Does damming streams alter the water use strategies of riparian trees? A case study in a subtropic climate

Understand the mechanism governing degradation of riparian forests in response to damming streams is needed for a comprehensive impact assessment of dams in relation to protection of river ecosystems. However, the ascription of riparian forest degradation to the hydrological changes induced by dams lacks of direct proof and the hypothesis that dam construction alters the water use strategy of riparian trees is yet to be tested. In this study, the water sources of riparian trees grown upstream and downstream of dams at SW China, were investigated by dual isotopes and a Bayesian model. We observed that xylem water in downstream Populus adenopoda had significantly lower line-conditioned excess values than upstream trees. The water source of upstream trees was barely subjected to evaporation. Upstream mature and young P. adenopoda trees both acquired more streamwater (47.2 +/- 9.8% and 44.8 +/- 7.9%) than groundwater (28.2 +/- 4.4% and 29.2 +/- 3.5%) or soil water (24.5 +/- 5.6% and 26.0 +/- 4.9%). Downstream mature and young P. adenopoda trees both largely used soil water (49.0 +/- 11.6% and 53.0 +/- 6.2%) as main water source. In contrast, Pterocarya stenoptera highly depended on streamwater, regardless of size and location, implying that damming had little effect on this species. Damming streams may increase water competition among downstream riparian trees, which could explain declines in growth rate, canopy dieback, tree death, and eventually downstream land degradation. We believe that this study is of significant value for research into riparian forest degradation resulting from hydrological changes, and in particular to apprehend the impact of small dams, which has barely been studied.