Decoupling between soil moisture and biomass drives seasonal variations in live fuel moisture across co-occurring plant functional types

Background: Wildfires are important global disturbances influencing ecosystem structure and composition. The moisture content of living and senescent plant components are key determinants of wildfire activity, yet our understanding of how seasonal fluctuations in water availability and biomass control live foliar moisture content (LFMC) across co-occurring plant functional types is limited in diverse forested landscapes. Results: We recorded root-zone volumetric water content (VWC) and sampled leaf mass area (LMA) and LFMC of three co-occurring plant functional types across six field sites. We used a linear mixed effects model to quantify the drivers of LFMC and understand whether LFMC dynamics were coupled or decoupled from site conditions. Both LMA and VWC were significant predictors of LFMC variability (p < 0.001), although the strength and direction of these relationships varied across functional types. LFMC dynamics of understorey plant functional types were strongly coupled to site conditions, where the site random effect explained 44.2% (shrub) and 74.8% (herb) of the variability in LFMC across the season respectively. In contrast, overstorey plants were decoupled from site conditions, which explained only 8.1% of the variability in LFMC. Conclusions: LFMC of understorey plants responded to changes in soil water availability (VWC), while overstorey trees responded to biomass fluctuations (LMA). We present a conceptual model describing the influence of these factors on LFMC, which aligns with our findings and draws on the broader literature. This knowledge and conceptual approach can be used to improve our ability to characterize seasonal LFMC variation across different plant functional types, in turn improving our capacity to predict wildfire risk.