Prescribed Fire Can Increase Multi-Species, Regional-Scale Resilience to Increasing Climatic Water Deficit

Abstract

Dry mixed conifer forests of southwestern North America are projected to be particularly vulnerable to ongoing persistent warm drought conditions, and related increases in wildfire frequency, size and severity, due in part to consequences of over a century of fire exclusion. Prescribed fire is applied actively in many landscapes to reduce hazardous fuel loads and continuity, restore forest community composition and structure, and increase tree resilience to drought stress. However, fire can also adversely affect tree growth by damaging cambial, root, and canopy tissues, leading to tradeoffs in the use of fire as a tool for forest resilience. Radial growth is an indicator of climatic and ecological stress and can thus provide a relative measure of resilience to stress and disturbances; but, the mechanisms driving tree resilience to prescribed fire and concurrent drought are poorly understood. Thinning effects of prescribed fire may increase tree resilience to drought by increasing water, light and nutrient availability and production of defense mechanisms. However, trends over the last century indicate warming temperatures are increasing tree sensitivity to fire by reducing post-fire growth (lower resilience) and increasing the likelihood of mortality. Trees can be resistant to fire exposure, and where growth changes occur they can be transient or persistent. We studied the interactions between tree- and stand-level fire effects on the growth responses of surviving Abies concolor, Pinus jefferyi, Pinus ponderosa, and Pseudotsuga menziesii over 24 years of variable climatic conditions in ten National Parks across the western and southwest United States. We used linear mixed effects models to identify mechanisms influencing resistance and resilience responses to fire and interannual climate, using climatic water deficit (CWD) as an index of climatic stress. Compared to pre-fire growth, trees exposed to fire increased growth during periods of greater water deficits. Tree growth responses were variable among and within species and size classes, but contingent on time-since-fire and the climate during the recovery period. Negative fire effects on tree resistance were generally transient, while climate and pre-existing stand conditions were persistent controls on tree resilience. These results suggest that antecedent and subsequent climate conditions modulate post-fire forest response. Consideration of climate variation could improve the strategic use of prescribed fire for tree resilience to drought, and a deeper understanding of factors contributing to prefire growth may elucidate the mechanisms driving post-fire growth responses.