Seasonal and Regional Variation in Ash-Free Net Heat Content of Common Native and Non-Native Surface Fuels in East Texas

Ash-free net heat content (AF-NHC) represents the combustible heat content of plant biomass and is an important parameter in fire behavior and fire effects modeling. Despite its widespread use, little information exists regarding seasonal and regional variation in AF-NHC among common woody fuels of the southeastern US. This study quantified seasonal and regional variation in AF-NHC among five common woody species in eastern Texas: yaupon (Ilex vomitoria), greenbrier (Smilax spp.), eastern red cedar (Juniperus virginiana), Chinese privet (Ligustrum sinense), and escarpment live oak (Quercus fusiformis). Foliage samples were collected during the dormant and growing seasons across the Pineywoods, Post Oak Savannah, and Blackland Prairie ecoregions and were analyzed using oxygen bomb calorimetry. Linear mixed-effects models evaluated species, season, and species × season effects while accounting for regional variation. AF-NHC ranged from 17.35 to 19.92 MJ kg−1 and differed significantly among species and seasons, with distinct species-specific seasonal trajectories (p < 0.05). Regional variation accounted for approximately 41% of total model variance, indicating that environmental conditions influence fuel thermal properties. AF-NHC was greatest in yaupon and red cedar, intermediate in privet and greenbrier, and lowest in live oak. Although AF-NHC likely exerts less influence on fire behavior than fuel consumption and the rate of spread, species-specific differences in combustible heat content may contribute to variation in potential heat release and fuel combustibility. These findings provide baseline AF-NHC values for common eastern Texas woody fuels and improve the understanding of spatial and temporal variation in fuel thermal properties relevant to fire effects and wildfire hazard assessment.