PS 42-38 - Past fertilization inhibits ground cover recovery in a fire-suppressed woodland

Background/Question/Methods

Fire exclusion in fire-dependent ecosystems leads to an alternate ecological state characterized by higher woody stem density, closed canopies, and lower ground cover diversity and biomass. Prescribed fire can restore ground cover plant communities, but outcomes depend on scale, site characteristics, and site history. Our study evaluated ground cover recovery in longleaf pine during the first five years of fire reintroduction. Frequent prescribed fire (1 – 2-year return intervals) was reintroduced after 14 years of fire suppression in half of our study plots (“Restoration”). Fire continued to be excluded in the rest of the plots (“Fire Exclusion”). Both fire treatments occurred along a soil moisture gradient from wet-mesic to xeric. A subset of plots in each treatment had a history of experimental fertilizer addition. We compared community composition and structure in Restoration and Fire Exclusion plots to either pre-fire-exclusion community metrics or to frequently burned reference plots to evaluate restoration progress. We measured ground cover community metrics including aboveground biomass, community composition, and species richness at multiple spatial scales. We analyzed changes in community structure and composition using mixed models, estimated marginal means, and nonmetric multidimensional scaling (NMDS).

Results/Conclusions

Reintroduction of frequent prescribed fire changed ground cover communities significantly over the course of our study. After four fires, Restoration plots were more similar to reference communities structurally and compositionally. However, fire reintroduction effects varied across sites, and restoration outcomes were limited in previously fertilized plots. The rate of increase in ground cover biomass was significantly higher (α = 0.01) in Restoration compared to Fire Exclusion plots at both the xeric and wet-mesic sites (160 and 190 kg ha-1 yr-1, respectively), but this effect was not observed in the wet-mesic fertilized treatment. Species richness also increased significantly over time in Restoration plots at both sites at intermediate scales, though richness did not increase in previously fertilized plots. Community composition in unfertilized Restoration plots shifted towards Reference communities in both sites based on NMDS, but previously fertilized Restoration plots showed little change in community composition relative to their pre-restoration state. Our study demonstrates that the reintroduction of frequent, prescribed fire can effect a rapid transition of ground cover communities towards pre-fire-exclusion conditions, but it also highlights the variability of recovery trajectories and how additional variables, like scale, site characteristics, and historic fertilizer use, can influence those trajectories.