Abstract
AbstractForests are an incredibly important resource across the globe, yet they are threatened by climate change through stressors such as drought, insect outbreaks, and wildfire. Trailing edge forests—those areas expected to experience range contractions under a changing climate—are of particular concern because of the potential for abrupt conversion to non‐forest. However, due to plant‐climate disequilibrium, broad‐scale forest die‐off and range contraction in trailing edge forests are unlikely to occur over short timeframes (<~25–50 yr) without a disturbance catalyst (e.g., wildfire). This underscores that explicit attention to both climate and disturbance is necessary to understand how the distribution of forests will respond to climate change. As such, we first identify the expected location of trailing edge forests in the intermountain western United States by mid‐21st century. We then identify those trailing edge forests that have a high probability of stand‐replacing fire and consider such sites to have an elevated risk of fire‐facilitated transition to non‐forest. Results show that 18% of trailing edge forest and 6.6% of all forest are at elevated risk of fire‐facilitated conversion to non‐forest in the intermountain western United States by mid‐21st century. This estimate, however, assumes that fire burns under average weather conditions. For a subset of the study area (the southwestern United States), we were able to incorporate expected fire severity under extreme weather conditions. For this spatial subset, we found that 61% of trailing edge forest and 30% of all forest are at elevated risk of fire‐facilitated conversion to non‐forest under extreme burning conditions. However, due to compounding error in our process that results in unknowable uncertainty, we urge caution in a strict interpretation of these estimates. Nevertheless, our findings suggest the potential for transformed landscapes in the intermountain western United States that will affect ecosystem services such as watershed integrity, wildlife habitat, wood production, and recreation.
Highlights
Forests across the globe provide numerous and important ecosystem services such as carbon sequestration, clean water, wood products, and recreation (Costanza et al 1997, Goodale et al.2002, Postel and Thompson 2005, Douglass 2016)
We focus our attention on wildland fire as the catalyzing disturbance agent in trailing edge forests in the intermountain western United States, a region that has experienced substantial fire activity and drought in recent decades
Trailing edge forests, on average, are warmer and drier compared to forests that are expected to remain climatically suitable for forest by mid-century (Fig. 6)
Summary
Forests across the globe provide numerous and important ecosystem services such as carbon sequestration, clean water, wood products, and recreation (Costanza et al 1997, Goodale et al.2002, Postel and Thompson 2005, Douglass 2016). Climate analog models have been used in conjunction with gridded climate data to evaluate potential shifts in vegetation distribution (Batllori et al 2017, Parks et al 2018a) and potential effects on ecosystem services such as crop yields (Pugh et al 2016). These and conceptually similar models are useful, they assume that the current and future distribution of any given species or vegetation type is in equilibrium with climate (Heikkinen et al 2006, Araujo and Peterson 2012; i.e., that changes in climate result in immediate change in vegetation). These models, have their own caveats since they incorporate assumptions about complex processes that are difficult to parameterize (Fisher et al 2010, Williams and Abatzoglou 2016)
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