The structure of the floodplain forests of the Middle Ebro River (NE Spain) was examined at patch and landscape scales along a three-step chronosequence defined according to the extent of flow regulation-induced hydrogeomorphic changes, with the ultimate purpose of producing baseline information to guide through management and restoration plans. At patch scale, a total of 6,891 stems within 39 plots were registered for species, diameter and health status. The stem density, size class distribution, canopy dieback and mortality were further compared by means of non-parametric tests. At landscape scale, the temporal evolution of the area occupied by forest stands of different ages in the floodplain along the chronosequence was evaluated using four sets of aerial photographs dated in 1927, 1957, 1981 and 2003. The within-patch structure of pioneer forests (<25-30 years old) was characterized by dense and healthy populations of pioneer species (Populus nigra, Salix alba and Tamarix spp.), but the area occupied by these forest types has progressively decreased (up to 37%) since the intensification of river regulation (ca. 1957). In contrast, non-pioneer forests (>25-30 years old) were characterized by declining and sparse P. nigra-S. alba-Tamarix spp. stands, where late-seral species such as Ulmus minor and Fraxinus angustifolia were frequent, but only as small-size stems. At landscape scale, these type of senescent forests have doubled their surface after river regulation was intensified. Populus alba only appeared in the oldest plots recorded (colonized before 1957), suggesting sexual regeneration failure during the last five decades, but usually as healthy and dense stands. Based on these findings, measures principally aimed at recovering some hydrogeomorphic dynamism are recommended to guarantee the self-sustainability of the floodplain forest ecosystem.
Flood and flow pulses are primary factors that regulate macroinvertebrate community structure in river-floodplain ecosystems. In order to assess the impacts of these important hydrological events in a regulated Mediterranean river, bimonthly water and macroinvertebrate samples were collected in 2006 in the Middle Ebro River channel and six floodplain wetlands (NE Spain). We found significant differences among the river sites (permanently connected), two constructed wetlands (groundwater seepage) and three natural oxbow lakes (surface connected at 400, 800 and 1200 m 3 /s). River sites were dominated by aquatic worms (e.g., Naididae) and showed a high degree of eutrophication. Constructed wetlands provided new habitat for predatory insects (e.g., Coenagrion scitulum and Trithemis annulata) that benefited from the absence of fish. Natural oxbow lakes were dominated by crustaceans (e.g., Atyaephyra desmarestii, Procambarus clarkii) and were highly overlapped in Correspondence Analysis. Canonical Correspondence Analysis, coupled with variance partitioning, showed that hydrological connectivity accounted for 28 % of the variability in the invertebrate community, followed by physicochemical (10 %) and trophic (7 %) factors. Differences in frequency and duration of flood pulses in natural oxbow lakes were not enough to generate distinctive macroinvertebrate assemblages in the different wetlands. Analysis of variance showed that richness and total abundance increased with hydrological connectivity, while diversity showed a rather unimodal distribution. Generalized Additive Models indicated that among the measured environmental variables, nitrate concentration strongly affected the composition, abundance and diversity of aquatic communities. Our data indicate that increasing the diversity of water body types in degraded floodplains enhances biodiversity and aids in the functional and ecological recovery of the riverine landscape.
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