Comparing wildfire recovery at a bog and a fen along a burn severity gradient

Abstract

Wildfires are increasing in intensity and severity, emitting carbon (C) stored in soil and biomass to the atmosphere. This is of increased importance in peatlands which have deep deposits of combustible soil. Carbon in peatlands has been accumulating for millennia, due to organic matter input exceeding decomposition and combustion. The presence of saturated soils inhibits the rapid oxidation of dead organic matter, thereby limiting C losses through decomposition. Carbon accumulation in peatlands is supported by the adaptations of characteristic vegetation assemblages, which can grow quickly and in high abundance, increasing the rate of C accumulation, or reducing rates of decay. As wildfires are becoming increasingly severe, collecting data over a range of burn severities, in an array of peatland types to better characterize rates of recovery is paramount. Thus, I measured C fluxes, plant functional traits, and plant community composition at a bog and a fen along a burn severity gradient, with the aim of gaining a better understanding of the influence of burn severity on recovery in different peatland types. I found that, six to seven years following wildfire, biomass accumulation was greater at the fen than the bog, especially the moderately burned fen with nearly 12-fold the biomass of the moderately burned bog; however, the plant community composition was dominated by opportunistic plants such as Betula glandulosa that were not characteristic of the unburned treatment. Plant functional traits suggested that response to disturbance differs among plant types along the burn severity gradient at each peatland type, where LDMC is regularly decreased along the burn severity gradient, and either SLA or height are increased. Understory GEP and ER are significantly greater at the fen than the bog, although NEE was not statistically different, as sequestration and efflux balance at each site to approximately 0 g C m-2 day-1. These results should be considered alongside tree and vegetation surveys, which suggest that while ground-level fluxes may be similar, overstory contributions to each site are crucial to consider as they contribute to the C storage capacity of the unburned sites, but are missed by ground-level fluxes. There is a great amount of C held in trees at the unburned sites, less at the moderately burned sites due to competition, and a high number of tree seedlings at the severely burned sites. Methane fluxes, however, appear to recover more slowly following deeper peat combustion, with moderate burns trending toward pre-burned conditions, while the severe burns have limited efflux of CH4, which may suggest that there has been a reduction in substrate quality or that the soil microbial community that has not yet recovered. The results of this study emphasize the need for more nuanced consideration of burn severity in peatland management and research. Severe burns are becoming more common with climate change, and implementation of burn severity into global C models is necessary to ensure accurate estimates of C losses from wildfire. This study also highlights the importance of distinguishing between bogs and fens in ecological modeling, as applying the same rates of C efflux or accumulation could lead to significant inaccuracies. Understanding these differences is crucial for prediction of C dynamics in peatland ecosystems, particularly in the context of increasing wildfire frequency and intensity.