ECOPHYSIOLOGICAL IMPACT OF SPRUCE BEETLE EPIDEMIC ON THE ANNUAL WATER CYCLE OF A HIGH ELEVATION ROCKY MOUNTAIN (WYOMING, USA) FOREST

Bark beetle disturbances in the forests of western North America have been linked to anthropogenic changes in land use and climate and to disruptions in ecosystem function and alterations in the hydrologic cycle of forests. Spruce beetle (Dendroctonus rufipennis) outbreaks, which have also become more common, impact subalpine forests that contribute to the headwaters of many western US watersheds. These epidemics influence annual water vapor fluxes by two distinct processes. (1) They cause an evapotranspiration response during the growing season as the plants respond to attack and (2) they create a more open canopy, which during the wintertime causes changes in sublimation due to changes in the exposed snow surface area and in the within-canopy radiation and wind flow. In this study, we analyze eddy-covariance flux data from the GLEES AmeriFlux site in southeastern Wyoming, USA, where a subalpine forest dominated by Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) recently experienced an outbreak of spruce beetle. The immediate impact of the outbreak was hydraulic failure in the affected spruce, which led to a reduction in canopy conductance and a 1/3 decrease in summer evapotranspiration (ET). The spruce maintained both their biochemistry and most of their canopy for several more years before ultimately dying and dropping their needles. This loss of canopy had minimal further influence on ET, but signaled a decrease in wintertime sublimation rates by 1/3 when normalized by snowfall amount. Through the use of a within-canopy eddy-covariance system and stable isotope analyses of snowfall and snowpack we observed that post-beetle sublimation originates from the surface of the snowpack and is in part driven by downward sensible heat flux plus transitions from cloudy to cloud-free days. Results suggest that summer ET and winter sublimation each account for approximately half of the total annual water vapor flux, and thus both must adequately be modeled to predict the ecosystem hydrologic response to bark beetles. But most critically, a physical model is required to describe the mechanics of sublimation in this ecosystem to determine the processes that drive sublimation and to test for mechanistic changes in these processes relative to the outbreak.