Abstract: Living plants represents a substantial proportion of the fuel complex for wildland fires. Foliar moisture content (FMC) is a critical component of vegetation flammability, yet our capacity to model seasonal and spatial fluctuations in FMC within and across species and integrate these dynamics into planning tools for ecosystem management is limited. Currently, FMC models have focused on changes to the water content of foliage alone. However, living vegetation regulates biomass and water content actively through processes including photosynthesis and transpiration, thus flammability is a function of both water- and carbon cycles. Focusing only on water content omits decades of ecophysiology, phenology, and trait-based ecology knowledge that describes vegetation dynamics. Importantly, it also limits our capacity to model inter- and intra-species variability from the site to landscape scale – which will become increasingly important as climate warming alters weather and shifts vegetation distributions. In this interdisciplinary presentation, we seek to integrate ecology and fire science to characterize water- and carbon-cycle drivers of FMC across different species. We will present a conceptual model, field-collected data, and a novel mechanistic model to predict seasonal dynamics of FMC.
Data were collected in conifer-dominated forests of the Northern Rocky Mountains between 2019-2022. Foliage samples from eleven conifers and two shrubs were analysed to determine FMC, relative water content, density, and leaf mass area. Our results suggest that FMC fluctuations are more strongly related to the carbon-cycle for overstorey trees, and the water-cycle for understorey shrubs, which is reflected in our conceptual model. The mechanistic FMC model predicted seasonal fluctuations well for new and old conifer foliage (r2=0.95) and annual shrub foliage (r2=0.87). The relative importance of water- and carbon-cycle fluctuations on FMC varied by species and foliage age. Our results demonstrate the efficacy of the mechanistic model for predicting foliar moisture, and the importance of capturing the different drivers of FMC fluctuations across and within a diverse range of ecosystems.
To date, our capacity to model inter- and intra-species variations in flammability at temporal and spatial scales relevant to ecosystem management has been limited. Understanding the relative contribution of water and carbon-cycle dynamics is a critical step in understanding vegetation flammability across diverse ecosystems, which will be increasing important under climate change. The conceptual and mechanistic models presented provide a framework for combining carbon- and water-cycle dynamics to characterize FMC for the purpose of understanding landscape fire risk to communities and ecosystems.
