In deserts, increased wildfire frequency is facilitating the invasion of exotic annual grasses. Post-fire plant community assembly is controlled by the interplay between the temporal water requirements of plants and the timing and availability of soil water. Previous studies provide a framework to understand how soil water availability is affected by climate change, recurrent wildfire events, and top-down trophic interactions. However, the interactive effects of these environmental factors on post-fire succession is unclear. We report the results of a field experiment in the Great Basin, USA where we measured soil water availability and plant density in response to factorial combinations of single and multiple wildfire events, precipitation manipulation, exclusion of small-mammals. We hypothesized: 1.) Increased fall precipitation would increase the abundance of exotic grasses, which would decrease spring soil water availability that maintains perennial grasses and annual forbs. 2.) The exclusion of small-mammals and recurrent wildfire events would increase the abundance of exotic grasses, magnifying the effect of the precipitation manipulation treatment. Soil water sensors deployed at the experiment site recorded roughly 5 million data points over a two-year period. Although this provided a novel view of soil water dynamics, it created a considerable data management problem.
Results/Conclusions
A fall water addition treatment designed to simulate the projected changes in climate increased the abundance of B. tectorum (exotic annual grass) and decreased the abundance of C. testiculata (annual forb) and E. elymoides (perennial grass). The rodent-exclusion treatment had the same effect on plant community structure, and the recurrent burn event had no effect on plant community structure. High-resolution soil water data revealed that B. tectorum had no effect on early spring water availability, suggesting that soil water usage by B. tectorum likely did not decrease germination, growth, or survivorship of winter annuals or perennial grasses. More likely, increased litter from B. tectorum in 2018 prevented the establishment of C. testiculata and E. elymoides in 2019. Soil water data also revealed that B. tectorum and C. testiculata decreased late-spring/summer soil water availability, which could have effects on plant community structure not measured in this experiment. These results document that exclusion of rodents in combination with projected changes in climate affect the trajectory of succession and may increase the long-term stability of the B. tectorum-dominated vegetation state.