Abstract: Anthropogenic climate change is causing increases in the severity of wildland fire globally. At the same time, post-fire succession is occurring under new temperature and precipitation regimes. Despite this, the combined effects of uncharacteristically high burn severity and increased ambient temperatures and altered precipitation regimes on post-fire community composition remain poorly understood. We ask how post-fire forest understory community composition is influenced by 1) burn severity 2) experimental warming 3) growing season weather and, 4) time since fire. To address these questions, we established 120 1m2 quadrats in unburned, low- and high-severity locations one year after a mixed-severity fire. Half of plots were subject to experimental warming via open-top warming chambers. Plant composition data have been collected annually since 2020. Relationships between community composition, burn severity, experimental warming, time since fire, mean growing-season temperature (MGT) and precipitation (MGP) were analyzed using PERMANOVA and PERMDISP in PRIMER version 7.0.21. Composition differed significantly between burn severities within (p < 0.05) and across years (p < 0.003). High severity plots had greater beta diversity than control (p = 0.002), and greater annual exotic cover. Composition changed more rapidly across years in high severity plots compared to low-severity and control. MGT and MGP significantly interacted with severity to influence composition (p=0.001 for both interactions). Variance partitioning revealed that MGT and severity had the largest impact on community composition, while our experimental warming did not induce significant changes. Our results suggest that anthropogenically driven high severity fire, growing season temperature and precipitation strongly influence understory community composition following fire, while experimental warming had little effect. High severity fire combined with extreme growing season temperature and precipitation drove large, rapid changes in plant composition compared to unburned controls or climate alone, favoring exotic annuals in a historically perennial-dominated plant community.
