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Alnus rhombifolia/Philadelphis lewisii

white alder/syringa

Vegetation Composition: This forest association is dominated by broad-leaved, deciduous woody species. The tree Alnus rhombifolia dominates, with high cover. The shrub layer, also broad-leaved deciduous, is dominated by Philadelphis lewisii, from 1.5 to 2.5 meters tall. Several other shrub species are commonly present, including the taller Crataegus douglasii, Cornus sericea, and Prunus virginiana, and the shorter Salix exigua. Occassional colonies of the introduced shrub Rubus discolor can be found, as well as the native liana Clematis ligusticifolia and the introduced Solanum dulcamera. There is almost complete cover of Alnus rhombifolia in the overstory. The shrub canopy in the understory can be very sparse or reach as much as 50% cover. The herbaceous component of this association has low cover due to shade and hydrologic disturbance (Miller 1976). The native graminoid Elymus triticoides was notably present in all Idaho stands (Moseley 1999).

Classification Comments: This association was originally described by Miller (1976) in west-central Idaho. Apparently, plot data was collected by Miller (1976) though stand tables are not available. The association is also reported in Oregon (Evans 1989). Seven stands with Alnus rhombifolia as the dominant overstory species were sampled in southwest Idaho (Moseley 1999). Two of the stands were tentatively classified as an Alnus rhombifolia/Cornus sericea association. Philadelphus lewisii was present in both of these stands and stands are considered part of the Alnus rhombifolia/Philadelphus lewisii plant association. Miller (1976) described several Alnus rhombifolia associations from west-central Idaho that are similar to this type. Diagnostic species in other associations include Amelanchier alnifolia, Betula occidentalis, Celtis reticulata, Prunus virginiana, Rhus glabra, Rosa woodsii, and Sambucus cerulea. The description for Alnus rhombifolia/Philadelphis lewisii, however, appears to fit most situations encountered.

Environment: The Alnus rhombifolia/Philadelphis lewisii association is found in a mountainous region dissected by major river canyons and tributary streams. There are elevational extremes in this region, from subalpine summits over 9000 feet, to canyon bottoms below 2000 ft. Topography is characterized by precipitous canyon walls along the rivers, steep upland slopes, and smaller tributary streams with cliffs, rocky slopes, and mass-wasting features such as slumps and mud-rock flows. The soils of the region are especially prone to erosion and slippage. Slopes often exceed the angle of repose and periodic severe thunderstorms with intense rains will supersaturate the soil mantle. The resulting slumping and mud-rock flows often deposit materials into stream channels and can destroy streambanks (Miller 1976). The climatic conditions in these deep canyon bottoms are strikingly different from surrounding uplands and mountains. In the canyons, summers are hot and dry, and winters mild, with only limited and ephemeral snows. Precipitation is variable, but generally the higher elevation reaches have higher precipitation. Annual precipitation is approximately 13 inches at 740 feet, and about 18 inches at 2000 ft. However, a rainshadow effect occurs further to the east, in upstream reaches, and annual precipitation is less than 10 inches (Miller 1976). Stands are mostly restricted to the flooding zone of perennial streams (USDA 2000) in canyon bottoms, from 900 to 2800 ft elevation. It is associated with very poor stream channel stability ratings, indicating it occupies sites frequently disturbed by flooding and deposition of mud-rock flow materials (Miller 1976).

Soils: Detailed soil survey data is not available. Descriptions from stands indicate that deposits of boulder and rocks dominate the floodplain (Moseley 1998).

Management Considerations: Alnus rhombifolia is rated as fair to useless browse for cattle, sheep, and horses. Livestock may use stands as loafing areas to escape the heat and browse on understory associates. The dominant overstory species has been used for revegetating disturbed riparian habitat. It has been planted to restore streams and to stabilize flood control channels. Since it requires a constant water supply, irrigation may be necessary to help transplants get established. Alnus rhombifolia is not fire resistant, though landscape position may allow stems to escape fire. Fires that expose mineral soils may provide seedbeds for establishment of Alnus rhombifolia (USDA 2000).

Successional Dynamics: Alnus rhombifolia is a riparian pioneer species that produces copious amounts of wind and water transported seed. The seeds will colonize sites that are frequently disturbed by flooding and deposition of mud-rock flow materials. Seed establishment seems to be restricted to areas with continuously moist substrates (Miller 1976; USDA 2000). Plants will regenerate form both seeds and sprouts.

Wildlife Functions: Wild ungulates will eat the bark, twigs, leaves, and buds of Alnus rhombifolia. Most consumption is likely in the fall, winter, and early spring. The bark of alder is used as food by beavers and stems are used for construction of lodges and dams. Seeds of alder are eaten by siskins, goldfinches, and other songbirds (USDA 2000).

Distribution: The association has been described from west-central Idaho, on the Snake River and its tributaries, in Washington, Adams and Idaho Counties, Idaho (Miller 1976, Moseley 1998). Stands have also been reported in Oregon (Evans 1989). The main range for Alnus rhombifolia is California (Johnson 1995). It then extends north to the interior foothills of the Coast Ranges and at low to mid elevations in the Sierra Nevada. Further north it is known from the eastern slopes of the Cascades of Oregon and Washington into southern British Columbia. In Idaho the species is at its eastern limit on the Clearwater and Snake Rivers (USDA 2000). Johnson (1995) also reports small stands of Alnus rhombifolia in the Owyhee Mountains of Idaho.

References:

Evans, S. 1989. Provisional riparian and aquatic wetland plant communities on the Columbia Plateau. Washington State Department of Ecology, Contract No. C0089098. 52 pp.

Johnson, F. D. 1995. Wild trees of Idaho. University of Idaho Press, Moscow. 212 pp.

Miller, T. B. 1976. Ecology of riparian communities dominated by white alder in western Idaho. Unpublished thesis, University of Idaho, Moscow. 143 pp.

Moseley, R. K. 1998. Riparian and wetland community inventory of 14 reference areas in southwestern Idaho. Conservation Data Center, Idaho Department of Fish and Game, Bureau of Land Management Technical Bulletin No. 98-5. 52 pp. plus appendices.

Moseley, R. K. 1999. Riparian and wetland communities in southwestern Idaho: second-year inventory results and preliminary catalog of community types. Conservation Data Center, Idaho Department of Fish and Game. 43 pp. plus appendices.

U.S. Department of Agriculture. 2000. Rocky Mountain Research Station, Fire Sciences Laboratory. Fire Effects Information System, [Online]. Available: http://www.fs.fed.us/database/feis/

Author/Date(Update): M. Reid/1993-11-11(2000-12-14)