Several recent studies have documented how fire severity affects the density and spatial patterns of tree regenerationin western North American ponderosa pine forests. However, less is known about the effects of fireseverity on fine-scale tree regeneration niche attributes such as understory plant composition and cover, surfacefuel abundance, and soil properties, or how these attributes in turn affect regenerating ponderosa pine growth.Using 1-m2 plots centered on 360 ponderosa pine seedlings that regenerated naturally after the Pumpkin Fire in2000 in Arizona, we quantified regeneration niche attributes 13 years post-fire and measured their associationswith seedling growth 11–16 years post-fire. Plots were established in a) unburned, b) moderate-severity, and twotypes of high-severity (100% tree mortality) burns, either c) adjacent to residual live forest edges (high-severityedge plots), or d) > 200 m from any residual live trees (high-severity interior plots). We found that all burnedplots had greater understory plant species richness, percent cover of forbs, exotic plants and coarse wood, as wellas higher soil pH, sand and gravel content, and lower soil clay content compared to unburned plots. Highseverityburn plots had the greatest total understory plant and shrub cover, the most herbaceous fine fuel biomass,and tended to have the highest soil nitrogen content compared to other burn severity classes. Ponderosapine growth (i.e., stem diameter at root collar (DRC) and length of terminal leader) was lowest in the unburnedcompared to burned plots, and ponderosa pine terminal leader growth was consistently greater in the highseverityedge plots compared to other severities. Finally, niche characteristics such as overstory tree canopycover (−), soil phosphate (+), and cover of coarse wood (+), forbs (+), and the native grasses, mountainmuhly (+) and squirreltail (−), were important explanatory variables of ponderosa pine growth. Exotic plantcover did not have a negative association with ponderosa pine growth. These results suggest that if ponderosapine seeds can disperse and germinate, and if seedlings can survive the first few critical years after germination,then low overstory canopy cover and abundant forbs or coarse wood may be associated with increased growthrates. Alternatively, forbs may be responding to the same site benefits as the seedling; and abundant forbs, coarsewood, and fine fuels might also put seedlings at increased risk of mortality from subsequent fire, at least untilthey are taller and more fire resistant.
Owen SM, Sieg CH, Fulé PZ, Gehring CA, .Baggett L, Iniguez JM, Fornwalt PJ, Battaglia MA. Persistent effects of fire severity on ponderosa pine regeneration niches and seedling growth. Forest Ecology and Management. 2020 ;477.