Increased burn severity and simulated climate warming drive shifts in ecosystem function and understory plant traits one year post-fire in northern Arizona

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Ethan M. Taber and Rachel M. Mitchell, School of Earth and Sustainability, Northern Arizona University, Flagstaff, AZ

Background/Question/Methods
A changing climate and altered fire regimes in the southwestern U.S. have led to questions regarding future forest understory community composition and ecosystem function. Communities that are fire adapted and have been historically shaped by fire are at risk of conversion by uncharacteristically severe fires. Trait-based plant ecology provides a means of assessing the impacts of varying levels of disturbance on community composition and ecosystem function and can be used to predict community trajectories. The aim of this study was to quantify the effects of increasing burn severity and simulated climate warming on two ecosystem functions and three plant functional traits (PFTs), approximately one year post-fire in a northern Arizona Pinus ponderosa forest understory. We asked the following research questions: 1) How do burn severity and warming influence litter production and decomposition rates? 2) How do burn severity and warming affect community composition and plant functional trait (specific leaf area (SLA), leaf dry matter content (LDMC), and plant height) diversity? 3) How do burn severity and warming affect PFT expression of the dominant bunchgrass species at the site? 120 plots were established along a burn severity gradient including unburned, low, and high burn severity areas. Open-top warming chambers were employed to produce just over 1°C of warming. Trait and function data were collected in Fall 2020, at the end of the first post-fire growing season.
Results/Conclusions
We found that the interaction of high burn severity and simulated climate warming significantly increased decomposition rates (p=0.036, F=3.4752), decreased community weighted means (CWMs) of LDMC (H (5)= 15.4, P=0.0086), and increased specific leaf area of a dominant C4 bunchgrass (Muhlenbergia montana; p=0.002, F=6.4212). Additional significant effects were observed according to severity alone. Our results indicate potential for divergent community composition, structure, and ecosystem functioning in response to fire and simulated climate warming. We conclude that herbaceous and shrub understory communities after future high severity fires may be on novel trajectories. Our conclusions support ongoing efforts to reduce the likelihood of high severity fires in this region, and provide information on trait expressions and species that may prove useful in future post-fire restoration efforts. Given the projected increase of high severity fires in the southwestern U.S., understanding the combined effects of climate warming and high burn severity will be critical for successful land management.