7388 - Laserfiche WebLink

Home Browse Search

1.3 RIPARIAN VEGETATION OF THE FLOOD- PLAIN Throughout the desert Southwest, a few plants are uniquely adapted to the floodplains of seasonally fluctuating streams. These riparian plants exist where their roots are in the capillary fringe of the water table. They may extend only as far from the channel as the stream exerts its influence through a water table. This strip of vegetation Is often used to define the floodplain of a river, and creates a marked contrast as a ribbon of green bisecting the desert uplands (Figure 6). WETLAND z Mes uite Cottonwood-willow Mesquite replacing z woo land forest cotton woo d?w i 11 o w z a mar sr?snA o F m c m U m ?j Q « U Flood old river Flow channel Flood terrace terrace channel High water channel FLOODPLAIN Figure 6. Semidiagrammatic represen- tation of riparian communities in warm temperate to subtropical habitats of the American Southwest. Adapted from Minckley and Brown (1982). The natural vegetation associa- tions along the lower Colorado River were well described by Grinnell (1914). Belts of riparian vegetation stretched for many kilometers and filled the broad alluvial valleys. The dominant riparian forest species were cottonwood (Po°?ulu_s fremontii) and black or Goodding willow (Salix gooddinavil). These occurred primarily on the "first bottom" and braided channels, in association with the understory shrub seepwillow (Baccharis sal icifolia [B. glutinosa]) and occasionally other willows (e.g., coyote willow, Salix extgua) and emer- gent species (e.g., cattail, [T=ha spp.] and bulrush [Scirgus spp.]) (Figure 7). As an adaptation to a frequently flooded environment, these plants were fast-growing and relative- ly short-lived. In fact, their exis- tence was ultimately dependent on the cycle of annual floods that created new silt beds for seed germination. However, these and other native plants cannot tolerate prolonged inundation. Where such long-term flooding persisted, such as in oxbow lakes, emergent marsh vegetation became established (Figure 8). This marsh vegetation consisted of cattails, bulrushes or tules, and in the southern portions of the valley, cane ( ragmites australis [P. communis]). Along the drier sites adjacent to the willow and cottonwood stands a shrub, arrowweed (Tessaria sericea [Pluchea sericea]), often formed dense monotypic belts or small strands in some areas (Figure 9). Where the floodplain of the first bottom escaped inundation for a number of years, the rare screwbean mesquite (Prosopis pubescens) grew in association with willows (Figure 10). A very different type of riparian vegetation occurred on the second bottoms than that which existed ad- jacent to the river. The dominant tree in the second bottom was honey mesquite (erosoRL gl andu loss [P.. juliflora]) (Figure 11). This tree formed relatively sparse monotypic woodlands. The long roots of the mesquite must find permanently moist soils to ensure survival, yet the tree itself apparently cannot tolerate inundations of even a few weeks. In addition to honey mesquite, several 9