The sensitivity of fuel moisture to forest structure effects on microclimate

An understanding of variation in dead fuel moisture content (FMC) is essential for accurate predictions of wildfire risk, particularly in productive wet forests where FMC is a primary control on fire activity. In these systems, forest structure and composition influence microclimate, which in turn effects FMC. However, changing disturbance regimes are altering forest structure and our understanding of the sensitivity of FMC to these variations is incomplete. To explore this, we quantified the relative importance of changes in microclimate resulting from altered forest structure to FMC variability. This was done by modelling FMC using microclimate scenarios extracted from observations at six field sites with contrasting structural profiles. The scenarios related to maximum, mean, and minimum fire-related microclimate conditions. To understand sensitivity in a fire management context, we summarised results using FMC thresholds of fuel availability (FA) across three seasons – corresponding to wildfire and prescribed burning conditions. Distinct differences in FA demonstrate the potential for altered structure to influence fire activity in wet forests. Structure effects on vapour pressure deficit (VPD) and longwave radiation (LWR) exerted the strongest control on FA, resulting in increases of 125% and 87% across all seasons respectively, while shortwave radiation (SWR) had a limited influence. However, FMC sensitivity to microclimate inputs changed with season. Critically, in summer, forest structure effects generating elevated VPD conditions resulted in 8 additional days (of 30) when FMC was less than 10%, a threshold for intense fire behaviour. Our research supports the hypothesis that FMC is sensitive to forest structure change, and that this sensitivity can be mechanistically linked to elevated VPD and LWR in the microclimate resulting from these changes. Given the potential for disturbances to alter forest structure in the future, the potential impact of this on microclimate and FMC should be considered in landscape predictions of wildfire risk.