EFFECTS OF GROUNDWATER DECLINE ON RIPARIAN VEGETATION OF SEMIARID REGIONS - THE SAN-PEDRO, ARIZONA

Groundwater depletion threatens many riparian ecosystems in arid and s emiarid regions of the world. The aquifer that sustains Arizona's San Pedro River riparian ecosystem, for example, is threatened by regional groundwater declines and localized pumping from the alluvial aquifer. This paper demonstrates the important role of shallow groundwater in structuring the San Pedro River plant community, portions of which fun ction as reference areas that indicate site potential for a globally r are forest type (Sonoran riparian Populus-Salix forests). Several ecol ogical indicators varied with depth to groundwater, including a weight ed average wetland indicator score calculated for herbaceous and woody plant species, cover of plants within wetland indicator groups, and f requency of indicator plant species. These relationships can be used i n a space-for-time substitution to predict consequences of groundwater decline. For example, the wetland indicator score changed sharply as depth to groundwater ranged from 0 to 4 m, and abundance of obligate w etland herbs (the group most sensitive to groundwater changes) decline d sharply at groundwater depths below approximate to 0.25 m. Such sequ ential ''desertification'' of the riparian flora (i.e., loss or reduct ion in cover of species based on their probability of occurrence in we tlands) is one predicted response to groundwater decline. Other predic ted impacts of groundwater decline include reduced establishment of Po pulus fremontii-Salix gooddingii forests, and reduced cover of herbace ous species associated with the fine-textured soils and shady conditio ns of floodplain terraces stabilized by these early seral tree species . High floodplain terraces (depth to groundwater of 5-8 m) had wetland indicator scores below those of upland sites and were vegetated by sp ecies (e.g., Prosopis velutina and Sporabolus wiightii) with low sensi tivity to groundwater changes.