Effects of low-severity fire on species composition and structure in montane forests of the Sierra Nevada, California, USA

Abstract

Climate, topography, and fire influence the species composition and structure of montane forest communities in the Sierra Nevada. This thesis uses data from 97, 0.1 ha plots (46 unburned since 1930; 51 burned at moderate-severity or below since 1946) in Yosemite and Sequoia & Kings Canyon National Parks to describe relationships between (1) climatic water balance and species composition, (2) recent fire and species composition, (3) climatic water balance and forest diameter distributions, and (4) recent fire and forest diameter distributions. These analyses are presented in two parts. Chapter I examines the effects of moderate-severity fire and below (hereafter, lower-severity fire) on individual tree species and characterizes the relationships between species composition and (1) modeled climatic water balance and (2) lower-severity fire. Chapter II parallels the approach of Chapter I but categorizes trees by diameter class rather than species. Chapter II examines the effects of lower-severity fire on individual diameter classes and characterizes the relationships between diameter class assemblages (i.e., structural communities) and (1) modeled climatic water balance and (2) lower- severity fire. This thesis recognizes species composition and structural composition as unique but interrelated attributes of forest systems. These analyses suggest that lower-severity fire reduces the density of smaller-diameter trees but that reintroduction of fire will not restore historical species composition given the abundance of shade-tolerant species that have attained fire- resistant size during the fire exclusion of the previous century. Land management strategies designed to conserve forest structure rather than species composition may therefore become increasingly important.