Are Post-Fire Soil Changes Responsible for Persistent, Non-Forested Scars in Colorado Forests?.

Pile burning is the most common method of disposing logging residue in Rocky Mountain forests yet the practice is known to alter and potentially degrade soil properties. Though extreme heating has short-term effects on soils beneath burn piles, it is uncertain if the practice has long-term, ecologically-relevant consequences. In Colorado lodgepole pine ecosystems we have established a 50-year time series of pile burn scars located within pine stands regenerating after clear cut harvesting. The burn scars support herbaceous vegetation but have resisted tree colonization. We hypothesize that lasting changes in 1) soil resources (i.e., nutrients, water relations or plant symbionts), 2) tree seed availability, 3) herbaceous plant competition, or 4) herbivory may explain the scarcity of pine recruitment within pile burn scars. To evaluate these factors we combine soil nutrient and ectomycorrhizal analysis, greenhouse bioassays, and in situ plant establishment trials to evaluate the underlying causes. Pile burning increased mineral soil pH (upper 10 cm depth) and a 0.8 pH unit difference persisted throughout the 50 year chronosequence. Mehlich-extractable plant nutrients (P, K, Ca and Mg) were 2 to 3 times higher in the burn scar openings on average with the largest differences the first decades after burning. Similarly, extractable soil nitrate and total soil N and C were all twice as high in the burn scars. In spite of generally higher nutrient pools within the burn scars, two indices of soil N availability to plants (anaerobic and aerobic mineralization assays) indicate that inorganic N supply is lower than in the surrounding forests. Lower seedling germination and more than double the incidence of damping off suggests that soil pathogen pressure and, or scarce root symbionts may affect tree recovery in burn scar soils. Our preliminary laboratory analyses provide evidence of both positive and negative soil changes within burn scars; further greenhouse and field trials will help disentangle the soil, plant and herbivory factors that regulate tree regeneration and growth in burn scars.