Multi-scale quantification of anthropogenic, fire, and drought-associated forest disturbances across the continental U.S., 2000–2014

Authors: WANG, M. - Los Alamos National Research Laboratory; XU, C. - Los Alamos National Research Laboratory; JOHNSON, D. - University Of Florida; ALLEN, C. - Us Geological Survey, Fort Collins Science Center; HAIN, C. - Goddard Space Flight Center; Anderson, Martha; WANG, G. - Southern Illinois University; QIE, G. - Southern Illinois University; MCDOWELL, N. - Pacific Northwest National Laboratory

Submitted to: Frontiers in Forests and Global Change
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/2022
Publication Date: 11/3/2022
Citation: Wang, M., Xu, C., Johnson, D., Allen, C., Hain, C., Anderson, M.C., Wang, G., Qie, G., McDowell, N. 2022. Multi-scale quantification of anthropogenic, fire, and drought-associated forest disturbances across the continental U.S., 2000–2014. Frontiers in Forests and Global Change. 5. Article e693418. https://doi.org/10.3389/ffgc.2022.693418.
DOI: https://doi.org/10.3389/ffgc.2022.693418

Interpretive Summary: Drought is a major source of disturbance and mortality in forests, in turn impacting carbon stores and exchanges with the atmosphere. The degree to which drought, specifically, impacts forests and carbon fluxes is difficult to evaluate, given the many other possible sources of natural and human-induced disturbance. This paper discusses a methodology for assessing the relative contributions of forest disturbance and related carbon stock loss from four major disturbance categories: fire, humans, drought, and other factors. The methodology starts with information from a global forest disturbance dataset, sampled over the continental United States (CONUS), and using other spatially distributed indicators to attribute disturbance between fire and drought. For identifying fire-impacted regions, the Monitoring Trends in Burn Severity (MTBS) fire database is used. Human impacts on tree loss are estimated from the Forest Inventory and Analysis (FIA) database. The remote-sensing based Evaporative Stress Index (ESI) is used to highlight periods and regions impacted by drought-induced stress. Results indicate that 1) the detected loss of forest carbon is largely caused by anthropogenic disturbance; 2) the potential carbon loss associated with drought exceeds that from fire during 2000-2014 in the CONUS, and varies largely across the region, suggesting the need to better understand how drought stresses and kills trees. This study provides valuable information for more accurately modeling and predicting vegetation responses to changing climate.

Technical Abstract: Drought is a key driver of forest mortality but continental-scale quantification of the impact of drought on forest mortality does not exist. This lack of data limits our understanding and predictions of broad scale carbon cycling to changing climate and extreme events. In this study, we quantified the area and potential biomass carbon loss in continental U.S. (CONUS) for the period of 2000-2014 from four categories of disturbance: 1) fire-induced, 2) human-made, 3) drought-associated, and 4) others. Our results showed that potential carbon loss associated with droughts accounts for approximately 10.6 -13.3% of total forest carbon loss in CONUS, which is about one-fifth of that resulting from anthropogenic disturbances. The potential carbon loss through tree mortality from natural disturbances for the same study period is approximately 70.2-73.7% of the total carbon loss from anthropogenic disturbance, suggesting that natural disturbances play a very important role on forest carbon turnover. Our results demonstrate the importance of drought impact on forest carbon stocks at the continental scale and provide critical data to evaluate predictive models of tree mortality under droughts.