Development , maintenance and role of riparian vegetation in the river landscape

ion from the channel and/or alluvial aquifer may reduce the height of the water table, killing riparian vegetation and increasing erosion potential of the stream banks (Kondolf & Curry, 1984). Conversely, ̄ow regulation by dams and diversions may allow encroachment of riparian vegetation into the channel which, in some cases, can cause increasing velocity and associated channel degradation (Johnson, 1994). L 510 E. Tabacchi et al. # 1998 Blackwell Science Ltd, Freshwater Biology, 40, 497±516 Linking stream, groundwater and landscape concepts to riparian plant ecology Contemporary stream ecology is multidisciplinary and includes regional characterization of the drainage basin (Martin & Bouchard, 1993), including longitudinal, lateral and vertical analyses of the ecology of the riparian zone and its subsurface components (Stanford & Ward, 1993). Synthetic understanding of riparian plant biodiversity and processes, like competition, colonization and nutrient and water uptake, require thorough grounding in geomorphology, landscape ecology and biogeochemistry of the stream and its valley. Many, perhaps most, studies in stream ecology (sensu lato) are still devoted to studies of the various ecological compartments per se, instead of trying to take into account the interactive effects of the structures and processes from one compartment to another and at different places within the stream network. We suggest that regional settings should be more rigorously included in freshwater ecosystem studies as input parameters. Using regional frameworks as templates allows better understanding of upland and instream controls on riparian vegetation and the in ̄uences of riparian vegetation on stream ecology. Site-speci®c studies of biodiversity and ecosystem processes are important and commonly performed (Schultze & Mooney, 1994; Mooney et al., 1996), but cascading effects between adjacent compartments or systems may be expected when ecological continua are examined (Forman, 1995; Mooney et al., 1996). Moreover, many of the processes examined in this paper are scale-dependent and similar analyses could lead to very different conclusions when scale is properly considered. Anthropogenic stream regulation elicits many edaphic, biogeochemical and biological problems that are seldom considered from a total system perspective. Assessing the consequences of water abstraction on stream ecosystem functioning should be restricted to considering instream effects, but also to the dynamics and linkages of the channel and riparian compartments. For example, as noted above, riparian soils develop and are sustained by linkages and dynamics of channel ̄ooding, sediment transport, riparian vegetation and groundwater supply or permanence, among a host of other potential controls, all of which are likely to be dramatically in ̄uenced by stream regulation. We believe that riparian vegetation can buffer pollutant loading to streams from upland sources. The evidence of riparian plant controls on stream chemistry is very sparse, however, and this subject is very clearly in need of innovative research from both localized (stream reach) and catchment perspectives, especially with respect to interactions between rooted plants, groundwater and soil microbiology and nutrient allocation. Qualitative changes accompany quantitative changes in riparian vegetation. We focused on exotic species because they represent good case studies of such changes. As a consequence of human activities along temperate streams, as well as over entire drainage basins, exotic species tend to invade the riparian zone where they can ®nd appropriate ecological niches. Exotic species may be expected to change the plant patch dynamics and associated biogeochemical cycling of ecotones such as riparian zones (Forman, 1995). Moreover, riparian and channel corridors are ef®cient vectors for spreading of many exotic plants and probably accelerate regional changes. Vegetation changes may stimulate other biota in other unexpected ways (e.g. by providing habitats that were not present before exotic plants established). Finally, a better understanding of control of riparian vegetation by animals is needed, such as large herbivore in ̄uences on the riparian corridor (see Naiman & Rodgers, 1997). The perspective in this paper is clearly ecosystem and landscape oriented. This does not mean that other approaches are not useful or appropriate. Our main point is that a spatially explicit theory explaining the occurrence and abundance of riparian vegetation remains elusive and is one of the great challenges for sustaining the integrity of stream ecosystems worldwide (sensu Meyer & Swank, 1996). We believe this challenge requires a synthetic view of the biophysical linkages between instream, groundwater and terrestrial components of the riparian corridor in a regional landscape (catchment) context.