Over the past three decades, wildfires in southwestern United States ponderosa pine forests have increased in size and severity, leaving large patches of tree mortality. Ponderosa pine evolved under fire regimes dominated by low- to moderate-severity wildfires, and they are poorly adapted to regenerating in large patches of high-severity fire. There is concern about these high-severity burn patches because the lack of seed-producing trees can prevent or significantly delay ponderosa pine regeneration.
We measured tree regeneration in large, high-severity burn patches from two 12+ year-old Arizona wildfires to determine if: 1) distance from forest edge influences the density and spatial patterns of regenerating ponderosa pines, 2) interactions with resprouting trees affect spatial patterns of ponderosa pine regeneration, and 3) species competition affects regenerating ponderosa pine height. Plots were located in high-severity burn patches (defined as 100 percent tree mortality) and either adjacent to unburned forest edges (edge plots), or more than 200 meters from any live trees (interior plots). We found higher ponderosa pine regeneration densities in the edge plots as compared to the interior plots, but no differences in spatial patterns between plot types. Ponderosa pine regeneration displayed patterns of small-scale spatial aggregation, possibly due to seed dispersal by rodents or birds. Native sprouting trees dominated tree regeneration on one fire, but they did not influence ponderosa pine spatial locations or height (Figure 2). 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.
Owen, S.M., C.H. Sieg, A.J. Sánchez Meador, P.Z. Fulé, J.M. Iniguez, L.S. Baggett, P.J. Fornwalt, M.A. Battaglia. 2017. Spatial patterns of ponderosa pine regeneration in high-severity burn patches. Forest Ecology and Management 405 (2017) 134-149. http://dx.doi.org/10.1016/j.foreco.2017.09.005