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Are soil changes responsible for persistent slash pile burn scars?

Date: July 28, 2021

Soil changes may contribute somewhat to sparse tree colonization in slash pile burn scars, but they do not appear to be a significant barrier.

Slash pile burning is the most common method of logging residue disposal in Rocky Mountain forests. Federal regulations stipulate that management activities on Forest Service land must not permanently degrade the productive capacity of soils. Land managers therefore need information about the long-term consequences of pile burning to evaluate potential rehabilitation needs and alternative practices.  

An aerial view of slash pile burn scars.
Persistent slash pile burn scars created in lodgepole pine clear cuts on the Medicine Bow – Routt National Forest. Clear cutting and pile burning occurred in the 1980s, about 30 years earlier.
We previously identified a 50-year time series of slash pile burn scars created after clear cut harvesting in lodgepole pine stands, and found that tree colonization in scars was sparse across the entire time period. We visually observed evidence of high soil burn severity in the scars, including layers of soil charcoal and hardened red soil across the time series, leading us to hypothesize that soil changes may be playing a role in tree colonization patterns. Here we explored this hypothesis by examining soil nutrients and chemistry and conducting in situ and greenhouse seedling bioassays.

Key Findings

  • Pile burning had a lasting effect on soil pH. However, 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 scar soils 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.
  • A preliminary study suggests that seed predation may strongly contribute to the low tree colonization in the burn scars, and warrants further investigation.

Featured Publications

Rhoades, Charles C. ; Fegel, Timothy ; Zaman, Tahir ; Fornwalt, Paula J. ; Miller, Susan P. , 2021

Principal Investigators - External: 
Susan Miller - Rocky Mountain Research Station
Tahir Zaman - University of Abbottabad