Damage and mortality patterns in young mixed conifer plantations following prescribed fires in the Sierra Nevada, California

Abstract

High-severity wildfires increasingly influence forests in the western United States. Extensive research has identified preventative practices including mechanical and prescribed fire treatments to reduce wildfire severity in mature stands. Yet limited research has investigated fuel management treatments in young stands which can be particularly vulnerable to even low intensity fire. To address this gap, we investigated how prescribed fire (conducted in both the spring and fall) and pre-treatment fuel modifications impacted individual tree damage (or injury) and mortality in nine 13–14year old mixed conifer stands in the central Sierra Nevada, California, USA. Prior to burning, randomly selected trees were pruned to 1.8m height above ground. Randomly selected trees were also raked to reduce surrounding surface fuel. Raking fuel from trees reduced the amount of crown volume scorched for all species, but not enough trees died to determine if raking influenced probability of survival. Pruning was associated with reductions in crown consumption height and percent crown volume consumed (5% of crown volume in pruned versus <1% of crown volume unpruned, p=0.02) but was not a significant predictor of percent of crown volume scorched brown. Pruned trees had only 27% of the mortality of unpruned trees when less than half of the crown was scorched (p=0.046). However the mortality of pruned trees showed no less sensitivity to fire damage at higher levels of crown scorch (p=0.0076); in fact extremely scorched trees appeared more likely to die when pruned. Mortality differed strongly by species; giant sequoia (Sequoiadendron giganteum) showed 97% post-fire survival while nearly half of sugar pines (Pinus lambertiana) died. Fires occurring in spring months killed seven times as many trees as those in autumn months. While the results demonstrate that prescribed burns can be feasible in stands as young as 13years old, the factors of post-fire damage and mortality are different compared to mature stands. Species composition, pre-fire fuel reduction treatments, and timing of prescribed burns are all important considerations for managers wanting to develop resilient young stands with prescribed fire.