Mortality due to pine beetle infestations, for example, has reached record levels, with over 4 million hectares affected in the US, causing immediate, sweeping changes in mountain ecosystems. The annual area affected by bark beetle infestations in North America is comparable to that burned by fire, with hydrologic impacts of presumably similar magnitude. Human disturbance can also regulate hydrologic processes.
Predictable temporal changes in biological controls on hydrology (e.g., transpiration, evaporation, interception and soil moisture) due to beetle kill likely occur during outbreaks. We can expect changes in evapotranspiration, soil moisture, and streamflow. The response of watersheds to these outbreaks will depend on how variable tree mortality changes the partitioning of precipitation and thus influences the path of water flow from the surface to the subsurface. To quantify these changes in the land surface and test our predictive understanding of how streamflow responds to changes in precipitation partitioning at the stand scale we propose ecological, geochemical, and geophysical measurements that will be fed into models. Total evapotranspiration will be measured with four new eddy covariance towers; tree and soil evaporation components will be measured with sap flux and water isotopes to check partitioning. We will test the effects of tree and beetle species and succession by comparing lodgepole pine and spruce fir forests using towers in intact forests, forests with bark beetle mortality in the last 1-3 years and 3-5 years representing key stages of bark beetle outbreaks. Geophysical transects (seismic, resistivity, and gravity) will be acquired across the boundaries between intact and infested stands to quantify contrasts in subsurface structure and moisture content.
Figure. WyCEHG resistivity image across the boundary between stands of live and dead (beetle-killed) trees. The image is approximately 50 meters wide and 6 meters deep. Note the sharp decrease in resistivity at about 50 cm depth, which likely represents the water table, and the contrast in resistivity in the upper 50 cm between the dead and live stands. These results are consistent with more soil moisture in the upper 50 cm beneath the dead stand than beneath the live stand, where soil moisture is used for transpiration.