Decades-old silvicultural treatments influence surface wildfire severity and post-fire nitrogen availability in a ponderosa pine forest

Abstract

Wildfire severity and subsequent ecological effects may be influenced by prior land management, via modification of forest structure and lingering changes in fuels. In 2002, the Hayman wildfire burned as a low to moderate-severity surface fire through a 21-year pine regeneration experiment with two overstory harvest cuttings (shelterwood, seed-tree) and two site preparations (scarified, unscarified) that had been applied in a mature ponderosa pine forest in the montane zone of the Colorado Front Range in 1981. We used this event to examine how pre-fire fine fuels, surface-level burn severity and post-fire soil nitrogen-availability varied with pre-fire silvicultural treatments. Prior to the wildfire, litter cover was higher under both shelterwood and unscarified treatments than seed-tree and scarified treatments. Immediately after the fire in 2002, we assessed burn severity under 346 mature trees, around 502 planted saplings, and in 448 4 m 2 microplots nested within the original experimental treatments. In one-fourth of the microplots, we measured resin-bound soil nitrate and ammonium accumulated over the second and third post-fire growing season. Microplots burned less severely than bases of trees and saplings with only 6.8% of microplot area burned down to mineral soil as compared to >28% of tree and sapling bases. Sapling burn severity was highest in unscarified treatments but did not differ by overstory harvest. Microplot burn severity was higher under the densest overstory (shelterwood) and in unscarified treatments and was positively related to pre-fire litter/duff cover and negatively associated with pre-fire total plant cover, grass cover and distance to tree. In both years, resin-bound nitrate and ammonium (NH 4 +-N) increased weakly with burn severity and NH 4 +-N availability was higher in unscarified than scarified plots. The lasting effects of soil scarification and overstory harvest regime on modern patterns of surface burn severity after two decades underscores the importance of historic landuse and silviculture on fire behavior and ecological response. Unraveling causes of these patterns in burn severity may lead to more sustainable fire and forest management in ponderosa pine ecosystems.