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Populus trichocarpa/Salix exigua

black cottonwood/coyote willow

Vegetation Composition: Populus trichocarpa/Salix exigua is an early seral plant association dominated by Populus trichocarpa. Stands are mostly even-aged with cohorts ranging from seedling and sapling thickets to older, pole size tree woodlands (Moseley and Bursik 1994; Hansen et al. 1995; Manning and Padgett 1995; Hall and Hansen 1997). Populus trichocarpa cover ranges from 40 to 98% with constancy over 50%. Other trees, such as Populus angustifolia, Populus deltoides, Populus hybrids, and conifers are occasionally present with low cover. There is a sparse and patchy tall-shrub layer composed of Salix exigua (or other species) and Alnus incana (each usually less than 10% cover) with occasional Amelanchier alnifolia and Cornus sericea. A few low shrubs, such as Rosa woodsii and Symphoricarpos albus, may also be present (usually with low cover and constancy) (Moseley and Bursik 1994; Hansen et al. 1995; Manning and Padgett 1995; Hall and Hansen 1997). The herbaceous understory usually has low cover. Instead, rock cover is typically over 30% with bare soil and gravel with over 20% cover combined. The understory is often dominated by exotic, weedy forbs and grasses which are usually rhizomatous perennials or biennials able to withstand, and thrive with annual flooding disturbance. Native forbs, all usually with low cover and constancy, often include Achillea millefolium, Artemisia ludoviciana, Aster spp., Castilleja miniata, Epilobium spp., Equisetum arvense, Fragaria vesca, Heracleum lanatum, Smilacina stellata, Solidago canadensis, and Thalictrum occidentale. Native grasses have even less cover and include Calamagrostis canadensis, Carex spp., Elymus glaucus, and Muhlenbergia richardsonis. Moss is occasionally present.

Classification Comments: The Populus trichocarpa/Salix exigua plant association is synomous with associations described as Populus trichocarpa/Recent alluvial bar. It is similar to the Populus/Stream bar association dominated by either Populus trichocarpa or Populus angustifolia described by Manning and Padgett (1995). In Montana and elsewhere, a similar situation exists where any one, or combination of, Populus spp. (Populus trichocarpa, P. angustifolia, and P. deltoides) may dominate stands. Though dominance is usually clear (species are generally separated by elevation and region), mixed Populus stands (or stands dominated by hybrids) are occasionally found, thus complicating classification (Hansen et al. 1995; Hall and Hansen 1997). This association is recognized by its occurrence on recent alluvial deposits along rivers and perennial streams that are seasonally flooded. Identification of this association is less clear when this community grades into less frequently flooded early or mid-seral stands (such as Populus trichocarpa/Rhamnus spp., Populus trichocarpa/Symphoricarpos albus, or Populus trichocarpa/Cornus sericea) (Moseley and Bursik 1994; Hansen et al. 1995; Hall and Hansen 1997).

Environment: The Populus trichocarpa/Salix exigua plant association is found on narrow to broad valley floodplains of major streams (often meandering, with low to high gradients) at low to mid-elevations. The association is found within floodplains and channels on point bars, side bars, mid-channel bars, deltas, and islands where frequent flooding (nearly every year) deposits fresh alluvium (Moseley and Bursik 1994; Manning and Padgett 1995; Hansen et al. 1995; Hall and Hansen 1997).

Soils: Soils range from coarse loamy or sandy-skeletal (Manning and Padgett 1995; Hansen et al. 1995; Hall and Hansen 1997) to predominantly river cobble and gravel with fines removed by scouring. Soils are shallow, well-drained Entisols (Fluvents) overlying gravel and cobble with the water table at the surface during spring flooding, dropping to 50 cm by late summer (Manning and Padgett 1995; Hansen et al. 1995; Hall and Hansen 1997).

Management Considerations: Early seral Populus trichocarpa/Salix exigua stands are very important for reproduction and maintenance of floodplain Populus trichocarpa stands. This community does not produce large amounts of forage, however, livestock do browse Populus trichocarpa, especially when there is little else to eat. Though Populus trichocarpa produces many suckers and seedlings after flood damage, overgrazing of young stems and foliage will eventually eliminate Populus trichocarpa. Thus, post-flood stands should be protected from livestock (Hansen et al. 1995; Manning and Padgett 1995; Hall and Hansen 1997). Equally important is protection of the hydrologic processes which produce alluvial bars essential for Populus trichocarpa reproduction. This community only persists on natural, free-flowing rivers (e.g. not dammed or diverted) with unconfined banks (e.g. not channelized, rip-rapped, or diked) where frequent flooding and associated channel migration, erosion, and deposition still occur. Likewise, other watershed activities, such as logging, road building, dredging, and development, can alter sediment inputs and the hydrologic system supporting this association. Elimination of these floodplain associations destabilizes the erodible streambanks and terrace soils because Populus trichocarpa effectively reduces flood erosion energy (Hansen et al. 1995; Hall and Hansen 1997). Populus trichocarpa communities, especially younger stands, are easily eliminated by wildfire (Moseley and Bursik 1994; Hansen et al. 1995; Hall and Hansen 1997). Cuttings can be planted on alluvial bars for long-term revegetation. Though soils are not easily compacted, frequent flooding precludes any development in this community.

Successional Dynamics: The Populus trichocarpa/Salix exigua plant association originates from, and is maintained by, frequent flooding (and ice flow damage) which removes understory vegetation and promotes Populus trichocarpa reproduction. Lateral migration (meandering) of stream channels, especially during floods, erodes banks and deposits fresh alluvium on point bars, side bars, and islands. Populus trichocarpa produces many reproductive suckers after flood damage on these bars. Its seeds and seeds of Salix spp. germinate on the sunny, moist, and barren alluvium forming the early seral Populus trichocarpa/Salix exigua plant association. The resulting pattern is one of even-aged stands which are progressively older (later seral states) as you go from active alluvial bars to higher bars further away on the floodplain (Moseley and Bursik 1994; Manning and Padgett 1995; Hansen et al. 1995; Hall and Hansen 1997). Older stands, on drier bars, terraces, and islands, are mid-seral Populus trichocarpa communities with understories dominated by Cornus sericea, Rhamnus spp., or Salix spp. (Moseley and Bursik 1994; Hansen et al. 1995; Manning and Padgett 1995; Weixelman et al. 1996; Hall and Hansen 1997). With further disturbance, by livestock grazing or floods, secondary succession will produce Populus trichocarpa types with understories of Symphoricarpos albus (or Symphoricarpos occidentalis), Rosa woodsii, Poa pratensis, or mesic graminoids or forbs. If stream downcutting continues the community may become a drier conifer or shrub-steppe (e.g. Sarcobatus vermiculatus and Artemisia tridentata var. tridentata) type. This association is perpetuated by channel migration and large-scale floods that remove older stands and replace them with Populus trichocarpa/Salix exigua (Moseley and Bursik 1994).

Wildlife Functions: Stands of Populus trichocarpa/Salix exigua are used for cover, shade, and food by numerous species of small mammals and ungulates such as deer (Hansen et al. 1995; Hall and Hansen 1997). Beaver utilize Populus trichocarpa for food and building. Waterfowl may nest on bars and neotropical migrant songbirds (and other birds) utilize Populus trichocarpa and understory shrubs for nesting and foraging (Hansen et al. 1995; Hall and Hansen 1997).

Distribution: Populus trichocarpa/Salix exigua is a widespread plant asociation occurring throughout the range of the tree species along larger naturally functioning rivers in Idaho, Montana, Washington, Oregon, California, British Columbia, Alaska, and elsewhere. High quality examples of the association are rare. Examples degraded by dams, diverstions, channelization, dredging, and weed invasion are not uncommon.

References:

Hall, J. B. and P. L. Hansen. 1997. A preliminary riparian habitat type classification system for the Bureau of Land Management Districts in Southern and Eastern Idaho. Bureau of Land Management Technical Bulletin No. 97-11. 381 pp.

Hansen, P. L., R. D. Pfister, K. Boggs, B. J. Cook, J. Joy, and D. K. Hinckley. 1995. Classification and Management of Montana's Riparian and Wetland Sites. Montana Forest and Conservation Experiment Station, School of Forestry, Missoula, MT. 646 pp.

Manning, M. E., and W. G. Padgett. 1995. Riparian community type classification for the Humboldt and Toiyabe National Forests, Nevada and eastern California. USDA Forest Service R4-ECOL-89-01. Intermountain Region, Ogden, UT. 274 pp.

Moseley, R. K., and R. J. Bursik. 1994. Black cottonwood communities of Spion Kop Research Natural Area, Coeur d'Alene River, Idaho. Cooperative Challenge Cost Share Project, Idaho Panhandle National Forests and Idaho Conservation Data Center, Idaho Department of Fish and Game. 14 pp. plus appendices.

Weixelman, D. A., D. C. Zamudio, and K. A. Zamudio. 1996. Central Nevada Riparian Field Guide. USDA Forest Service R4-ECOL-96-01. Intermountain Region, Ogden, UT. 90 pp. plus appendices.

Author/Date(Update): IDCDC/NODATE(2002-11-02)