The increasing size and severity of wildfires in the Western United States may have long term effects on species composition. A Fact Sheet published this month by Northern Arizona University and the U.S. Forest Service looks at ponderosa pine regeneration in patches of high-severity areas of the 2000 Pumpkin and the 2002 Rodeo-Chediski wildfires in the Southwest. Below are excerpts from the document written by Suzanne Owen, PhD student, School of Forestry, Northern Arizona University:
Introduction
Over the past three decades, wildfires in southwestern U.S. ponderosa pine (Pinus ponderosa) forests have increased in size and severity, leaving large, contiguous patches of tree mortality. Ponderosa pines evolved under fire regimes dominated by low- to moderate-severity wildfires. They are poorly adapted to regenerate in large patches of high-severity fire because they are not a sprouting species and do not have serotinous cones or long-lived soil seedbanks. Consequently, the lack of seed-producing trees in high-severity burn patches may prevent or significantly delay ponderosa pine regeneration. Previous studies have documented low ponderosa pine regeneration densities in large high-severity burn patches, but less is known about the spatial patterns of ponderosa pine regeneration and interactions with sprouting species near residual live forest edges or the interiors of high-severity burn patches.
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Results
- Ponderosa pines were re-establishing in all of our study plots, however regeneration densities were lower farther from forest edges.
- Ponderosa pines seedlings were found in areas more than 980 feet from potential parent trees on all interior study plots.
- Regenerating ponderosa pines displayed patterns of small-scale spatial aggregation in all plots, except one edge and one interior plot on the Pumpkin Fire, which displayed random distributions.
- Dense resprouting trees dominated tree regeneration on the Rodeo-Chediski Fire, but did not influence the spatial location or height of regenerating ponderosa pine.
- Regenerating ponderosa pine height was positively correlated with neighboring ponderosa pine densities and height.
Implications
- Tree regeneration densities and species composition in high-severity burn patches are highly variable in different geographic locations.
- Regeneration patterns suggest both short- and long-distance dispersal may play important roles in ponderosa pine regeneration in high-severity burn patches.
- Ponderosa pine regeneration could be more strongly influenced by intraspecific facilitation than interspecific competition from dense sprouting species.
- Future forest spatial patterns and composition are still unclear, but at this stage of development, these heterogeneous patches, characterized by drought-tolerant sprouting species or low pine densities, could be more resilient to climate change and severe wildfires than the overly dense ponderosa pine forests that were present before the wildfires.
- Managers may want to use a “wait and see” approach before replanting in some areas to monitor natural regeneration over time.
