Decrease in ecosystem multifunctionality in response to altered disturbance in a fire-adapted system

Rachel M. Mitchell, School of Earth and Sustainability, Northern Arizona University, Flagstaff, AZ, Justin P. Wright, Biology, Duke University, Durham, NC and Cari D. Ficken, University Program in Ecology, Duke University

Background/Question/Methods
Understanding impacts of altered disturbance regimes on ecosystem function is a key goal for ecology. Ecosystem multifunctionality integrates across multiple ecosystem functions to provide a holistic understanding of ecosystem response to disturbance. At present, it is unclear whether increased fire frequency in fire-adapted systems strongly impacts ecosystem multifunctionality. Using a longleaf pine/wiregrass community in North Carolina, USA, we investigated the impact of altered fire regime on ecosystem multifunctionality. Understory plots were burned zero, one, two, or four times between 2011 and 2014. Seven ecosystem functions were assessed in 2014 to quantify the impact of the number of burns experienced on a plot: soil C, aboveground biomass production, belowground biomass production, litter biomass production, canopy openness, and community weighted means of two functional traits related to litter quality, decomposition rates, and relative growth rates, specific leaf area and leaf dry-matter content. All ecosystem functions were standardized site maxima. Using ecosystem functions measured on 83 understory plots, we ask three questions: 1) Does multivariate functional space differ between burn treatments? 2) Does average ecosystem multifunctionality differ between burn treatments, and 3) How does burn treatment impact the number of ecosystem functions exceeding threshold values set between 5-95% of site maxima?
Results/Conclusions
Multivariate functional space differed significantly between burn regimes (p=0.001). Average ecosystem multifunctionality decreased with increasing number of fires (p<0.001, R2=0.12). Importantly, we found that, regardless of threshold value, increasing numbers of fires led to fewer functions meeting or exceeding threshold values. At threshold levels of 90, 75, 50%, and 25% of maximum, plots that had burned two or fewer times had more functions above threshold values than plots that had burned four times. At the highest threshold value, 90%, plots that had burned four times had no functions above threshold, while all other treatments had at least some plots that with functions that exceeded threshold values. These results indicate that, even in a fire-adapted ecosystem, perturbations to disturbance regimes can significantly reduce ecosystem multifunctionality.