Fuel loads and composition across a pine dominance gradient within fire-dependent pine–oak mixedwoods of the southeastern U.S. undergoing hardwood encroachment

Fire exclusion and encroachment by shade-tolerant, often fire-sensitive, and/or opportunistic tree species threaten dominance of fire-dependent, open pine ( Pinus spp.) forests throughout the southeastern U.S., largely due to encroaching species’ traits that reduce flammability. To better understand fuels within fire-dependent pine ecosystems undergoing encroachment, we measured loads of varying fuel types (herbaceous, shrub, leaf litter, duff, and woody debris) in 96 plots with little to no recent management across a pine dominance gradient at five sites in east-central Alabama, USA in 2022. Across pine-, mixedwoods-, and hardwood-dominated plots, we found similar mean basal area (36.5–40.2 m 2 ha −1 ), canopy cover (>90%), and total fuel loads (20–22 Mg ha −1 ). Encroaching species had lower basal area and density in pine-dominated plots, but their mean leaf litter fuel loads were similar among plot types (0.9–1.5 Mg ha −1 ). However, encroaching species’ relative contribution to leaf litter fuelbeds declined with increasing pine basal area due to a significant linear increase in pine leaf litter fuel loads. Overall, leaf litter was the only fuel to co-vary with pine dominance, suggesting fuel homogenization with encroachment. Future studies experimentally manipulating leaf litter fuels could determine thresholds where encroaching species’ fire-suppressing leaf litter negates pine’s fire-promoting effects.