PS 28-144 - Relationships between live fuel moisture, water potential, and tissue-level flammability: the impact of species’ turgor loss point

Abstract: Pyro-ecophysiology bridges plant physiology and fire ecology, seeking to understand mechanisms that drive landscape-level wildfire dynamics. An ongoing challenge of pyro-ecophysiology revolves around the quantification of plant hydration, as fire ecologists and plant physiologists use different metrics. Typically, fire ecologists use live fuel moisture (LFM), a mass-based metric of water and dry matter content, while plant physiologists use xylem water potential, a pressure-based metric indicative of physiological drought stress. The relationships between these metrics have started to be investigated; however, integrative studies relating these two metrics to flammability and analyzing functional differences between LFM and water potential in predicting leaf-level flammability are missing from the literature. To fill this gap, we explored relationships between the two plant hydration metrics and tissue-level flammability. Over the course of a benchtop drydown, we burned tissue samples of common chapparal species, Adenostoma fasciculatum (chamise) andCeanothus megacarpus (big-pod Ceanothus) using an epiradiator flammability chamber. During the analysis, we explored linear and thresholding relationships between the different metrics of plant hydration and flammability. Both ‘raw’ flammability metrics (e.g., time to ignition, flame height, flame duration, glow duration, maximum temperature, and three other glow-related metrics, as measured using the flammability chamber) and flammability proxies (determined using principal component analyses conducted upon the ‘raw’ flammability metrics) were used to quantify flammability. We predicted that the turgor loss point (TLP), a physiological trait that reflects a plant’s capacity to maintain turgor pressure under drought stress, might coincide with identified thresholds in flammability. Our results show LFM thresholds in flammability that, in some cases, align with physiological drought response: for both A. fasciculatum and C. megacarpus, the mean value of identified tissue-flammability thresholds aligned with each species mean LFM value at the TLP (LFM_TLP). In contrast to LFM, thresholds were largely absent when examining water potential; however, water potential predicted flammability better than LFM when assessing the relationship linearly. Our findings are two-fold: (1) the turgor loss point could be a key trait for predicting thresholds in flammability and (2) water potential and live fuel moisture have distinct relationships with tissue-flammability. Amplifying landscape-level monitoring efforts, especially for water potential, would allow future pyro-ecophysiology research to distinguish whether our findings could track to the landscape and improve predictions of drought and wildfire dynamics.