Are soil changes responsible for persistent slash pile burn scars in lodgepole pine forests?

Abstract

Pile burning is the most common method of logging residue disposal in Rocky Mountain forests. Though the high temperatures reached during burning affect numerous soil properties in the short term, the longer-term effects of the practice are less clear. We previously identified a 50-year time series of burn scars created after clear cut harvesting in lodgepole pine stands where we reported sparse tree colonization across the entire chronosequence. Here we analyzed soil nutrients and chemistry and conducted in situ and greenhouse seedling bioassays to determine whether edaphic factors or poor seedling performance explain the pattern. Pile burning had a lasting effect on soil pH, but nutrient availability was 2–3 times higher in burn scars compared to unburned forest soils for many constituents and planted pine seedlings had good survival and growth. However, seedling growth was slightly less in burn scars compared to unburned soils indicating suboptimal soil pH or other belowground factors may contribute to sparse tree colonization of the openings. For example, seedling survival and ectomycorrhizal fungi colonization were both lowest in the most recently created scars where soils were alkaline and improved with time as pH declined, suggesting gradual amelioration of post-fire growing conditions. Survival in burn scars was comparable for unprotected trees and those in protective mesh tubes, indicating that herbivory was not a significant impediment to seedling establishment. However, a preliminary study suggests that seed predation may have contributed to the low tree colonization into the openings. Though large burn pile scars may require soil rehabilitation, and soil changes may have a lasting effect on understory plant composition, we found that they were not a significant barrier to tree establishment in these moderate-size burn scars.