Drivers of conifer regeneration in severely burned ponderosa pine – dominated forests of the Colorado Front Range

Abstract

Wildfires have increased in size and severity in ponderosa pine (Pinus ponderosa) – dominated forests in recent decades, and the ability of ponderosa pine and other co-occurring conifers to regenerate in severely burned portions of such fires is unclear. I collected post-fire conifer regeneration and other data within and surrounding five 11 to 18 year-old Colorado Front Range wildfires to examine whether severely burned patches are regenerating, and how regeneration density in these patches is governed by biotic and abiotic factors. Data were collected in plots distributed along transects originating within surviving forest and extending into high severity burn areas, and in plots located outside the fire perimeters. My results indicate that conifers have regenerated in severely burned areas (mean density of 118 stems ha-1, 81% of which is ponderosa pine), but at densities that were more than five times lower than those in unburned and lightly to moderately burned areas. Generalized linear mixed model analyses revealed that as distance from surviving forest increased, conifer regeneration decreased; estimates of conifer regeneration were ~100 stems ha-1 50 m from surviving forest but <10 stems ha-1 ≥200 m from surviving forest. These analyses also identified elevation as an important predictor of conifer regeneration in high severity burn patches, with densities decreasing with decreasing elevation. Regression tree analyses likewise found distance from surviving forest and elevation to be important predictors of regeneration, where within 50 m of live trees mean regeneration was 150 stems ha-1 at elevations ≤2490 m and 1120 stems ha-1 at elevations >2490 m, but at distances ≥50 m from live trees mean regeneration was 49 stems ha-1. Extrapolating my regression tree results to the 2002 Hayman Fire, 84% of the now unforested portion of this fire is predicted to have regeneration densities of 150 stems ha-1 or less. Taken as a whole, these findings suggest that activities such as tree planting may be warranted in high severity patches of Colorado Front Range wildfires if managers wish to return these areas to ponderosa pine – dominated forest in the foreseeable future, particularly where surviving forest is not in close proximity or where elevation is low.