Recovery of small pile burn scars in conifer forests of the Colorado Front Range

• We studied small pile burn scars (<4 m diameter) at 20 conifer forest sites.
• Inorganic soil N and native herbaceous plant cover recovered within 3 years.
• Rehabilitation is warranted for sites with water quality or exotic plant concerns.
• Woodchip mulch dampens post-fire increases in plant-available soil N.
• Establishment of more diverse plant communities is enhanced by seeding.

The ecological consequences of slash pile burning are a concern for land managers charged with maintaining forest soil productivity and native plant diversity. Fuel reduction and forest health management projects have created nearly 150,000 slash piles scheduled for burning on US Forest Service land in northern Colorado. The vast majority of these are small piles (<5 m diameter). Similar to larger piles, we found that burning small piles had significant immediate effects on soil nutrients and physical and chemical properties and native plant cover. To evaluate the need to rehabilitate small piles and compare the effectiveness of treatment options, we examined soil and plant responses to treatments designed to alter soil nutrients, moisture and temperature and to increase seed availability. We compared four surface treatments (soil scarification, woodchip mulch, tree branch mulch, untreated scars), with and without addition of a native seed mixture. Natural recovery and treatment effects were examined for 2.5 years after pile burning at 19 conifer forest sites along the Colorado Front Range. Woodchip mulch had dramatic effects on soil moisture, temperature, decomposition and inorganic soil N compared to the other treatments, untreated scars or unburned areas; woodchip mulch also suppressed plant establishment. Seeding increased total native species richness as expected, but had marginal effects on forb cover and no effect on graminoid cover. Soil N availability and plant cover did not differ from unburned areas in the absence of surface or seeding treatments within two years of pile burning. Neither reduced seed availability nor altered soil properties following burning hindered revegetation of these small burn scars by native herbaceous plants. Our findings indicate that rehabilitation may not be required for small burn pile scars except in sensitive areas, such as those with water quality and invasive plant concerns.