The Combustion of Large Downed Wood: Initial Impacts of Burn
Intensity on Soil Nutrients and Ectomycorrhiza Communities of
Ponderosa Pine Seedlings
Ariel D. Cowan, Department of Forest Ecosystems and Society, College of Forestry, Oregon State University, Corvallis, OR; USDA Forest Service, PNW Research Station, Corvallis, OR; Stephen A. Fitzgerald, Department of Forest Engineering,
Resources, and Management, College of Forestry, Oregon State University, Corvallis, OR
Abstract—Climate shifts in conjunction with fuel accumulation are predicted to increase the frequency of
high-severity fires in the ponderosa pine forests of central Oregon. Exposure of soils to intense heating from
high-severity fire alters soil properties and causes mortality of soil microbes. This outlook has created a need
to comprehend the potential ecological and timber resource impacts. Ectomycorrhizal fungi (EMF) are considered essential for conifer seedling growth and survival by aiding seedlings in water and nutrient uptake.
Previous studies showing EMF improve ponderosa seedling establishment after fire did not compare EMF
communities at different levels of soil heating intensity in a field setting. For this study, burn intensity impacts
on soil nutrients and EMF species composition were compared in a thinned ponderosa stand at Pringle Falls
Experimental Forest, La Pine, Oregon. A total of twelve replicate sites were used, each with three treatments
applied using a prescribed burn: high intensity burn (HB), low intensity burn (LB), and unburned control (UB).
HB treatments were created by the combustion of downed logs stacked together and LB treatments were
applied through the broadcast burning of pre-existing ground fuels. Thermocouple probes were used to record burn temperatures at multiple depths within the soil. Ponderosa pine seedlings planted post-burn were
harvested after four months and analyzed for EMF species richness, constancy, and relative abundance using
morphotyping and molecular techniques. Preliminary results show: a) lower soil nutrient concentrations were
present in HB soils compared to UB soils; b) EMF species composition differed significantly between HB and
UB soils but not between LB and UB soils or LB and HB soils; c) there were no detectable differences in EMF
species richness across treatments; and d) correlations exist among burn treatments, pH, carbon, and organic matter content when compared with species composition. These results suggest fire-related alterations
to the physical and chemical environment influence EMF species composition. Changes to soil nutrients and
EMF species composition detected in high intensity burned soils were otherwise undetectable in low intensity
burned soils. Despite lethal temperatures throughout the depths of soil commonly occupied by EMF, HB soils
were recolonized within four months with nearly the same number of species though species dominance
was altered. This research aims to advance our understanding of EMF primary succession and its role in fire
ecology. The importance of mixed-severity fire in providing soil refugia for EMF species may influence fuel
reduction practices in ponderosa pine forest management.
In: Keane, Robert E.; Jolly, Matt; Parsons, Russell; Riley, Karin. 2015. Proceedings
of the large wildland fires conference; May 19-23, 2014; Missoula, MT. Proc.
RMRS-P-73. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky
Mountain Research Station. 345 p.
330
The content of this paper reflects the views of the authors, who are responsible for the
facts and accuracy of the information presented herein.
USDA Forest Service Proceedings RMRS-P-73. 2015.