Fuel dynamics after reintroduced fire in an old-growth Sierra Nevada mixed-conifer forest

Abstract

Background: Surface fuel loadings are some of the most important factors contributing to fire intensity and fire spread. In old-growth forests where fire has been long excluded, surface fuel loadings can be high and can include woody debris ≥100 cm in diameter. We assessed surface fuel loadings in a long-unburned old-growth mixedconifer forest in Yosemite National Park, California, USA, and assessed fuel consumption from a managementignited fire set to control the progression of the 2013 Rim Fire. Specifically, we characterized the distribution and heterogeneity of pre-fire fuel loadings, both along transects and contained in duff mounds around large trees. We compared surface fuel consumption to that predicted by the standard First Order Fire Effects Model (FOFEM) based on pre-fire fuel loadings and fuel moistures. We also assessed the relationship between tree basal area-calculated for two different spatial neighborhood scales-and pre-fire fuel loadings. Results: Pre-fire total surface fuel loading averaged 192 Mg ha −1 and was reduced by 79% by the fire to 41 Mg ha −1 immediately after fire. Most fuel components were reduced by 87% to 90% by the fire, with the exception of coarse woody debris (CWD), which was reduced by 60%. Litter depth in duff mounds were within 1 SD of plot means, but duff biomass for the largest trees (>150 cm diameter at breast height [DBH]) exceeded plot background levels. Overstory basal area generally had significant positive relationships with pre-fire fuel loadings of litter, duff, 1-hour, and 10-hour fuels, but the strength of the relationships differed between overstory components (live, dead, all [live and dead], species), and negative relationships were observed between live Pinus lambertiana Douglas basal area and CWD. FOFEM over-predicted rotten CWD consumption and under-predicted duff consumption. Conclusions: Surface fuel loadings were characterized by heterogeneity and the presence of large pieces. This heterogeneity likely contributed to differential fire behavior at small scales and heterogeneity in the post-fire environment. The reductions in fuel loadings at our research site were in line with ecological restoration objectives; thus, ecologically restorative burning during fire suppression is possible.

Local forest structure variability increases resilience to wildfire in dry western U.S. coniferous forests

“…Greater NDVI corresponds to high canopy cover and vegetation density (Rouse et al ) which translates directly to live fuel loads in the forest canopy and can increase high‐severity fire (Parks et al ). Overstorey canopy cover and density also correlate (though weakly) with surface fuel loads (Lydersen et al ; Collins et al ; Cansler et al ), which can play a large role in driving high‐severity fire in these forests (Agee ). Thus NDVI is likely a strong predictor of fire severity because it is correlated with both surface fuel loads and canopy live fuel density.…”

Wildfire and drought moderate the spatial elements of tree mortality

“…Local neighborhood structure was directly related to probability of immediate mortality, presumably because higher stem density was associated with increased fuel loadings (Cansler et al 2019) that elevated fire intensity (Miller and Urban 1999 b , Thaxton and Platt 2006) and induced greater damage to trees. Surprisingly, forest structure was also related to probability of direct fire mortality when we included crown scorch as a predictor variable to control for variability in fire intensity (Table 2), suggesting that forest spatial structure also influenced probability of direct mortality by reducing tolerance of individual trees to direct fire damage (perhaps by modifying local water availability and competitive stress; van Mantgem et al 2018).…”

“…2). Surface fuel consumption was >90% for litter, duff, and small fuels (<1000 h), and 61% for coarse woody debris (Larson et al 2016, Cansler et al 2019). Fire effects were heterogeneous with patches of low, moderate, and high tree mortality (Furniss et al 2020).…”

Local forest structure variability increases resilience to wildfire in dry western U.S. coniferous forests

“…Greater NDVI corresponds to high canopy cover and vegetation density (Rouse et al 1973) which translates directly to live fuel loads in the forest canopy and can increase high-severity fire (Parks et al 2018). Overstory canopy cover and density also correlate (though weakly) with surface fuel loads (Lydersen et al 2015;Cansler et al 2019), which can play a large role in driving high-severity fire in these forests (Agee 1996). Thus NDVI is likely a strong predictor of fire severity because it is correlated with both surface fuel loads and canopy live fuel density.…”