Forest fire legacy effects are more pervasive at low elevations

Background/Question/Methods

In forests where fire is an infrequent disturbance, such as the northeastern US, an increase in fire frequency can cause significant changes in the ecosystem.  The 2007 Intergovernmental Panel on Climate Change report forecasts higher forest fire frequency for this region.  However, as fires are infrequent in this region, there are few opportunities to study the effects of increased fire frequency.  A severe fire in the Adirondacks Mountains of upstate New York 13 years ago presents the unique opportunity to investigate three questions; 1) what are the legacy effects of fire on soil carbon and nitrogen pools and fluxes, 2) how do fire legacy effects interact with elevation, and 3) what are the fire legacy effects on microbial and plant communities?  Soil samples were collected in the summer of 2011 and analyzed for differences in total nitrogen, total carbon, nitrate, and ammonium pools as well as microbial community composition.  A companion greenhouse study is being conducted to investigate whether a change in soil properties at high elevation favors the growth of low elevation Populus tremuloides or high elevation Betula cordifolia. 

Results/Conclusions

Unburned site soil nitrate pools are smaller and ammonium levels are higher at high elevations.  Fire recovery site ammonium pools are 12% or less of unburned sites across all elevations.  Nitrate in fire recovery sites decreased by 64% at low elevations but was unaffected at high elevation.  Total soil carbon and nitrogen pools at fire recovery sites are 30% and 50% respectively of unburned sites.  Fire recovery sites have carbon to nitrogen ratios less than 20, while unburned sites have a ratio higher than 20.  Forthcoming results from molecular analyses will help clarify the legacy effects of fire on microbial community composition.  Fire legacy effects are most pronounced at low elevations.  More frequent fires could compound these effects with serious implications for ecosystem processes and properties.  Soil nitrogen and carbon pools would diminish with more frequent fires which could alter ecosystem processes, such as reducing productivity.  Species composition could directly be affected by more frequent fires as well as fire-regime altered ecosystem properties.