Ecohydrologic response and atmospheric feedbacks from beetle-induced transpiration losses in the Colorado headwaters

Abstract

The mountain pine beetle (MPB), Dendroctonus ponderosae, has resulted in the largest recorded tree mortality caused by insect in North America. Existing literature has documented complex ecohydrologic response to infestation-induced transpiration losses, including changes to local soil moisture, snow accumulation and ablation, evapotranspiration partitioning, groundwater storage, and the surface energy budget. Potential atmospheric feedbacks have not yet been thoroughly investigated as a possible compensating factor, despite modeled evidence that changes to latent and sensible heat flux at the surface could propagate into the atmosphere in the form of increased surface temperatures and planetary boundary layer height. Here we present controlled numerical experiments that resolve complicated feedbacks from land disturbance to atmosphere, using the Weather Research and Forecasting (WRF) mesoscale meteorological model. WRF is coupled to ParFlow, a physically-based, integrated hydrologic model, through the land surface model Noah. The model was run at high meteorological resolution (1-km lateral grid spacing) over the Colorado headwaters, a region paramount to domestic and agricultural resources and heavily influenced by MPB. Vegetation parameters for evergreen needleleaf trees were adjusted to reflect beetle-induced reductions in stomatal conductivity and LAI, and an ensemble methodology was used to represent a measure of uncertainty in initial atmospheric conditions. Results suggest that MPB signal is retained in atmospheric processes with distinct seasonal and diurnal signatures. However, atmospheric responses, particularly for precipitation, are inconsistent and often insignificant when compared to ensemble spread. Changes to the land surface energy budget and to ground and near-surface air temperatures are damped when compared to meteorological models that lack a lateral flow hydrology component. This work presents the applicability of a deterministic, integrated climate-hydrologic model to identify complicated physical interactions occurring with forest disturbance, which may not be discernable with simpler models or observations.