“Excess Water” Following Deforestation by Beetle Kill?

Abstract:

Deforestation resulting from tree mortality by insects and disease may reduce transpiration demand and increase available water in mountain environments throughout. We tested this hypothesis using three large catchments (97-407 km²) located in the Snowy Mountains of Wyoming where hydrology is snowmelt dominated. An epidemic of spruce bark beetle and associated tree mortality emerged in 2006 and has since impacted 60 to 80% of basal area of the spruce-fir and mixed conifer forests. A 25-year continuous record (1998-2013) of daily snowfall, temperature, and stream discharge data between 1 April and 30 September of each year were available for each catchment. We used quantile regression and multivariate time series analysis first to control for the effects of temperature and snow water equivalent on the timing and magnitude of discharge and then to test for changes in discharge trends since 2006. We found no compelling evidence of changes in discharge trends associated with the onset of the beetle epidemic independent of snowmelt trends. Several factors could explain this apparent lack of “excess water” following tree mortality by insects and disease. Any increases in water may be scale dependent, a local phenomenon that does not transfer through large catchments. Other vegetation including young cohorts of affected tree species, shrubs, and herbaceous cover may respond robustly to the open canopy and utilize soil water previously consumed by the infected trees.