Bark beetle impacts on forest evapotranspiration and its partitioning

Authors: KNOWLES, J. - California State University; BJARKE, N.R. - University Of Colorado; BADGER, A.M. - University Of Colorado; BERKELHAMMER, M. - University Of Illinois; Biederman, Joel; BLANKEN, P.D. - University Of Colorado; BRETFELD, M. - Kennesaw State University; BURNS, S.P. - Kennesaw State University; EWERS, B.E. - University Of Wyoming; FRANK, J.M. - Us Forest Service (FS); HICKE, J.A. - University Of Idaho; LESTAK, L. - California State University; LIVNEH, B. - University Of Colorado; REED, D.E. - University Of Oklahoma; Scott, Russell - Russ; MOLOTCH, N.P. - University Of Colorado

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2023
Publication Date: 4/6/2023
Citation: Knowles, J., Bjarke, N., Badger, A., Berkelhammer, M., Biederman, J.A., Blanken, P., Bretfeld, M., Burns, S., Ewers, B., Frank, J., Hicke, J., Lestak, L., Livneh, B., Reed, D., Scott, R.L., Molotch, N. 2023. Bark beetle impacts on forest evapotranspiration and its partitioning. Science of the Total Environment. 880. Article 163260. https://doi.org/10.1016/j.scitotenv.2023.163260.
DOI: https://doi.org/10.1016/j.scitotenv.2023.163260

Interpretive Summary: Insect outbreaks affect many forested watersheds, with unknown impacts on the balance of water among evaporation, plant water use, and streamflow. Here we combined direct measurements of water fluxes, satellite remote sensing imagery, and simulation modeling to assess the impacts of insect outbreaks on the water cycle in the southern Rocky Mountains, USA. Overall, bark beetle outbreaks reduced evaporation and plant water use, increasing the water available for streamflow. Effects lasted at full strength for 6-8 years and then dwindled, with water balance returning to original values after 10-15 years.

Technical Abstract: Insect outbreaks affect forest structure and function and represent a major category of forest disturbance globally. However, the resulting impacts on evapotranspiration (ET), and especially hydrological partitioning between the abiotic (evaporation) and biotic (transpiration) components of total ET, are not well constrained. As a result, we combined remote sensing, eddy covariance, and hydrological modeling approaches to determine the effects of bark beetle outbreak on ET and its partitioning at multiple scales throughout the 144,462 km2 Southern Rocky Mountain ecoregion, USA. At the eddy covariance scale, water year ET as a fraction of precipitation decreased by 17% relative to a control site, with changes in transpiration accounting for 40% of the peak growing season reduction during the epidemic phase. At the ecoregion scale, corresponding ET reductions derived from…model? RS? of 9-15% occurred 6-8 years post-disturbance, with peak growing season processes accounting for 44-47% of the reduction, and there was an associated 9-18% increase in the modeled ecoregion runoff ratio. Coupled long-term (16-18 year) ET and vegetation mortality datasets extended the length of previously published analyses and allowed for clear characterization of the forest recovery period. During that time, transpiration recovery outpaced total ET recovery, which was lagged in part due to persistently reduced sublimation of canopy-?intercepted snow. At a process level, the sensitivity of ET to potential ET recovered post-disturbance, but with a lower intercept due to the loss of mature trees, and there was associated evidence of increasing late summer moisture stress to vegetation. Overall, comparison of three independent methods and two partitioning approaches demonstrated a net negative impact of bark beetle disturbance on ET that was driven by changes in transpiration. These multi-scale results suggest that a nuanced interpretation is required to accurately predict post-disturbance hydrological impacts in areas of complex terrain.