Pre-fire hydroclimate and plant-community composition influence post-fire soil resource availability and resistance and resilience in the sagebrush steppe

Toby M. Maxwell, Biological Sciences, Boise State University, Boise, ID, Marie-Anne de Graaff, Department of Biological Sciences, Boise State University, Boise, ID and Matthew J. Germino, Forest and Rangeland Ecosystem Science Center, US Geological Survey, Boise, ID

Background/Question/Methods
The exotic-annual grass and fire cycle is degrading ecosystem structure and function across millions of acres of semiarid landscapes in the Western US, and our understanding of the factors that impact native plant community recovery following fire remains limited. This study addresses several key knowledge gaps by experimentally separating pre-fire hydroclimate, plant-community composition, and soil depth, and testing for their control over post-fire recovery. With this study we ask (1) how legacy effects of hydroclimate and community composition impact soil resource availability after fire, and (2) how differences in soil water and nutrient availability affect resistance to invasion and resilience of desirable plant communities following wildfire. This work leverages a 25-year manipulative ecohydrological experiment in the sagebrush-steppe of Eastern Idaho, that burned in a 2019 megafire, to identify mechanisms that govern resistance and resilience in the sagebrush steppe. Our results are based on pre- and post-fire plant community surveys, demographic measurements, monitoring of soil nitrogen and moisture content, and stable-isotope tracer studies of nitrogen and deuterium.
Results/Conclusions
Early observations of plant community recovery showed distinct spatial patterning that linked recovering vegetation to interspace dynamics, where the large bare soil patches left by burned shrubs supported a longer growing season for adjacent perennial grasses. These interspaces maintained heightened levels of water and nitrogen for at least one month longer into the growing season, making them hot spots for invasive grass establishment. In accordance with our research questions, we found that (1) the legacy of climate and plant community treatments show strong interactive effects that impact soil resource availability, and (2) that soil resource entrainment by resprouting perennial herbs provides resistance to invasive annual grasses. The magnitude of these effects, however, is overridden by the importance of interspace size, where resprouting perennial grasses could not access the center of large interspaces and thus were limited in conferring resistance to these plots. Plots populated by an assembly of native grasses had the largest interspaces, while crested wheatgrass (an introduced perennial grass) dominated plots were more homogenous with smaller interspaces. Across all plots, post-fire resistance to cheatgrass invasion was closely linked to the quality of bare soil patches (i.e. size of gaps) rather than the amount of bare soil (i.e. % cover of bare soil). Our data confirm the importance of pre-fire plant community composition in determining post-fire recovery, while revealing how this effect is altered by hydroclimatic setting and soil depth.