Decreased streamflow in semi-arid basins following drought-induced tree die-off: A counter-intuitive and indirect climate impact on hydrology

Drought- and infestation-related tree die-off is occurring at regional scales and is projected to increase with global climate change. These large-scale changes in vegetation are expected to influence hydrological responses, but the ecohydrological consequences of die-off have rarely been studied empirically and consequently remain uncertain. Here we evaluate observed hydrologic responses to recent regional-scale die-off of pinon pine (Pinus edulis) in Southwestern USA. Basins with the most tree die-off showed a significant decrease in streamflow over several years following die-off, and this decrease was not attributable to climate variability alone. The results are counterintuitive compared to responses to reductions in tree cover by harvest that have shown an increase in streamflow, although such increases are more substantial for locations with higher precipitation than where the pinon pine die-off occurred. We are unable to isolate the cause of the increase, but note that it is consistent with a reported increase in understory herbaceous cover post-die-off and associated increase in solar radiation reaching near-ground (below the tree canopy overstory), which together would be expected to reduce overland flow. Our study highlights the need to more fully evaluate hydrological responses to drought-induced tree die-off empirically, in addition to modelling studies. More generally, the result illustrate potential indirect effects of climate on hydrological responses mediated through ecohydrological changes in vegetation, which will need to be considered in future water resources assessments. (C) 2011 Elsevier B.V.. All rights reserved.