, second-growth stands. Early publications from the levels-of-growing-stock study include Barrett (1973) and Barrett (1983).

The frequency, intensity, and spatial patterns of wildfire in old-growth ponderosa pine stands were examined, and the genetic characteristics of the remaining ponderosa pine were investigated. Research initiated in the 1980s and through the early 1990s evaluated the response of dwarf mistletoe-infected pine to thinning, interactions between fire and dwarf mistletoe, and development of stands in response to various silvicultural practices.

Andrew Youngblood utilized the Pringle Butte unit in 1999 to quantify old-growth ponderosa pine attributes for guiding the design of forest restoration prescriptions. The old-growth ponderosa pine reference conditions outlined in Youngblood (2004) provide an estimated range of natural variability in composition and structure in this ecosystem, as well as a baseline to measure departures within the ecosystem and a criterion for measuring the success of restoration treatments. Several valuable ecological lessons regarding management of old-growth ponderosa pine and young ponderosa pine have been learned from ongoing research within the experimental forest and are outlined in Youngblood (2011).

Since 2010, most of the research has been designed to increase ecological understanding on the following topics:

1. The long-term processes that regulate or influence the structure, composition, and pattern of forests.
2. The effect of forest management practices on the same.
3. The resulting resiliency of managed forests to insects, pathogens, and fire disturbance.

Silvicultural research has been focused on evaluating the following:

4. Forest restoration treatments designed to increase resiliency of dry conifer forests and mitigate compounded forest disturbances.
5. The role of shrub competition in ponderosa pine recruitment after treatment.
6. The effect of prescribed fire to control competing shrubs for the same.
7. Soil thermal and moisture properties, and gap production from high temperature fires on the success of ponderosa pine recruitment from natural seed fall. 

The Lookout Mountain Study

Today, low elevation dry conifer forests of the western United States face a greater risk of succumbing to fire and bark beetles, increased environmental stress, and their interactions. More than 10 million ha of forests in the western United States are classified as having moderate to high fire hazards. As a result, national and regional initiatives and legislation have identified the need for large-scale fuel reduction and forest restoration treatments to reduce environmental stresses and the risk for fire uncharacteristic to the ecosystem.

The Lookout Mountain Study provides a unique setting for long-term investigation of a range of silvicultural treatments designed to reduce surfac

e fuels and decrease canopy density while retaining large trees of fire-resistant conifer species. At its core, the study is designed to characterize the response of the structure and composition of the forest overstory, understory, and coarse downed wood to these treatments. The core study sets the experimental foundation for several ancillary studies that address additional ecological and biophysical responses, such as wildlife habitat, microclimate, and soil. Highlights from these studies can be found below.

The Lookout Mountain Study represents a rare opportunity to investigate silvicultural treatments at operational scales providing information directly applicable to similar management plans and their implementation in dry conifer forests in the western United States.

A more detailed description of the implementation of treatments and immediate forest responses are described the 2023 report, Forest Dynamics After Thinning and Fuel Reduction in the Pringle Falls Experimental Forest—Establishment and Early Observations of the Lookout Mountain Thinning and Fuels Reduction Study. 

Bark Beetle Incidence After Forest Treatments

Endogenous bark beetles facilitate important disturbances in forests in the western United States by accelerating mortality of stressed trees and creating small gaps in the overstory. Tree mortality due to bark beetles is influenced by numerous factors, including the level and duration of drought and hot drought experienced, tree to tree competition, and initial tree vigor. Forest treatments that reduce stand density were designed to improve individual tree vigor and resource availability, and thus the likely effectiveness of applied pheromones for control of beetle populations.

Fettig et al. (2021)(link is external) investigated the responses of bark beetles immediately after forest thinning and fuel reduction in the experimental forest. Bark beetle-related tree mortality was low when compared with similar studies in other ecologically related experimental forests. Bark beetle-related tree mortality is has temporally lagged, the lag should be captured by the 2019-2020 re-measure of forest treatment response, as yet unquantified. Future re-assessments will determine long-term tree mortality, snag recruitment (relevant to woodpecker habitat), and gap formation attributable to bark beetles.

Fire Effects on Soil Nutrients and Fungi

Frequent, low severity fire reduces dead and accumulated downed wood, and dead and live vegetation which enriches the soil by releasing nutrients bound in litter, and temporarily increases light levels conducive for conifer recruitment. The suppression of wildfire in the western United States resulted in an accumulation of downed wood. Large, downed wood burns hot and creates patches of bare mineral soil which is conducive for ponderosa pine seedling establishment if sufficient light filters through the canopy. Although hot burns are lethal to, and may eliminate soil fungi, re-population can occur from adjacent areas with lower temperature fire (fungal refugia), common in prescribed fires.

Forest Treatment Effects on Microclimate

The variation in elevation and aspect across a landscape leads to climatic gradients and microclimates (Burnett and Anderson, 2019(link is external)) which are important drivers of forest composition and structure. Forest treatments altered microclimate and microsite conditions by opening the canopy and increasing sun radiation and air flow at the ground level, which then altered air temperature, humidity, and soil temperature and moisture.

The spatial and temporal variation of microclimates within the Lookout Mountain study area were mapped and modeled before and after forest treatments using data collected from an array of sensors and data loggers. Seasonal differences in air temperature were revealed between treated and untreated areas, with relevance for fuel moisture and both prescribed fire and wildfire behavior when it occurs.

Woodpecker Habitat

As a guild of birds that nests in dead trees, woodpeckers passively bring fungi to their cavity excavation activities, and in turn, the fungal community facilitates wood decay, likely increasing the speed of cavity excavation, nesting, and energy spent on raising their broods. Nine species of woodpeckers are known within the experimental forest, including the white-headed woodpecker, a species of concern in dry conifer forests of the Pacific Northwest. Woodpecker nest sites were surveyed and mapped before and after forest treatments and showed an increase in active nest sites post-treatment.

Wood samples were taken from trees without nests, as well as active woodpecker nest trees to determine the diversity of decay fungi using genetic sequencing. Fungal communities differed between samples taken from woodpecker nest cavities and control snags, with nest cavities having the highest diversity of fungal communities. Analyses of decay pathways in wood samples are in progress to resolve which fungi facilitate woodpecker nest cavity excavation.

Future research may include determining snag density, proportion and characteristics of snags chosen by woodpeckers, and the length of time used before abandonment.

Songbird Habitat

Songbirds respond to habitat at multiple scales and in complex ways. Little is known about their response to dry conifer forest thinning and fuel reduction treatments. In an ongoing study of songbird habitat in the experimental forest, we are evaluating how prescribed thinning and fuels reduction treatments alter, maintain, create, or eliminate habitat attributes at multiple spatial scales, and how resultant forest structure and composition is associated with and supports bird species composition and relative abundance. Through a combination of songbird surveys, habitat attribute surveys, and modeling, the associations between habitat attributes, suitable habitat available within each forest treatment type, and avian functional groups will be developed.

Northern Spotted Owl Prey

A pair of nesting northern spotted owls was detected in the Lookout Mountain Unit in 2012. Their detection led to surveys of northern spotted owl prey, small mammals, and before and after forest treatments. A mark-recapture approach was used to determine how changes in forest structure were related to changes in owl prey species. The abundance and density of prey varied in their response to treatment type by prey species. Posttreatment, Siskiyou chipmunks had the highest density in unmanaged areas and had the lowest density in heavily thinned areas. Golden-mantled ground squirrels showed the reverse pattern.

Future research may investigate how small mammal populations shift dynamically with forest succession.

Research Opportunities

The Pringle Falls Experimental Forest provides many opportunities for investigative field research in established, replicated forest treatments in the Lookout Mountain unit or in the unmanaged Natural Area Reserve in the Pringle Butte unit. Forest tree structure and composition of the ponderosa pine- dominated forest, and the mixed conifer forest in the experimental forest is similar to forests on the eastern flank of the southern Cascades, the eastern and western slope of the Sierra Nevada, and the eastern third of the Transverse Range. Shrub species change across this gradient, with ceanothus shrubs replaced by ceanothus sub-shrubs, and an increase in bitterbrush dominance and sage.

Approved research is supported by on-site housing and WiFi, and the town of La Pine, Oregon, is nearby for supplies. Some potential studies are suggested above, including ponderosa and lodgepole pine silviculture, forest treatment effects on insect and disease frequency, fuel reduction and effects on fire behavior in repeated treatments, habitat use of species of concern and their prey, and the effects of natural disturbance regimes, such as drought and hot drought on tree and shrub growth in forest treatments.

In 1995, an annotated bibliography covering 63 years of research at Pringle Falls Experimental Forest was published: Research Publications of the Pringle Falls Experimental Forest, Central Oregon Cascade Range, 1930 to 1993.

Publications published after 1993 that are relevant to the Pringle Falls Experimental Forest are included in the 2023 report, Forest Dynamics After Thinning and Fuel Reduction in the Pringle Falls Experimental Forest—Establishment and Early Observations of the Lookout Mountain Thinning and Fuels Reduction Study (PNW-GTR-1015).