Near-Future Forest Vulnerability to Drought and Fire Varies Across the Western United States
Vulnerability highest in the Southwest and Sierra Nevada, lowest in the Pacific Northwest, U.S.
The study assessed forest vulnerability to drought and fire across the western United States during 2020–2049 using the Community Land Model (CLM4.5), which simulates forest growth and wildland fire given prescribed climate conditions. Researchers used future climate conditions that are based on the current trajectory of greenhouse gas emissions. Defining multiple forest types and environmental and climate conditions at a fine spatial resolution, regionally relevant fire fuel limits, and enhanced tree response to drought were important model improvements. The improved model allowed the researchers to assess the potential for tree mortality from short- and long-term drought, and the potential for future fire.
Forests identified as having low vulnerability could be targeted for preservation as carbon sequestration preserves. Research in high-vulnerability forests can identify management and environmental conditions that could delay or avoid ecosystem transformation. Communities in high–fire vulnerability areas may want to assess or improve their fire preparedness. These drought vulnerability metrics could be incorporated as probabilistic mortality rates in Earth system models, enabling more robust estimates of the feedbacks between the land and atmosphere under future conditions.
A research team from Oregon State University used the Community Land Model (CLM4.5) to determine forest vulnerability to mortality from drought and fire by the year 2049. They modified CLM to represent 13 major forest types in the western United States and ran simulations at a 4-km grid resolution, driven with climate projections from two general circulation models under one emissions scenario (RCP 8.5). The study developed metrics of vulnerability to short-term extreme and prolonged drought based on annual carbon allocation to stem growth and net primary productivity. They calculated fire vulnerability based on changes in simulated future area burned relative to historical area burned, for all forested grid cells. Projections indicate that water-limited forests in the Rocky Mountains, Southwest, and Great Basin regions will be the most vulnerable to future drought-related mortality, and vulnerability to future fire will be highest in the Sierra Nevada and portions of the Rocky Mountains. High–carbon density forests in the Pacific coast and western Cascades regions are projected to be the least vulnerable to either drought or fire. Importantly, differences in climate projections lead to only 1% of the domain with conflicting low and high vulnerability to fire and no area with conflicting drought vulnerability.
Oregon State University
This work was supported by the Office of Biological and Environmental Research (BER) within the U.S. Department of Energy (DOE) Office of Science (DE-SC0012194) and the U.S. Department of Agriculture National Institute of Food and Agriculture (NIFA) (2013-67003-20652, 2014-67003-22065), and DOE Oregon AmeriFlux Sites. High-performance computing resources on Cheyenne (https://doi.org/10.5065/D6RX99HX) were provided by the National Center for Atmospheric Research (NCAR) Computational and Information Systems Laboratory, sponsored by the National Science Foundation.
Buotte, P.C. et al. “Near-future forest vulnerability to drought and fire varies across the western US.” Global Change Biology Online 25(1), 290–303 (2018). [DOI:10.1111/gcb.14490.]