Patterns of conifer regeneration following high severity wildfire in ponderosa pine – dominated forests of the Colorado Front Range

Abstract

Many recent wildfires in ponderosa pine (Pinus ponderosa Lawson & C. Lawson) – dominated forests of the western United States have burned more severely than historical ones, generating concern about forest resilience. This concern stems from uncertainty about the ability of ponderosa pine and other co-occurring conifers to regenerate in areas where no surviving trees remain. We collected post-fire conifer regeneration and other data within and surrounding five 11–18year-old Colorado Front Range wildfires to examine whether high severity burn areas (i.e., areas without surviving trees) are regenerating, and how regeneration density is related to abiotic and biotic factors such as distance from surviving forest, elevation, and aspect. We found that some conifer regeneration has occurred in high severity burn areas (mean and median of 118 and 0stemsha−1, respectively), but at densities that were considerably lower than those in unburned and in low to moderate severity burn areas. Generalized estimating equation analyses revealed that distance from surviving forest was the most important predictor of conifer regeneration in high severity burn areas, with regeneration declining as distance from surviving forest increased; estimates of conifer regeneration were 211stemsha−1 immediately adjacent to surviving forest but only 10stemsha−1 200m from surviving forest. These analyses also revealed that conifer regeneration densities declined as elevation decreased. Regression tree analyses likewise showed that distance from surviving forest and elevation were important predictors of conifer regeneration in high severity burn areas; within 50m of surviving forest mean (median) regeneration was 150 (0)stemsha−1 at elevations ⩽2490m and 1120 (1000)stemsha−1 at elevations >2490m, but at distances ⩾50m from surviving forest mean (median) regeneration was only 49 (0)stemsha−1, regardless of elevation. Applying regression tree results spatially to the 2002 Hayman Fire, Colorado’s largest and most severe known wildfire, we found that 70% of the area without surviving forest exceeded this 50m threshold. These patterns of conifer regeneration suggest that Colorado Front Range ponderosa pine – dominated forests may not be resilient to high severity wildfire, particularly where surviving forest is not in close proximity. We recommend that land managers consider planting conifers within the interiors of large high severity burn patches, as well as implementing treatments to reduce the risk of uncharacteristic high severity wildfire in unburned forests, where maintaining a forested condition is desired.