Abstract. Riparian zones are habitats of critical conservation concern worldwide, as they are known to filter agricultural contaminants, buffer landscapes against erosion, and provide habitat for high numbers of species. Here we test the generality of the notion that riparian habitats harbor more species than adjacent upland habitats. Using previously published data collected from seven continents and including taxa ranging from Antarctic soil invertebrates to tropical rain forest lianas and primates, we show that riparian habitats do not harbor higher numbers of species, but rather support significantly different species pools altogether. In this way, riparian habitats increase regional (␥-) richness across the globe by Ͼ50%, on average. Thus conservation planners can easily increase the number of species protected in a regional portfolio by simply including a river within terrestrial biodiversity reserves. Our analysis also suggests numerous possible improvements for future studies of species richness gradients across riparian and upland habitats. First, Ͻ15% of the studies in our analysis included estimates of more than one taxonomic group of interest. Second, within a given taxonomic group, studies employed variable methodologies and sampling areas in pursuit of richness and turnover estimates. Future analyses of species richness patterns in watersheds should aim to include a more comprehensive suite of taxonomic groups and should measure richness at multiple spatial scales.

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Environmental Services, Salt River Project, Tempe, AZ, U.S.A. SUMMARY 1. Riparian vegetation in dry regions is influenced by low-flow and high-flow components of the surface and groundwater flow regimes. The duration of no-flow periods in the surface stream controls vegetation structure along the low-flow channel, while depth, magnitude and rate of groundwater decline influence phreatophytic vegetation in the floodplain. Flood flows influence vegetation along channels and floodplains by increasing water availability and by creating ecosystem disturbance. 2. On reference rivers in Arizona's Sonoran Desert region, the combination of perennial stream flows, shallow groundwater in the riparian (stream) aquifer, and frequent flooding results in high plant species diversity and landscape heterogeneity and an abundance of pioneer wetland plant species in the floodplain. Vegetation changes on hydrologically altered river reaches are varied, given the great extent of flow regime changes ranging from stream and aquifer dewatering on reaches affected by stream diversion and groundwater pumping to altered timing, frequency, and magnitude of flood flows on reaches downstream of flow-regulating dams. 3. As stream flows become more intermittent, diversity and cover of herbaceous species along the low-flow channel decline. As groundwater deepens, diversity of riparian plant species (particularly perennial species) and landscape patches are reduced and species composition in the floodplain shifts from wetland pioneer trees (Populus, Salix) to more drought-tolerant shrub species including Tamarix (introduced) and Bebbia. 4. On impounded rivers, changes in flood timing can simplify landscape patch structure and shift species composition from mixed forests composed of Populus and Salix, which have narrow regeneration windows, to the more reproductively opportunistic Tamarix. If flows are not diverted, suppression of flooding can result in increased density of riparian vegetation, leading in some cases to very high abundance of Tamarix patches. Coarsening of sediments in river reaches below dams, associated with sediment retention in reservoirs, contributes to reduced cover and richness of herbaceous vegetation by reducing water and nutrient-holding capacity of soils. 5. These changes have implications for river restoration. They suggest that patch diversity, riparian plant species diversity, and abundance of flood-dependent wetland tree species such as Populus and Salix can be increased by restoring fluvial dynamics on floodsuppressed rivers and by increasing water availability in rivers subject to water diversion or withdrawal. On impounded rivers, restoration of plant species diversity also may hinge on restoration of sediment transport. 651 6. Determining the causes of vegetation change is critical for determining riparian restoration strategies. Of the many riparian restoration efforts underway in south-western United States, some focus on re-establishing hydrogeomorphic processes by restoring appropriate flows of surface water,...

As global climate change affects recharge and runoff processes, stream flow regimes are being altered. In the American Southwest, increasing aridity is predicted to cause declines in stream base flows and water tables. Another potential outcome of climate change is increased flood intensity. Changes in these stream flow conditions may independently affect vegetation or may have synergistic effects. Our goal was to extrapolate vegetation response to climate-linked stream flow changes, by taking advantage of the spatial variation in flow conditions over a 200 km length of the San Pedro River (Arizona). Riparian vegetation traits were contrasted between sites differing in low-flow hydrology (degree of stream intermittency) and flood intensity (stream power of the 10-year recurrence flood). Field data indicate that increased stream intermittency would cause the floodplain plant community to shift from hydric pioneer trees and shrubs (Populus, Salix, Baccharis) towards mesic species (Tamarix). This shift in functional type would produce changes in vegetation structure, with reduced canopy cover and shorter canopies at drier sites. Among herbaceous species, annuals would increase while perennials would decrease. If flood intensities increased, there would be shifts towards younger tree age, expansion of xeric pioneer shrubs (in response to flood-linked edaphic changes), and replacement of herbaceous perennials by annuals. Woody stem density would increase and basal area would decrease, reflecting shifts towards younger forests. Some effects would be compounded: Annuals were most prevalent, and tree canopies shortest, at sites that were dry and intensely flooded. Vegetational changes would feedback onto hydrologic and geomorphic processes, of importance for modeling. Increased flood intensity would have positive feedback on disturbance processes, by shifting plant communities towards species with less ability to stabilize sediments. Feedbacks between riparian vegetation and stream lowflow changes would be homeostatic, with reduced evapotranspiration rates ameliorating declines in base flows arising from increased aridity.

Flood-control levees are generally thought to increase flood height and velocity for a given discharge. While extensive areas of floodplain in the United States are leveed, the ecological impacts of levees have largely been ignored relative to other anthropogenic impacts to large river floodplains. We examined a century of flood control along the Wisconsin River by comparing simulated flood regimes under ''levee'' and ''leveeremoval'' scenarios. We also used field sampling to determine if levees had altered the distribution of dominant floodplain forest trees. Increases in flood stage (height) due to levees were minor, only a few centimeters. This was primarily due to the location of the levees, set back hundreds of meters into the floodplain in some areas. Increases in overbank flood velocities due to levees were minimal compared to increases caused by channel constriction and by increased flood magnitude. Generally, levees had a greater impact on stage and overbank flood velocities of larger magnitude events. The mean number of floods and number of days flooded were lower in areas outside (on upland sides) of levees, and stream power was zero in these areas due to a lack of any inundation. These areas also had lower importance values (IV) for several flood-tolerant tree species (Acer saccharinum and Fraxinus pennsylvanica) and higher IVs for some flood-intolerant species (Quercus velutina and Q. ellipsoidalis). Furthermore, areas inside levees (between the levee and the channel) were no different from completely unleveed areas in the number of floods, number of days flooded and in IVs of several dominant tree species. The levee location (set back into the floodplain) resulted in a similar historic flood regime, and thus, similar abundances of floodplain tree species in areas inside levees as compared to completely unleveed areas. Setback levees can provide an important compromise by maintaining the relative abundance of tree species normally found in unleveed areas, while also allowing some flood control. Floodplain restoration involving levee removal should generally target the removal of mainline levees (those adjacent to the channel) rather than removal of setback levees.