Towards predicting flammability of Sierra Nevada mixed conifer forests: drought stress and fuel moisture are strongly linked in angiosperms but decoupled in gymnosperms

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Abstract Background Drought and wildfire are linked by their relationships with plant hydration, yet drought- and fire-focused research use discipline-specific hydration metrics to capture plant stress and fuel flammability. We investigated potential drought–wildfire dynamics in angiosperms and gymnosperms common to Sierra Nevada mixed conifer forests by relating plant drought status (water potential) to water content (live fuel moisture, LFM) and to flammability. We conducted laboratory flammability tests coupled with a benchtop drydown and measurements of physiological drought response (the turgor loss point, TLP). We measured water potential, LFM, and phenology during a seasonal drydown in the field. Results Gymnosperms showed inconsistent relationships between water content and water potential across drydown type (field vs. lab), complicating how we measure their potential drought-wildfire relationships. In contrast, angiosperms had consistent hydration relationships, showing promise for relating drought stress to flammability. Physiological adjustments occurring near the TLP and phenological patterns impacting dry matter development drove differences across functional groups. Decreased plant hydration increased flammability, and LFM predicted flammability better than water potential—likely because LFM captured both hydration and shifts in leaf and branch dry matter associated with phenology. Conclusions Our results suggest that phenological development and drought response mediate the relationship between drought stress, tissue water content, and tissue flammability in seed-producing plants (i.e., angio- vs gymnosperms). This work links key ecological processes, drought and wildfire, and advances our ability to predict wildfire risk using species composition and drought stress.

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