Laserfiche WebLink
modifying driving factors such as streamflow and the potential benefits of achieving specific <br />goals with respect to the function and form of riparian areas. <br />A variety of factors influence the structure and composition of riparian vegetation growing along <br />a particular reach of river. Geologic setting, short-term weather patterns and long-term climatic <br />fluctuations, and direct human activities all influence the composition and dynamics of riparian <br />plant communties. For example, the width of a river valley between confining canyon walls and <br />depth of valley fill influence groundwater patterns and determine the extent of available habitat <br />for riparian plants, natural wet and dry cycles can be reflected for centuries in the population <br />stn~cture of woody vegetation, and the building and maintenance of roads and other human- <br />activities can serve to remove, constrain, or cause significant shifts in riparian vegetation. <br />Riparian plants are limited by many of the same factors that influence plants growing in uplands: <br />fire, grazing and insect herbivory, water availability-drought, competition with other plants, and <br />availability of suitable habitat. Plants growing along rivers are also subjected to disturbances that <br />are unique to streamside areas, primarily fluvial processes (those related to flowing water such as <br />scour and burial) and stresses associated with flooding (anaerobic conditions and periodic <br />submergence). As a consequence, riparian vegetation along free-flowing arid-land streams is <br />often dominated by species adapted to (and in some cases reliant upon) such processes. Because <br />fluvial processes distinguish riverine habitats from other ecosystems, a discussion of the <br />processes that structure and support riparian vegetation hinges upon an understanding of the <br />interplay between hydrology, geomorphic processes, and the life history attributes (requirements <br />at various stages of development) of plants. <br />For plants, each stage of development from seed to reproductively active adult must be met (at <br />least occasionally) for self perpetuating populations to persist. Completion of the life cycle for <br />plants requires that: 1) suitable habitat must occasionally become available (newly formed or <br />cleared of existing vegetation) for colonization, 2) seeds must reach such habitat and germinate, <br />and 3) seedlings must persist and mature to reproductive age (Figure 1). The same principle <br />applies for species that reproduce vegetatively (by root sprouts, stolons, etc.). For many species <br />that are only found in riparian areas, these stages are tied to some aspect of river flow regime. In <br />the arid West few obligate riparian species can persist solely on the water provided by the local <br />climate; groundwater and or streamflow is necessary at certain times during the year. Of course, <br />extreme or prolonged drought affects both riparian and upland species, whereas longterm <br />human-caused dewatering of streams may result in riparian plant communities becoming more <br />similar over time to the plant communties characteristic of adjacent uplands. <br />43 <br />