Rivers are conduits for materials and energy; this, the frequent and intense disturbances that these systems experience, and their narrow, linear nature, create problems for conservation of biodiversity and ecosystem functioning in the face of increasing human influence. In most parts of the world, riparian zones are highly modified. Changes caused by alien plants -or environmental changes that facilitate shifts in dominance creating novel ecosystems -are often important agents of perturbation in these systems. Many restoration projects are underway. Objective frameworks based on an understanding of biogeographical processes at different spatial scales (reach, segment, catchment), the specific relationships between invasive plants and resilience and ecosystem functioning, and realistic endpoints are needed to guide sustainable restoration initiatives. This paper examines the biogeography and the determinants of composition and structure of riparian vegetation in temperate and subtropical regions and conceptualizes the components of resilience in these systems. We consider changes to structure and functioning caused by, or associated with, alien plant invasions, in particular those that lead to breached abiotic-or biotic thresholds. These pose challenges when formulating restoration programmes. Pervasive and escalating human-mediated changes to multiple factors and at a range of scales in riparian environments demand innovative and pragmatic approaches to restoration. The application of a new framework accommodating such complexity is demonstrated with reference to a hypothetical riparian ecosystem under three scenarios: (1) system unaffected by invasive plants; (2) system initially uninvaded, but with flood-generated incursion of alien plants and escalating invasion-driven alteration; and (3) system affected by both invasions and engineering interventions. The scheme has been used to derive a decision-making framework for restoring riparian zones in South Africa and could guide similar initiatives in other parts of the world.

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Groundwater depletion threatens many riparian ecosystems in arid and semi‐arid 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 function as reference areas that indicate site potential for a globally rare forest type (Sonoran riparian Populus‐Salix forests). Several ecological indicators varied with depth to groundwater, including a weighted average wetland indicator score calculated for herbaceous and woody plant species, cover of plants within wetland indicator groups, and frequency of indicator plant species. These relationships can be used in 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 wetland herbs (the group most sensitive to groundwater changes) declined sharply at groundwater depths below ≈ 0.25 m. Such sequential desertification of the riparian flora (i.e., loss or reduction in cover of species based on their probability of occurrence in wetlands) is one predicted response to groundwater decline. Other predicted impacts of groundwater decline include reduced establishment of Populus fremontii‐Salix gooddingii forests, and reduced cover of herbaceous species associated with the fine‐textured soils and shady conditions 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 species (e.g., Prosopis velutina and Sporobolus wrightii) with low sensitivity to groundwater changes.

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,...

River damming and flow regulation can alter disturbance and stress regimes that structure riparian ecosystems. We studied the Bill Williams River in western Arizona, USA, to understand dam‐induced changes in channel width and in the areal extent, structure, species composition, and dynamics of woody riparian vegetation. We conducted parallel studies along a reference system, the Santa Maria River, an unregulated major tributary of the Bill Williams River. Flood magnitude on the Bill Williams River has been dramatically reduced since the closure of Alamo Dam in 1968: the 10‐yr recurrence interval flood in the pre‐dam era was 1397 m3/s vs. 148 m3/s post‐dam. Post‐dam average annual flows were higher due to increased precipitation in a few years, but increases in post‐dam May–September flows are largely attributable to dam operation. An analysis of a time series of aerial photographs showed that channels along the Bill Williams River narrowed an average of 111 m (71%) between 1953 and 1987, with most narrowing occurring after dam closure. Multiple regression analysis revealed significant relationships among flood power, summer flows, intermittency (independent variables), and channel width (dependent variable). The pattern of channel width change along the unregulated Santa Maria River was different, with less narrowing between 1953 and 1987 and considerable widening between 1987 and 1992. Woody vegetation along the Bill Williams River was denser than that along the Santa Maria River (27 737 stems/ha vs. 7559 stems/ha, P = 0.005), though basal areas were similar (14.3 m2/ha vs. 10.7 m2/ha, P = 0.42). Patches dominated by the exotic Tamarix ramosissima were marginally (P = 0.05) more abundant along the Bill Williams River than along the Santa Maria River, whereas the abundance of patches dominated by the native Populus fremontii or Salix gooddingii was similar across rivers (P = 0.30). Relative to Populus and Salix, Tamarix dominates floodplain vegetation along the Bill Williams River (P < 0.0001). Most stands of the dominant pioneer trees on both rivers became established in the 1970s and 1980s. Recent seedling establishment occurred in wider bands along the Santa Maria River (15.3 m wide vs. 5.4 m wide on the Bill Williams River, P = 0.0009), likely due to larger floods and associated seedbed formation along the Santa Maria River. Seedling survival rates were generally higher along the Bill Williams River, perhaps due to higher summer flows.