The Southern Great Plains have experienced some of the greatest riparian habitat losses over the past forty years attributed to anthropogenic land cover change and the regulation of rivers. River regulation can have potentially cascading effects on the trajectory of riparian vegetation community development as new tree recruitment declines or is replaced by non-native plants. This study aims to assess the impacts of varying hydrologic conditions on native riparian tree establishment and growth. Our sites were located along the Brazos River, Colorado River and Guadalupe River in Central Texas. On each river, two sites were established at developing meanders within the active floodplain, and tree cores were collected from the target species: black willow (Salix nigra), eastern cottonwood (Populous deltoides), boxelder (Acer negundo), American sycamore (Plantanus occidentalis), green ash (Fraxinus pennsylvanica), and bald cypress (Taxodium distichum). These species were selected because their life cycle is known to depend on floods or wetland conditions. Annual ring-width chronologies were developed to determine both germination year and incremental growth patterns, which were then compared to the timing and seasonality of peak flows, stage height, flood magnitude, and flood duration.
Results/Conclusions
Germination was associated with periods of high flows in some species, indicating closer dependency on ideal flow conditions for recruitment in these wetland dependent trees. The more highly regulated rivers were dominated by target species whose life strategies were most compatible with the flow regime: either those with long seed viability or greater dryness tolerance after germination. In contrast, less regulated rivers with more variable flow and periodic high flows possessed a greater diversity of target species, especially those with short seed viability. Species also differed in relative importance of hydrologic and climatic drivers of annual growth. These findings suggest that rivers with increased regulation develop riparian communities that lose both species and age-structure heterogeneity over time. Our results are useful to identify modified flow conditions that are more optimal for riparian tree recruitment and growth on each river. The end goal is to set prescriptive targets for high flows that can be implemented throughout the Southern Great Plains