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Title: Recent tree die-off has little effect on streamflow in contrast to expected increases from historical studies

Author
item Biederman, Joel
item Scott, Russell - Russ
item SOMOR, A.J. - University Of Arizona
item HARPOLD, A.A. - University Of Nevada
item GUTMANN, E.D. - National Center For Atmospheric Research (NCAR)
item GOCHIS, D.J. - National Center For Atmospheric Research (NCAR)
item BRESHEARS, D.D. - University Of Arizona
item TROCH, P.A. - University Of Arizona
item BROOKS, P.D. - University Of Utah
item MEDDENS, A.J.H. - University Of Idaho

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2015
Publication Date: 12/7/2015
Citation: Biederman, J.A., Scott, R.L., Somor, A., Harpold, A., Gutmann, E., Gochis, D., Breshears, D., Troch, P., Brooks, P., Meddens, A. 2015. Recent tree die-off has little effect on streamflow in contrast to expected increases from historical studies. Water Resources Research. 51:9775-9789. https://doi.org/10.1002/2015WR017401.
DOI: https://doi.org/10.1002/2015WR017401

Interpretive Summary: Over the last two decades, beetle infestation has killed billions of trees and affected millions of acres of forest in North America, many of which are critical sources of human water supply. Here we quantified annual streamflow responses in the decade following mortality and compared them to long-term pre-mortality records (~25-40 years). In contrast to streamflow increases predicted by historical paired catchment studies and recent modeling, observed streamflow changes were weak, variable, and more frequently showed declines than increases. Although initially surprising, these results are consistent with the growing body of literature documenting increased snow sublimation and evaporation from the subcanopy following die-off in semiarid forests.

Technical Abstract: Recent bark beetle epidemics have caused regional-scale tree mortality in many snowmelt-dominated headwater catchments of western North America. Initial expectations of increased streamflow have not been supported by observations, and the basin-scale response of annual streamflow is largely unknown. Here we quantified annual streamflow responses during the decade following tree die-off in eight infested catchments in the Colorado River headwaters and one nearby control catchment. We employed three alternative empirical methods: (i) double-mass comparison between impacted and control catchments, (ii) runoff ratio comparison before and after die-off, and (iii) time-trend analysis using climate-driven linear models. In contrast to streamflow increases predicted by historical paired catchment studies and recent modeling, we did not detect streamflow changes in most basins following die-off, while one basin consistently showed decreased streamflow. The three analysis methods produced generally consistent results, with time-trend analysis showing precipitation was the strongest predictor of streamflow variability (R2'='74–96%). Time-trend analysis revealed post-die-off streamflow decreased in three catchments by 11–29%, with no change in the other five catchments. Although counter to initial expectations, these results are consistent with increased transpiration by surviving vegetation and the growing body of literature documenting increased snow sublimation and evaporation from the subcanopy following die-off in water-limited, snow-dominated forests. The observations presented here challenge the widespread expectation that streamflow will increase following beetle-induced forest die-off and highlight the need to better understand the processes driving hydrologic response to forest disturbance.