Living on the edge: trailing edge forests at risk of fire‐facilitated conversion to non‐forest

Abstract

Forests 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.

Limitations to recovery following wildfire in dry forests of southern Colorado and northern New Mexico, USA

“…Given the importance of canopy cover and 30‐yr average CWD in predicting densities of both ponderosa pine and Douglas‐fir, results from the current study imply that under continued warming and altered fire activity, some forested areas in the Southern Rocky Mountains are likely to undergo fire‐driven conversions to non‐forested vegetation (Parks et al. ).…”

Climate, Environment, and Disturbance History Govern Resilience of Western North American Forests

“…As temperatures continue to rise, and interannual precipitation variability remains high, the potential exists for ongoing, widespread tree mortalitya phenomenon that has occurred during prior hot droughts (Allen, 2007;Williams et al, 2013). Further, increasingly large high-severity fires can trigger vegetation shifts in concert with ongoing climate change (Allen, 2014;Coop et al, 2016;Parks et al, 2019). The societal implications of large, high-severity fires are already being realized in terms of impacts on water supply (Smith et al, 2011), carbon sequestration, and air quality; more managed fire can ameliorate all of these impacts (Hurteau et al, 2014).…”

Wildfire severity and postfire salvage harvest effects on long‐term forest regeneration

“…Similarly, periods of benign weather following fire can provide opportunities for trees to regenerate (Littlefield 2019). However, long‐term regeneration success is not guaranteed, particularly where trees develop at the edge of their physiological tolerance (Parks et al 2019). Understanding the conditions conducive to regeneration and factors that limit long‐term success is key to postfire management, especially as the climate changes (Tepley et al 2017).…”