Aim A regional analysis was used to explore the influence of river regulation on the dominance of non-native, invasive shrubs and trees. We addressed the following questions: (1) How do large dams affect hydrological parameters that influence riparian vegetation? (2) How do flow regimes affect the dominance of non-native woody species? (3) How do changes in flow regimes affect the dominance of non-native woody species?
Location South-western USA.
Methods We sampled the canopy cover of woody species on 179 point bars along seven non-dammed and thirteen dammed river segments. Wilcoxon rank sum tests were used to determine differences between flow parameters in dammed and non-dammed rivers. We used correlation analyses and generalized linear model comparisons to examine associations of flow parameters and canopy cover of native (Populus and Salix) and non-native (Tamarix and Elaeagnus) taxa. An index of flow alteration that was created using principal components analysis was regressed with vegetation cover.
Results Tamarix cover was positively related to drainage area, flow constancy, August and May median flow and flow recession rate, but Elaeagnus cover was unrelated to flow variables. River segments with peak flows in late summer or high constancy had the highest Tamarix cover. Populus cover was positively influenced by low maximum temperatures and frequent high pulses. Flow alteration was negatively related to Populus cover and positively related to Tamarix cover. Total non-native, Elaeagnus and Salix covers were not correlated with flow alteration.
Main conclusions Rivers with a large drainage area and low flow variability are inherently more vulnerable to invasions. River regulation does not necessarily increase the cover of non-native, invasive species. Instead, changes in flow allow proliferation of species that have life-history traits suited to modified flow regimes. River restoration projects that aim to reinstate natural flow regimes should be designed with knowledge of native and non-native species' life history strategies.