Abstract
BackgroundSurface 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 mixed-conifer forest in Yosemite National Park, California, USA, and assessed fuel consumption from a management-ignited 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.ResultsPre-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.ConclusionsSurface 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.
Highlights
Surface fuel loadings are some of the most important factors contributing to fire intensity and fire spread
Resultados: La carga total de combustibles superficiales en el pre fuego promediaron 192 Mg ha−1 y fue reducida en un 79% por el fuego a 41 Mg ha−1 inmediatamente luego del fuego
We modeled the mass of litter and duff in P. lambertiana duff mounds within 1 m and 2 m of trees as a function of tree diameter at breast height (DBH) using a linear model
Summary
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. Understanding the variability of pre-fire fuel loadings at spatial scales similar to that of prescribed fires (i.e., 25 ha to 250 ha) can be important both to managers seeking to reintroduce fire (Collins et al 2010) and for calculating the likely effects of burning on tree survival (Lutes et al 2009; Furniss et al 2019; Hood et al 2018). While overstory structure determines the fluxes and distribution of fuel loadings at broad spatial scales, the tall trees and heterogeneous spatial patterns characteristic of dry, mixed-coniferous forests can obscure this relationship at sub-stand scales
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