Recent wildfires in southwestern ponderosa pine forests have created unprecedented large patches of tree mortality. Ponderosa pine regeneration is likely dispersal-limited in large burn patches because of lack of seed sources, non-serotinous cones and large seeds. Persistence of pine-dominated forests depends on post-wildfire pine regeneration, and may be influenced by available plant-symbiotic ectomycorrhizal (EM) fungi. In large burn patches with 100% tree mortality, we examined the question: does distance from unburned forest edges affect patterns of ponderosa pine regeneration and communities of associated EM fungi? Our study sites were located in two large wildfires, the 2000 Pumpkin Fire and the 2002 Rodeo-Chediski Fire, Arizona, USA. We quantified post-wildfire seedling densities, spatial patterns, and interactions with sprouting species within replicated 4-ha plots in severely burned areas both adjacent to forest edges (edge plots) and >200 m from live trees (interior plots); and EM fungi colonization and community composition in unburned to severely burned plots.
Ponderosa pine seedlings were highly aggregated in the burn patches, and regeneration ranged from 13-154 seedling/ha on the edge plots and 12-124 seedling/ha over 200 m from any live seed tree on the interior plots, indicating long-distance dispersal (likely from scatter-hoarding by corvids and other birds) plays an important role in regeneration after high-severity wildfires. In addition, sprouting species were common on the Rodeo-Chediski Fire, with an average of 242 alligator junipers/ha and 736 Gambel oaks/ha. Populations of ponderosa pine and sprouting species were mostly spatially independent. Also, similar EM colonization and community composition were found across burn severities and unburned areas, suggesting EM communities are resilient after 10+ years in severely burned areas. Pine seedlings are re-establishing, but at densities lower than historical ranges, and the long-term forest composition is still unclear. However, these large burn patches support vegetation types that could be more resilient to future climate change and severe wildfires than overly-dense ponderosa pine forests that were present before the wildfires we studied. We recommend managing dense stands by increasing thinning and fire use to reduce the risk of high-severity wildfires.