Structure, diversity and health of Sierra Nevada red fir forests with reestablished fire regimes

Abstract

The reestablishment of natural fire regimes may benefit forest ecosystems by restoring their fundamental structural, compositional or functional attributes. We examined the influence of fire on the structure, understorey diversity and health of red fir (Abies magnifica) forests by comparing burned and unburned stands in 22 separate, paired fires of Yosemite, Sequoia and Kings Canyon National Parks and the Giant Sequoia National Monument. Burned red fir plots were characterised by lower tree densities and canopy cover, restored spatial heterogeneity and higher understorey species richness than unburned plots. Densities of large trees and large snags and red fir regeneration were similar between burned and unburned sites. Forest health indicators were similar between burned and unburned sites, and red fir crown loss ratings were primarily associated with topographic variables indicative of increased moisture stress or reduced soil moisture availability (i.e. lower elevations, south-facing slopes). Our results suggest that fire does not improve the health of red fir trees especially in areas of greater moisture stress, but it can restore red fir forest structure, increase understorey diversity and enhance adaptive capacity in landscapes with reestablished fire regimes.

Do forest fuel reduction treatments confer resistance to beetle infestation and drought mortality?

“…The negative effect of burning on tree survival is somewhat surprising given that the fire regime under which these forests developed was characterized by frequent (i.e., 11–17 yr) low‐ to moderate‐severity fire (North et al 2005, Safford and Stevens 2017), and that the prescribed burn occurred approximately a decade prior to the drought. Further, van Mantgem et al (2016) observed decreased tree mortality associated with prescribed fire elsewhere in the Sierra Nevada following the initial two years of California’s drought, and Meyer et al (2019) found no difference in mortality between paired burned and unburned plots in red fir forests during the middle and late periods of the drought. The forests Meyer et al (2019) sampled were at higher elevations than Teakettle where soil moisture is substantially higher and temperatures lower.…”

“…Further, van Mantgem et al (2016) observed decreased tree mortality associated with prescribed fire elsewhere in the Sierra Nevada following the initial two years of California’s drought, and Meyer et al (2019) found no difference in mortality between paired burned and unburned plots in red fir forests during the middle and late periods of the drought. The forests Meyer et al (2019) sampled were at higher elevations than Teakettle where soil moisture is substantially higher and temperatures lower. The results presented here could be unique to the Teakettle Experimental Forest, but we suspect they are more likely attributable to the historic severity of the 2012–2016 drought.…”

Do forest fuel reduction treatments confer resistance to beetle infestation and drought mortality?

“…given that the fire regime under which these forests developed was characterized by frequent (i.e., 11-17 years) low-to moderate-severity fire (North et al 2005, Safford and, and that the prescribed burn occurred approximately a decade prior to the drought. Further, van Mantgem et al (2016) observed decreased tree mortality associated with prescribed fire elsewhere in the Sierra Nevada following the initial two years of California's drought, and Meyer et al (2019) found no difference in mortality between paired burned and unburned plots in red fir forests during the middle and late periods of the drought. The forests Meyer et al…”

Natural range of variation of red fir and subalpine forests in the Sierra Nevada Bioregion

“…For example, Taylor and Solem (2001) and Taylor (2000) estimated presettlement (1735-1849), settlement , and fire-suppression fire rotations of 76, 117, and 577 years, respectively, in red fir and other upper montane forests in the southern Cascades. The absence of fire over the past century has also increased the backlog of red fir forests that require fire for ecological benefits (e.g., surface fuels reduction, increased biodiversity and structural heterogeneity), as indicated by an increase in fire return interval departure (FRID) values in these forests (Caprio and Graber 2000, North et al 2012, Meyer et al 2019. However, most Sierra Nevada red fir forests have missed only one to three fire cycles (i.e., mostly low to moderate FRIDs), suggesting that the ecological effects of fire suppression in these forests are not as extreme as in the fire-frequent mixed-conifer and yellow pine forests (Miller and Safford 2012, North 2014, Safford and Van de Water 2014, van Wagtendonk et al 2002.…”