Fire and fungi: Changes in soil fungal abundance and community composition across a fire chronosequence in an Alaskan boreal forest

Background/Question/Methods: Wildfires are a chronic disturbance in boreal ecosystems and the frequency and intensity of wildfires is predicted to increase substantially under global change conditions. Understanding the microbial consequences of boreal wildfires is important given that these ecosystems contain an estimated 30% of global soil carbon. Since soil fungi regulate organic matter decomposition, fire-induced changes in soil fungal communities may have consequences for carbon cycling and storage in boreal forests. We hypothesized that wildfires would alter the abundance and community composition of soil fungi, with corresponding consequences for soil CO₂ emissions. We tested this hypothesis using a fire chronosequence in upland boreal forests of Alaska. This chronosequence includes sites that burned in 2010, 2004, 1999, 1987, and 1956. As a control, we also sampled from a mature black spruce stand that has not burned in over 85 years. We characterized fungal abundance at each site by measuring the length of fungal hyphae in soil. To understand changes in fungal community composition along the chronosequence, we used pyrosequencing of 28S rDNA. We also measured microbial respiration over the course of a 30-day laboratory incubation to examine changes in soil CO₂ emissions at each site.

Results/Conclusions: Fungal abundance differed significantly across the fire chronosequence (P<0.0001). In support of our hypothesis, 6 to 25 years following fire, fungal abundance was significantly lower in burned stands than in the control site (P=0.05). However, immediately following fire (2010 burn site) there was no reduction in fungal biomass in comparison to the control site. Changes in microbial respiration across the chronosequence followed a similar pattern, increasing from 61 to 1125 μg C g^-1 soil in the 2004 burn site and control site, respectively. Forests in the later stages of recovery from fire (1987, 1956) harbored diverse soil fungal communities, while more recently burned sites (2010 and 2004) were dominated by fewer fungal species. Taken together, these data suggest that fire has short- and long-term effects on soil fungi. In the short-term, fire primarily affects the composition of soil fungal communities. On longer time scales, wildfires also lead to significant reductions in soil fungal abundance, which may in turn result in concurrent declines in soil CO₂ emissions. Overall, severe wildfires in boreal forests may lead to a decreased flux of CO₂ from soils to the atmosphere that persists for the first 25 years of forest recovery and constitutes a negative feedback to climate warming.