COS 9-4 - The effects of nutrient addition on post-fire community dynamic in arid grassland

Abstract: Understanding how resource availability affects recovery from pulse disturbances becomes increasingly important under global environmental change. For example, wildfires may increase in both frequency and intensity in the future in regions with increasing levels of atmospheric nutrient deposition. However, the effects of chronic resource alteration on the composition and dynamics of communities are difficult to predict due to different nutrient utilization and acquisition strategies of plant functional groups following disturbances, such as wildfire.

We surveyed species composition and aboveground biomass in a long-term nutrient addition experiment for a decade following a lightning-initiated wildfire at the Sevilleta National Wildlife Refuge (SNWR) in central New Mexico, USA to address the following questions. (1) How does nutrient addition interact with fire to influence species reordering? (2) Does nutrient addition slow post-fire compositional recovery by altering the interaction between annual plants and dominant perennial species? (3) To what degree do interspecific interactions drive species reordering and community responses to disturbance?

Following the trajectories of post-fire communities for a decade, the recovery processes observed in our study were primarily driven by the abundance of perennial grasses. The decrease in the relative abundance of perennial species caused a destabilizing effect on postfire community composition. Although nutrient addition had no effect on the recovery of black grama after the wildfire, N addition negatively impacted postfire community stability. Interspecific interactions between functional groups were assessed by convergent cross mapping, which was performed to determine the causal relationships using time series data. We found that the causal effect of black grama weakened with N addition, likely because nutrients increased the competitiveness of annuals and lowering the competitive effects of dominant perennial grasses.

The co-occurrence of fire and nutrient addition in this long-term experiment provides new insights into the effects of eutrophication on desert grasslands. In summary, our findings extend those of previous studies reporting that fire has a much greater impact on community structure and function when interacting with a single chronic press, such as nutrient addition. Our results further demonstrated that the negative effect of N addition on compositional variation postfire in this semi-arid grassland was driven by the strong precipitation response of annuals rather than altered interactions between functional groups.