Plains cottonwood
Plains cottonwood (Populus deltoides var. occidentalis) also has other common names with geographical and topographical connotations: Texas cottonwood, river cottonwood, western cottonwood, and plains poplar.
Distribution
The range of plains cottonwood encompasses a broad, irregular-shaped band approximately 800 km (500 mi) wide and 2400 km (1,500 mi) long, extending south southeasterly from the southern prairie provinces of Canada into the high plains of northern Texas. This range spans approximately 20° in longitude (92° to 115° W.) and 25° in latitude (30° to 55° N.) (14,19).
Specifically, plains cottonwood grows from southern Alberta, central Saskatchewan, and southwestern Manitoba in Canada, south through the Great Plains in North Dakota, South Dakota, Nebraska, Kansas, western Oklahoma into northcentral Texas and extreme northeastern New Mexico; north in Colorado, eastern Wyoming, and eastern Montana. The eastern limit of the range is not well defined because it integrades with the western limit of the range of the typical variety, eastern cottonwood (var. deltoides) (13,14).
Environmental Associations
The climate of the Great -Plains, the region in which plains cottonwood grows, is distinctly continental. The region is characterized as dry subhumid to semiarid, with extremes and rapid fluctuations in temperature, unpredictable and limited precipitation, frequent and cyclic droughts, and strong persistent winds (3).
Average annual precipitation varies from about 250 mm (10 in) in the northern and western Great Plains to about 760 mm (30 in) in the extreme southeastern part of the species range. About 75 percent of the annual precipitation occurs during the growing season. Drought periods of 35 to 60 consecutive days may be expected annually, and periods of 60 to 70 days without rainfall may occur once in 10 years. Infrequent drought periods of 90 to 120 days have been recorded in the northern and southern plains, respectively. Drought hazard is greatest in the autumn and winter in the northern plains, and in the winter in the southern Great Plains where snowfall is less. High-velocity winds occur in all seasons but are strongest and most persistent during winter and early spring (29).
Average January temperatures vary from -15° C (5° F) in the North to 4° C (40° F) in the South. Minimum temperatures range from -46° C (-50° F) in the north to -18° C (0° F) in the South, with maximum temperatures of 38° C (100° F) to 46° C (115° F) throughout the region. The frost-free period varies from 100 d in the North to 220 days in the south (29).
Plains cottonwood grows along most of the rivers and streams that flow through the loessial soils of the Great Plains on sites that are 2.4 to 3.7 m (8 to 12 ft) above the water table. The taxon predominates on the level, narrow stringers of the river floodplains and stream bottom lands that cross the region. It is common in pure stands on river sandbars and on overflow land in the bends of large rivers but is also found in the beds of intermittent streams (1).
Plains cottonwood grows on soils of the order Entisols, mainly along alluvial streams, and on soils of the orders Mollisols, Alfisols, and Inceptisols on stream terraces, in drainage ways, and in bottom lands and subirrigated valleys. Best development is on deep, rich, well-drained loams; however, the species also grows on level subirrigated uplands of deep, sandy soils (1). Soil texture and fertility seem to be of lesser importance than moisture, however, in determining its occurrence.
Plains cottonwood grows between elevations of about 300 m (1,000 ft) near its eastern limit to about 1830 m (6,000 ft) in the foothills of the Rocky Mountains. It is seldom found above 2130 m (7,000 ft) (27).
Sexual Reproduction
Plains cottonwood's dioecious with only occasional deviations. Staminate and pistillate flowers are borne on twigs of the previous year's growth, appearing in early spring (April and May) before the leaves develop (20,27). Pollination is by wind. Following anthesis, the staminate catkins dry and fall within 2 weeks. Four to 6 weeks, ranging from June through August, are required for seed maturation (9,20).
The flowering period seems to be regular within the limits of geographic zones, but differences can occur in time of anthesis between stands and among trees within stands. Differences in date of flowering from year to year apparently depend upon temperatures. The ratio of staminate to ovulate trees is believed to be about 1 to 1. In the lower Mississippi Valley the ratio was reported to be 54 to 46 percent (10).
Minimum seed-bearing age of plains cottonwood is about 10 years, and fair to large seed crops can be expected annually. The seeds are very small, yet relatively large for the genus; they range from 551,000 to 1,056,000 seeds per kilogram (250,000 to 479,000 lb). Seeds have a tuft of "cottonlike" hairs attached and are dispersed primarily by wind, but also by water, over long distances a few days after ripening. Seedfall among trees within a locality varies greatly and may extend for 6 weeks or longer (9,20).
The viability of fresh seeds is high; 98 percent germination has been attained during the first 5 days following dispersal (9,20). Longevity of poplar seeds under natural conditions has been reported to be 2 weeks to 1 month. Vitality of fresh, unstored seed drops rapidly, however, if they are not kept moist. There is no evident dormancy.
Poplar seeds can be stored successfully and viability is prolonged if the moisture content is reduced to 4 or 5 percent. Air-dried P. deltoides seeds, stored in sealed containers at 1° to 4° C (34° to 39° F), were 100 percent viable after 6 months (9,20). Seeds of some poplar species have been similarly stored in vacuum-packed jars at 0° C (32° F) for as long as 3 years (11). Recently, it was demonstrated that P. deltoides seed stored at -20° C (- 4° F), either at normal air pressure or under vacuum, showed significantly higher germination than the same seed stored at 5° C (41° F) under vacuum (in sealed containers), after 6 months of storage. Thereafter, germination at -20° C (-4° F) remained unchanged during a 6-year study period, whereas germination of seed stored at 5° C (41° F) was considered unsatisfactory after 3.5 years. Eastern cottonwood seed should always be stored at below freezing temperature, even for short-term storage (28). Germination is epigeal.
Plains cottonwood seed germinates within 48 hours after dispersal on proven mediums such as moist silt, sand, or fine gravel in full sunlight. The aboveground portion of the seedling develops rapidly and vigorously. Constant moisture is required for at least several weeks to ensure the establishment and survival of the slower developing root systems of the seedlings (5,9).
The best planting sites are moderately welldrained, permeable, and fertile deep loam soils on bottom lands. Very sandy soil is suitable if the water table is within 3.7 to 4.6 rn (12 to 15 ft) of the surface. Even upland sites are satisfactory if they are fertile, not too shallow, and if rainfall is abundant and well-distributed. Full sunlight, freedom from competition of weeds and grass (particularly sod), and abundant moisture throughout at least the first growing season are essential to seedling survival and establishment (27).
Although initial establishment is usually good and growth is rapid on coarse sands and gravels of river bottom lands, periods of drought and fluctuating water tables make subsequent development uncertain (17). Establishment of plains cottonwood on meander lobes of rivers in southern Canada is positively correlated with flood flows during seed dispersal (June 1-July 10) (2). Floods during the seed-dispersal period recur in southern Alberta and northern Montana about every 5 years.
Asexual Reproduction
Plains cottonwood is easily reproduced by stem cuttings from 1-year-old "ripened" wood. Since this species, like other members of the genus, sprouts vigorously from both roots and stumps of young trees, clonal "stool" beds are commonly established for the production of these cuttings. Cuttings can also be taken from pollards, 1-year-old plants, or epicormic branches of old plants (11).
Healthy, straight, lignified wands without bark injuries approximately 2 m (6.6 ft) long and 3 to 30 mm (0.1 to 1.2 in) in diameter are cut from stool beds with a sharp knife during the dormant season (October to March), treated with fungicide, and placed in cool 50 C (410 F), moist storage (11). The wands are divided into cuttings approximately 25.4 cm (10 in) long and 10 to 20 mm (0.4 to 0.8 in) in diameter at midpoint and inserted in a mist-sprayed greenhouse rooting bench containing moist sand as a medium.
Recent greenhouse and field tests in Nebraska demonstrated that cuttings taken from the basal end of wands produce significantly more roots than those taken from the upper portion. Also, cuttings from clones of Nebraska and Minnesota-Wisconsin origins produced significantly higher numbers of roots than those of other geographic sources (33). About 4 to 6 weeks are required for rooting and subsequent field establishment. Rooted cuttings of R deltoides are not root-pruned when field-planted.
Unrooted cuttings can also be field-planted. In wetter climates and in heavier soils, 25.4 cm (10 in) cuttings are satisfactory. In drier climates and in sandy soils, cuttings 50 to 80 cm (20 to 31 in) long have been more successful. In both situations, dormant cuttings are planted in the early spring and are completely buried except for the top bud and 3 to 5 cm (1 to 2 in) of the wand. Difficult-to-root clones can also be grafted (11).
Experiments in Utah have shown that Populus deltoides (of unknown, but presumed eastern origin), R balsamifera, and R angustifolia, as well as R tremuloides, produce abundant shoots (suckers) and roots from root cuttings (segments). New shoots and roots originate from pre-existing suppressed buds embedded in the periderm along the surface of the root cutting and from the region of the exposed cambium at the cut ends. The presence of lateral root increased shoot growth, and the development of shoots and lateral roots responded to the inherent polarity of the root segments (18). The probability that the closely related plains cottonwood will react similarly would seem to be high.
Plains cottonwood can grow in pure stands, but it is frequently found as an associate in three forest cover types: Bur Oak (Society of American Foresters Type 42), Cottonwood (Type 63), and Cottonwood-Willow (Type 235) (22). Black willow (Salix nigra) and peachleaf willow (S. amygdaloides) are the most common associates. Other associates on the better sites include American elm (Ulmus americana), slippery elm (U. rubra), hackberry (Celtis occidentalis), boxelder (Acer negundo), green ash (Fraxinus pennsylvanica), red mulberry (Morus rubra), black walnut (Juglans nigra), American sycamore (Platanus occidentalis), eastern redcedar (Juniperus virginiana), and silver maple (Acer saccharinum) (1,17,30).
Associated shrubs and vines include sandbar willow (S. exigua), red-osier dogwood (Cornus stolonifera), indigobush (Amorpha fruticosa), coralberry (Symphoricarpos orbiculatus), wild grape (Vitis spp.), poison-ivy (Toxicodendron radicans), smooth sumac (Rhus glabra), and American plum (Prunus americana). In the western Plains, shrubs are scarce in the cottonwood stands, and several species of grasses and forbs are found in their place. These include sand dropseed (Sporobolus cryptandrus), buffalograss (Buchloe dactyloides), sunflowers (Helianthus spp.), lambs-quarters (Chenopodium album), and Russian-thistle (Salsola pestifer) (1).
Early diameter and height growth of plains cottonwood surpasses that of other species native to the Great Plains region (17). Growth and penetration of poplar seedling roots immediately following germination is reported to be relatively slow, however. About 5 days are required after germination for the primary root to begin downward growth, and after 12 days the root may be only 1.5 mm (0.06 in) long (19). Growth continues slowly for 3 weeks to 1 month, at which time taproots of the cultivar Petrowskyana, for example, grown indoors in fairly strong light, averaged only 2.5 cm Q in) in length at the end of I month. This growth pattern explains the critical need for continuous moisture during the seedling stage. Subsequent root growth is much more rapid.
Ninety-eight percent of the roots of a 43-year-old northern cottonwood (Populus monilifera), 19.8 m (65 ft) tall, were found to be in the top 1.2 m (4 ft) of a prairie clay soil near Fargo, ND (31). Roots of this and other non-drought-tolerant species formed shallow roots on dry sandy sites but had a tendency to grow deep vertical roots on very moist (nonsaturated) sandy sites. Similar trends of root development were revealed in excavations of plains cottonwood trees growing in silty loam soils in eastern Nebraska, where (1) a 14-year-old tree, 18.3 m (60 ft) tall, developed only shallow, widespread, and fibrous roots over and down to a water table 0.8 m (2.5 ft) deep; (2) a 16-year-old tree, 11.3 m (37 ft) tall, developed a moderately heavy root system downward to a water table 4.3 m (14 ft) deep and then branched outward; and (3) a 49-year-old tree, 21.3 m (70 ft) tall, developed a distinct and heavy taprooted pattern over an unreachable water table 18.3 m (60 ft) deep (24).
Young plains cottonwood trees grow 1.8 to 3.7 m (6 to 12 ft) per year in height under favorable conditions, surpassing other native species of the Great Plains region in height and diameter growth. Growth is most rapid in the first 25 to 30 years, by which time the trees can reach 15.2 to 22.9 m (50 to 75 ft) in height and 61.0 to 91.4 cm (24 to 36 in) in diameter. Cottonwood sources from Missouri (P. deltoides) and Nebraska (Sioux-land), along with silver maple (Acer saccharinum), ranked highest among seven species tested for the production of biomass during a 2-year study in Kansas (12).
Plains cottonwood usually attains maximum development in about 40 to 50 years. Mature trees can be 24.4 to 27.4 m (80 to 90 ft) tall, with diameters of 1.8 to 2.4 m (6 to 8 ft), and with clear holes for 9.1 m (30 ft) or more. The trees are usually single-stemmed with an open, spreading, symmetrical crown of massive horizontal branches and stout, more or less angled branchlets and twigs. While plains cottonwood is relatively short-lived, it can remain vigorous for 80 to 90 years under favorable conditions (21,27).
Survival and growth of cottonwoods on the Great Plains is directly dependent upon availability of moisture. Mortality during the drought of the mid-1930's was 59 percent along intermittent streams, 55 percent near springs that failed during the drought, and only 6 percent along continuously flowing streams (1).
Fully stocked cottonwood stands along creek and river channels and overflow land in Kansas are estimated to yield 168.0 to 210.0 m³/ha (12,000 to 15,000 fbm/acre) at 25 to 30 years of age (21). In North Dakota, 30-and 50-year-old plantation yields were 59.5 to 219.8 m³/ha (4,250 and 17,500 fbm/acre) gross merchantable volume, respectively, Scribner log rule (26).
Plains cottonwood 94 cm (37 in) in diameter outside bark and 19.8 to 22.9 m (65 to 75 ft) tall, growing in the South Platte River bottom, Morgan County in eastern Colorado, attained gross volumes of 8.0 m³ (283 ft³) inside bark (8). One could expect trees growing on more fertile and wetter sites along the Platte and Missouri River bottom lands in the eastern part of the range to achieve volumes in the magnitude of 11.3 m³ (400 ft³).
Management with Natural Regeneration
Plains cottonwood requires full sunlight for maximum growth. It is classed as very intolerant of shade and intolerant of root competition (21). It grows either in pure stands, which thin naturally and rapidly, or in open mixed stands, both of which are nearly always even-aged (17). After pioneering on alluvial sites, often with the willows, it is gradually replaced with other broadleaf species that can then become established under the forest conditions so created. Cottonwood does not normally regenerate until the overstory has broken up.
Within the large and climatically diverse north-south range of plains cottonwood, subtle but recognizable differences in the population have evolved by natural selection. What is here called Populus deltoides var. occidentalis has received at least nine names denoting either specific or varietal rank over the past two centuries (14).
Recently, the eastern cottonwood (Populus deltoides) complex has been treated as a group of three intergrading subspecies showing random or clinal variation or both within each subspecies (7). Plains cottonwood, on the basis of taxonomic affinities to the poplars of the Great Lakes region, was recognized as P. deltoides ssp. monilifera (Ait.) Eckenwalder. This treatment, supported by recent provenance evaluations in Nebraska, seems to be a more tenable interpretation. In these provenance evaluations the poplars of Kansas, Nebraska, and South Dakota, with smooth bark, small branches, small leaves with a few or no glands and few serrations, and prolific rooting habit tended to be similar to poplars of Minnesota. and Wisconsin origin (32).
Genetic Variation
The eastern members of plains cottonwood (Populus deltoides var. occidentalis) intergrade with western-most eastern cottonwoods (P. deltoides var. deltoides); therefore, the literature reveals no named hybrids between these very closely related populations (14).
Interspecific hybrids have been reported, however, between plains cottonwood and named species to the north and west. In southern Alberta, Canada, plains cottonwood is reported to cross and introgress readily with balsam poplar (Populus balsamifera L.), narrowleaf cottonwood (P angustifolia James), and possibly quaking aspen (P tremuloides Michx.) (4). Lanceleaf cottonwood (P. x acuminata Rydb.) is regarded as an interspecific hybrid between narrowleaf cottonwood (P. angustifolia James), which occurs from northern New Mexico, Nebraska, and North Dakota to southern Alberta, and plains cottonwood (P deltoides var. occidentalis) (14). Other reported interspecific hybrids involving plains cottonwood include Populus x jackii Sarg. (P balsamifera x deltoides var. occidentalis), and Populus x polygonifolia Bernard (P balsamifera x deltoides var. occidentalis x tremuloides (4).
Prolonged periods of severe environmental stress, such as drought, weaken trees physiologically and increase their susceptibility to disease and insect pathogens. Plains cottonwood, with its high water requirement is especially vulnerable (1). P. deltoides var. occidentalis trees, considered water-tolerant, showed 47 percent high stress rate and 18 percent mortality when inundated late in the growing season in the Central Plains (15).
Leaf rusts and stem cankers are the most widespread and damaging diseases. Leaf rusts cause premature defoliation of trees. This defoliation not only causes growth losses; it weakens the trees and increases their susceptibility to infection by other pathogens, which cause cankers and mortality.
Melampsora leaf rust caused by Melampsora medusae is one of the most serious leaf diseases of plains cottonwood. Others include Septoria leaf spot, caused by Septoria musiva, Marssonina brunnea leaf spot, and Alternaria leaf and stem blight, caused by Alternaria tenuis (16).
The most serious of the canker pathogens is Cytospora canker (Cytospora chrysosperma), which often results in wind-breakage at the wound area. Other canker pathogens include those caused by Septoria musiva, Fusarium solani, Phomopsis macrospora, Botryodiplodia theobromae, Cryptosphaeria populina, and Pleurotus ostreatus. Root and butt rots may be due to Ganoderma lucidum, Armillaria tabescens, and Scytinostroma galactinium (16).
The insects most damaging to plains cottonwood are the defoliators and wood borers; the former cause loss of vigor, the latter reduce the quality of lumber. Some of the more important defoliating insects include the cottonwood leaf beetle (Chrysomela scripta), cottonwood dagger moth (Acronicta lepusculina), forest tent caterpillar (Malacosoma disstria), poplar leaffolding sawfly (Phyllocolpa bozemani), fall cankerworm (Alsophila pometaria), and the fall webworm (Hyphantria cunea) (25).
Important boring insects include the poplar borer (Saperda calcarata), cottonwood borer (Plectrodera scalator), flatheaded wood borer (Dicerca divaricata), carpenterworm (Prionoxystus robiniae), poplar-and-willow borer (Cryptorhynchus lapathi), and the bronze poplar borer (Agrilus liragus) (25).
Several species of mites and aphids infest plains cottonwood, but their effects are not usually fatal.
Goods
The wood of plains cottonwood is coarse, odorless, soft, and lightweight, yet relatively strong. The heartwood is pale yellowish brown, the sapwood nearly white. The wood frequently warps on drying and is not durable in contact with soil and other moist conditions. It nails without splitting, is clean appearing, and takes printing and stenciling well.
The wood is used primarily for pallets, rough construction lumber (farm buildings), interior parts of furniture, excelsior, crating, and wood pulp (21,27). The pulp produces a very high-grade gloss paper.
New and potentially important commercial uses of the wood include roughage food for livestock and the production of fiber and reconstituted wood products derived from short-rotation (2- to 8-year) biomass operations (6,23).
Ecosystem Services
Plains cottonwood is an important component of windbreak plantings in the Great Plains. It is frequently used as an ornamental to provide quick, if rather temporary, esthetic and protective effects. Plains cottonwood can produce an effective windbarrier 12.2 to 15.2 in (40 to 50 ft) tall in 15 to 20 years on stream lowlands and on deep, sandy, subirrigated lands (17).
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