Pre‐fire drought and competition mediate post‐fire conifer mortality in western U.S. National Parks

Abstract

Tree mortality is an important outcome of many forest fires. Extensive tree injuries from fire may lead directly to mortality, but environmental and biological stressors may also contribute to tree death. However, there is little evidence showing how the combined effects of two common stressors, drought and competition, influence post-fire mortality. Geographically broad observations of three common western coniferous trees subjected to prescribed fire showed the likelihood of post-fire mortality was related to intermediate-term (10 yr) pre-fire average radial growth, an important component of tree vigor. Path analysis showed that indices of competition and drought stress prior to fire can be described in terms of joint effects on growth, indirectly affecting post-fire mortality. Our results suggest that the conditions that govern the relationship between growth and mortality in unburned stands may also apply to post-fire environments. Thus, biotic and abiotic changes that affect growth negatively (e.g., drought stress) or positively (e.g., growth releases following thinning treatments) prior to fire may influence expressed fire severity, independent of fire intensity (e.g., heat flux, residence time). These relationships suggest that tree mortality may increase under stressful climatic or stand conditions even if fire behavior remains constant.

Fire and tree death: understanding and improving modeling of fire-induced tree mortality

“…For example, in a large study of unburned trees, trees that died had reduced tree growth in years preceding death compared to surviving trees (Cailleret et al 2016). These above lines of evidence, coupled with studies showing post-fire mortality may be worsened by pre-fire competition (van Mantgem et al 2018), suggest that low pre-fire growth may be an indicator of susceptibility to fire-caused injuries.…”

“…Drought is a common stressor for conifers in the western US, and pre-fire drought has been shown to increase the likelihood of tree death following fire (van Mantgem et al 2013, van Mantgem et al 2018). This may become an important consideration in dry forests, where acute and chronic droughts are increasingly coupled with high temperatures and have been linked to forest die-backs even without fire (Allen et al 2015, Adams et al 2017a).…”

“…Density can influence fire-induced tree mortality through two main ways: by affecting local fire behavior and through competition with neighboring trees. Forest structure influences fuel arrangement, local moisture, and fire-atmosphere interactions, thereby also influencing fire behavior that causes direct injury to trees ( van Mantgem et al 2018), with slower growing trees more likely to die given the same level of fire-caused injury than faster growing trees (van Mantgem et al 2003, Nesmith et al 2015. Alternatively, decreases in tree density and competition after fire can increase resource availability, potentially compensating for the short-term impacts of injury to release surviving trees (Alfaro-Sánchez et al 2016).…”

“…classification and regression trees, path analysis) could help identify interactions and provide insight into the mechanisms of fire-induced tree mortality, laying a foundation for future advances in process-based models of fire-induced mortality. Some attempts to model fire-induced mortality with path analysis have been made (Menges and Deyrup 2001, Youngblood et al 2009, van Mantgem et al 2018. These models allow better accounting of the strength and direction of direct and indirect influences on post-fire tree mortality, but also require a priori hypotheses of effects and interactions.…”

Scaling Ecological Resilience

“…Seasonal patterns of precipitation, snowpack soil water content and persistence, seasonal timing of streamflow, and humidity are changing in ways that are demonstrably unprecedented for thousands of years. In many regions of the world, widespread drought-related stress driven by rapidly rising seasonal temperatures is already causing elevated tree mortality, indicating the potential for substantial contraction of forested area, or replacement of current forested communities with species better adapted to persistent drought conditions (van Mantgem et al, 2009(van Mantgem et al, , 2018Williams et al, 2013).…”