Responses of riparian guilds to flow alterations in a Mediterranean stream

Abstract

Questions: Do Mediterranean riparian guilds show distinct responses to stream water declines? If observed, which are the most sensitive and resilient guilds and their most affected attributes?Location: Tiétar river below the Rosarito dam, central-western Spain. Methods:We identified riparian guilds based on key woody species features and species distribution within this Mediterranean river corridor, and evaluated similarity of their responses to long-term flow alteration (i.e. stream water declines since dam construction in 1959). Hierarchical cluster analysis was used to group surveyed vegetation bands according to species composition. The groups were designated as riparian guilds where each vegetation group comprising a guild: (1) contains species sharing similar features (using PCA); and (2) shares a similar environment (using DCA). Changes in several guild attributes (i.e. dominance and species composition, diversity and establishment patterns) during the regulated period were compared statistically. We used pre-and post-dam established vegetation bands identified based on old (1956) and modern (2006) aerial photographs and field measurements of woody species diameter. Results:Responses to flow alterations varied between guilds according to ecological requirements of their species. The ability to survive water stress (i.e. 'Xeric' guilds) and drag forces caused by floods ('Torrential' guilds) allowed certain pioneer shrub-dominated guilds (e.g. Flueggea tinctoria and Salix salviifolia) to spread on newly emerged surfaces downward to the main channel after flow alterations, although new shrubland had less species diversity than pre-dam shrubland. In contrast, new hydromorphological conditions following damming limited recruitment of native late-successional tree guilds sensitive to floods (to drag forces, inundation and anoxia; i.e. 'Slow-water' and 'Flood-sensitive', respectively) and those with greater water requirements (i.e. 'Hydric') (e.g. Alnus glutinosa and Celtis australis), although species diversity increased in this mature forest through co-existence of remaining riparian species and new arrival of upland species.Conclusions: Changes in several riparian attributes after flow alterations differed between guilds. Stream water declines after damming caused shifts in species-poor pioneer shrubland downwards to the watered channel, resulting in severe declines of mature native forest. Understanding vegetation guild responses provides information about general trends in plant populations and assemblage structures expected to occur during river development and flow regulation, increasing our capacity to detect and synthesize complex flow alteration-riparian ecosystem response relationships, and anticipate irreversible impacts.

Riparian vegetation research in Mediterranean-climate regions: common patterns, ecological processes, and considerations for management

“…In med-region riparian zones, important sources of change include (1) groundwater extraction and associated water table decline (Shafroth et al, 2000;Stromberg & Patten, 1996;Scott et al, 1999); (2) flow diversion and regulation, with effects on channel morphology, and riparian species extent, complexity, and function (Trush et al, 2000;Bombino et al, 2007); and (3) land uses changes in the watershed, and associated modification in sediment delivery and morphological processes (Piégay et al, 2004;Stella et al, in press). In Mediterranean and arid climates, modification of the water regime from dams, diversion, or land cover changes can also affect soil conditions, nutrient cycling (Sampaio et al, 2008;González et al, 2010), and/or salinity (Akeroyd et al, 1998;Callow & Smettem, 2007), with significant effects on ecosystem functions and vegetation patterns (Bejarano et al, 2011).…”

Riparian vegetation patterns according to hydrogeomorphological factors at different spatial and temporal scales in Mediterranean rivers.

“…The definition of riparian guilds may be used as a tool for assessment of the ecological integrity of rivers, for instance determining shifts in the structure of the guilds in response to river regulation (Bejarano et al, 2011;Bejarano et al, 2012a;Bejarano et al, 2012b). Another application may be the definition of the most sensitive guilds, i.e., the group of species that share life history strategies that are highly adapted to specific riverine conditions and may decline far more than generalist species.…”

“…To continue further exploring the aggregation of species into riparian guilds in other rivers and correlate them with physiological attributes (Merritt et al, 2010;Bejarano et al, 2012a) due to the importance of considering the riparian forest in the water management.…”

“…In the same way, plants with different life-history strategies may respond to disturbances and stressors differently. Therefore, the definition of functional groups or guilds of plants with the same behaviour under specific environmental variables has been established as one of the keys in the development and validation of vegetation models (USDA-NRCS, 2002;Merritt et al, 2010;Bejarano et al, 2012a). For this task, it is necessary to distinguish if different species are affected in the same way by certain environmental variables, and which species show significant differences.…”

“…This approach has been applied to different components of the fluvial ecosystem, such as birds and mammals (Croonquist and Brooks, 1991), macroinvertebrates (Luczkovich et al, 2002), fish (Aarts and Nienhuis, 2003) and more recently to riparian species (Merritt et al, 2010;Bejarano et al, 2012a).…”