Physiognomy and Structure: No Data Available

Floristics: These shrublands are characterized by stands of medium-tall and tall, deciduous shrubs and a thick herbaceous undergrowth of wetland-indicator grasses, and little to no overstory tree canopy. Total shrub cover is usually over 50% and is dominated by Alnus incana, the diagnostic shrub. Alnus incana ssp. tenuifolia dominates the upper canopy with 10-90% cover. Other shrubs occasionally present include Salix exigua, Salix monticola, Rubus deliciosus, Salix bebbiana, Salix drummondiana, Rosa woodsii, and Cornus sericea. Occasionally, trees may be scattered throughout the shrubland, or occur along one edge. Tree species include Abies lasiocarpa, Pinus contorta, Populus angustifolia, Populus balsamifera ssp. trichocarpa, Populus deltoides, Populus tremuloides, Picea engelmannii, Pseudotsuga menziesii, and Salix fragilis (Padgett et al. 1989, Manning and Padgett 1995, Kittel et al. 1999b, Carsey et al. 2003a).

The herbaceous layer is a thick carpet of grasses. Native graminoids include Calamagrostis canadensis, Carex aquatilis, Carex utriculata, Glyceria striata, Carex pellita (= Carex lanuginosa), Elymus glaucus, and Festuca rubra. Heavily disturbed stands are dominated by introduced, non-native grasses, including Poa pratensis, Agrostis stolonifera, and Bromus inermis. Forb cover is usually low relative to the amount of graminoid cover but can include a high variety of species, including Thalictrum fendleri, Mertensia ciliata, Mentha arvensis, Geranium richardsonii, Erigeron peregrinus, Cardamine cordifolia, Aconitum columbianum, and Caltha leptosepala (Kittel et al. 1999b, Carsey et al. 2003a).

Dynamics:  Alnus incana ssp. tenuifolia is a long-lived, early-seral species. It is one of the first species to establish on fluvial or glacial deposits as well as the spoils of placer mining (Viereck 1970, Van Cleve et al. 1971, Hansen et al. 1989, Chapin et al. 1994). After establishment, young stands of Alnus incana are continually flooded. As stands mature, the stems can slow flood waters and trap sediment. Fine-textured sediments accumulate on top of the coarser alluvial material, and the land surface eventually rises above annual flood levels. Flooding is then less frequent and soils begin to develop (Padgett et al. 1989).

Alnus incana is shade-intolerant (Viereck 1970, Chapin et al. 1994), and many mature stands in Colorado are restricted to streambank edges, possibly because these are the only sites where light can penetrate the neighboring overstory canopy. Alnus incana has been observed on high-gradient streams and is thought to require well-aerated water (Hansen et al. 1988b, Padgett et al. 1989).

Undisturbed Alnus incana stands may become dominated by Salix (willow) species or conifers (Hansen et al. 1989). In Alaska, thick stands of alders inhibit succession by competing with spruce for nutrients and light (Chapin et al. 1994). In Utah, Acer negundo (box-elder) often becomes the dominant canopy species on more xeric sites (Padgett et al. 1989).

Alnus incana fixes atmospheric nitrogen through a symbiotic relationship with the bacteria Frankenia and increases the ecosystem nitrogen supply with the deposition of nitrogen-rich leaf litter (Binkley 1986). The annual input of nitrogen to soils from alder species ranges from 10 to 150 times the amount deposited by atmospheric precipitation alone (Binkley 1986, Bowman and Steltzer 1998). Nitrogen rich detritus is an important source of nutrients for the aquatic ecosystem as well.

In Nevada, Utah, southeastern Idaho, Montana, and Wyoming, the Alnus incana / mesic graminoid type is considered a grazing-induced community, derived from the Alnus incana / mesic forb plant association (Padgett et al. 1989, Manning and Padgett 1995, Jones 1992c). In Colorado, most stands of this plant association appear to be disturbed by improper grazing and have an abundance of non-native graminoid species. A few stands, however, appear undisturbed and have an undergrowth dominated by native grasses.