Recovering lost ground: Effects of soil burn intensity on nutrients and ectomycorrhiza communities of ponderosa pine seedlings

Fuel accumulation and climate shifts are predicted to increase the frequency of high-severity fires in pon-derosa pine (Pinus ponderosa) forests of central Oregon. The combustion of fuels containing large downedwood can result in intense soil heating, alteration of soil properties, and mortality of microbes. Previousstudies show ectomycorrhizal fungi (EMF) improve ponderosa seedling establishment after fire but didnot compare EMF communities at different levels of soil burn intensity in a field setting. For this study,soil burn intensity effects on nutrients and EMF communities were compared at Pringle FallsExperimental Forest, La Pine, Oregon. Twelve replicate sites were used, each with three treatments: highintensity soil burn from large downed wood combustion (HB), low intensity soil burn (LB), and unburnedcontrol (UB). Temperatures lethal to fungi were detected at 0-cm, 5-cm, and 10-cm depths in HB soils and0-cm depth in LB soils. Ponderosa pine seedlings planted post-burn were harvested after four months forEMF root tip analysis. We found: (a) greater differences in soil properties and nutrients in HB soils com-pared to LB and UB soils; (b) no differences in EMF richness and diversity among treatments; (c) weakdifferences in community composition based on relative abundance between UB and either burn treat-ments; and (d) EMF composition in HB and LB treatments correlated with soil carbon and organic mattercontents. These results support the hypothesis that the combustion of large downed wood can alter thesoil environment directly beneath it. However, an EMF community similar to LB soils recolonized HB soilswithin one growing season. Community results from both burn treatments suggest an increase in patchyspatial distribution of EMF. We hypothesize that quick initiation of EMF recolonization is possibledepending on the size of high intensity burn patches, proximity of low and unburned soil, and survivalof nearby hosts. The importance of incorporating mixed fire effects in fuel management practices willhelp to provide EMF refugia for ponderosa pine forest regeneration