Background/Question/Methods Increasing disturbance severity and novel growing conditions due to climate warming have the potential to alter terrestrial ecosystems worldwide. In coniferous forests of the Greater Yellowstone Ecosystem, U.S.A., severe fire and high soil moisture deficits may limit plant species establishment and delay conifer tree canopy closure. Furthermore, minimal canopy closure may prolong harsh growing conditions in post-fire sites, but whether these conditions persist over multiple decades following fire has not been previously examined. We have investigated successional development of subalpine forests on permanent plots (n =275) over 30 years following the 1988 Yellowstone fires. Plots were classified by burn status (burned or unburned) and soil moisture regime (mesic or xeric) in each of two study areas. At ~30 years after fire, we found major differences in post-fire conifer regeneration density, a proxy for tree canopy closure, between study areas. We asked whether low canopy closure results in divergence in microclimate and soil nutrient concentrations between burned and unburned communities? During one growing season at each study area, we measured photosynthetically active radiation (PAR), soil temperatures, and soil moisture and analyzed soil cores for C and N concentrations. Results/Conclusions Canopy closure was strongly associated with similarities in PAR and soil temperatures between burned and unburned communities. At the study area with higher canopy closure, growing season PAR and soil temperatures were similar (p > 0.05) between burned and unburned plots. In contrast, at the study area with low canopy closure, average PAR was four-fold higher, and average soil temperatures were 14°C warmer, in burned than in unburned plots. Soil nutrient concentrations, however, were comparable between burned and unburned plots at both study areas, suggesting no relationship with canopy closure. Organic C (loss on ignition) ranged from 1.2 – 31%, NH4 ranged from 0.0008 – 0.014 mg N/g soil, and NO3 ranged from 0.0002 – 0.0005 mg N/g soil. Furthermore, soil C and N were correlated with plot soil moisture. Our results suggest that canopy closure ameliorates site microclimate following fire, resulting in microclimatic convergence between burned and unburned communities. Conversely, minimal canopy closure appears to prolong microclimates typical of early successional communities. Although canopy closure does not appear to directly influence soil nutrient concentrations, it may indirectly influence biogeochemical cycling in seral forests through its effects on plant litter inputs and microbial species diversity.
