Research Ecologist USGS Boise, Idaho, United States
Abstract: Plant community recovery after wildfire is subject to legacy effects of pre-fire climatic conditions which shape the plant community impacted by wildfire and patterns of post-fire soil resource cycling. Understanding the relationship between pre-fire hydroclimate conditions and post-fire plant community recovery is important in light of invasive species encroachment that can disrupt the recovery of structure-function relationships after disturbance. Yet there is little experimental evidence that quantitatively connects the effects of pre-fire conditions on post-fire soil and plant processes, especially due to natural wildfires. 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, which received three hydroclimate treatments: ambient, winter (+200mm), or summer (+200mm) irrigation across two community types: perennial grassland (crested wheatgrass), and a diverse sagebrush-steppe assemblage, that burned in a 2019 fire. Our results are based on pre- and post-fire plant community surveys, demographic measurements, and monitoring of soil nitrogen and moisture content.
Preliminary data show that high canopy cover of sagebrush ( >~35%) before the wildfire, driven by the winter irrigation treatment, corresponded to larger bare soil patches (interspaces) after the fire. This, in turn corresponded to higher soil inorganic nitrogen concentrations and more exotic annual invasive plants (EAs) at the plot scale. At the microsite scale, soil inorganic nitrogen and EA plant density (per m2) were elevated in interspace microsites compared to plots as a whole. Pre-fire foliar cover of nitrogen fixing plants was also tightly correlated to EA plant invasion. Our results contrasted with the expectation that higher resistance to EAs would be associated with wetter hydroclimates. When plant community type and hydroclimatic conditions combined to promote high foliar cover of shrubs, plots were less resistant to EA invasion because of elevated soil nitrogen in large interspaces. In contrast, high pre-fire perennial grass cover improved resistance to EAs by promoting smaller interspaces with lower soil nitrogen content. Our findings challenge the paradigm that wetter conditions always enhance resistance and resilience in the sagebrush steppe by suggesting that the larger shrubs that tend to populate wetter regions lead to post-fire conditions that are less resistant to invasion by EAs.
