Can low‐severity fire reverse compositional change in montane forests of the Sierra Nevada, California, USA?

Abstract

AbstractThroughout the Sierra Nevada, nearly a century of fire suppression has altered the tree species composition, forest structure, and fire regimes that were previously characteristic of montane forests. Species composition is fundamentally important because species differ in their tolerances to fire and environmental stressors, and these differences dictate future forest structure and influence fire regime attributes. In some lower montane stands, shade‐tolerant, fire‐sensitive species have driven a threefold increase in tree density that may intensify the risk of high‐severity fire. In upper montane forests, which were historically characterized by longer fire return intervals, the effects of fire exclusion are both less apparent and less studied. Although land managers have been reintroducing fire to lower and upper montane forests for >4 decades, the potentially restorative effects of these actions on species composition remain largely unassessed. We used tree diameter and species data from 51 recently burned and 46 unburned plots located throughout lower and upper montane forests in Yosemite National Park and Sequoia & Kings Canyon National Parks to examine the effects of low‐ to moderate‐severity (hereafter, lower‐severity) fire on the demography of seven prevalent tree species. The density of Abies concolor <30 cm diameter at breast height (dbh) was significantly lower in burned plots than unburned plots; densities of A. concolor 30–45 cm dbh, A. magnifica <30 cm dbh, and Calocedrus decurrens <15 cm dbh were lower in burned plots than unburned plots at a marginal level of significance. These diameter thresholds represent the maximum size at which each species is likely to experience significant mortality from lower‐severity fire. We overlaid these thresholds on historical and contemporary diameter distributions to show that: (1) lower‐severity fire has reduced mean tree density to historical levels for A. concolor but not for C. decurrens, and (2) variability in tree density among plots that burned at lower severity exceeded the range of tree densities reported in historical data sets. High proportions of shade‐tolerant species in some postfire stands may increase the prevalence of shade‐tolerant species in the future, a potential concern for managers who seek to minimize ladder fuels and promote forest structure that is less prone to high‐severity fire.