Establishment Patterns of Riparian Tree Populations

Michael L. Scott National Biologic(ll Service I n much of western North America, riparian (streamside) environments are the only part of the landscape moist enough to allow survival of trees (Fig. 1). Riparian landscapes are usually defined as ecotones or corridors between terres ­ trial and aquatic realms (Malanson 1993). In spite of their limited areal extent, riparian ecosystems are essential habitat for many verte­ brate species and provide critical physical and biological linkages between terrestrial and aquatic environments (Gregory et al. 1991). Because of their association with scarce sur­ face water resources, western riparian ecosys­ tems have long been influenced by human activ­ ities. Human-caused perturbations can alter energy and material flow in riverine ecosys­ tems, thus modifying riparian plant communi­ ties (Brinson 1990). Among the most serious impacts to riparian ecosystems are water impoundmept and diversion, groundwater pumping from alluvial aquifers, livestock graz­ ing, land clearing for agriculture or to increase water yield, mining, road development, heavy recreational demand, fire, the elimination of native organisms (e.g., beaver [Castor canaden­ sis]) or the introduction of exotics , and overall watershed degradation (Stromberg 1993).