Spatial dynamics of tree group and gap structure in an old-growth ponderosa pine-California black oak forest burned by repeated wildfires

Knowledge of how tree groups and gaps are formed and maintained in frequent-fire forests is key to managing for heterogeneous and resilient forest conditions. This research quantifies changes in tree group and gap spatial structure and abundance of ponderosa pine (Pinus ponderosa) and California black oak (Quereus kelloggii) with stand development after wildfires in 1990 and 1994 in an old-growth forest in the Ishi Wilderness, southern Cascades, California. Forest demography and tree group and gap structure were quantified by measuring, mapping, and aging trees in six 1-ha permanent plots in 2000 and 2016. Tree recruitment, mortality, and growth were estimated using demographic models and spatial characteristics including gap structure were identified using an inter-tree distance algorithm and the empty space function. Potential fire behavior and effects in 2016 were estimated to determine if the current forest would be resilient to a wildfire in the near future. Stand density and basal area in both 2000 and 2016 resembled reference conditions for pre-fire suppression frequent-fire forests in the western United States. Wildfires initially promoted California black oak regeneration via sprouting, but oak regeneration from seed declined relative to ponderosa pine over the post-fire period. In 2000, similar to 15% of trees were classified as single tree groups and an average tree group had 6 trees (range 2-38) which increased to 9 trees (range 2-240) in 2016. Small groups (2-4 trees) had similar-aged trees while larger groups were multi-aged. By 2016, single tree groups decreased by similar to 30%, and the size, density, and intensity of clustering within tree groups increased, with an average tree group size of 9 (range 2-240) in 2016. Rates of post-fire regeneration, particularly the high rate for ponderosa pine, drove spatial dynamics in tree group and gap structure. Although the size and frequency of canopy gaps were similar in 2000 and 2016, the density of seedlings and saplings in gaps was higher in 2016, and large gaps were being fragmented by gap filling. Potential fire behavior predicted surface fire and low overstory tree mortality, suggesting the current forest would be resilient to a wildfire. However, burning will be necessary in the future to reduce the demographic pressure of ponderosa pine, promote black oak, and to maintain and create future spatial heterogeneity. Cumulatively, this research demonstrates that wildland fires under certain conditions can maintain and restore fire resilience in ponderosa pine forests reducing the negative ecological consequences related to past fire exclusion.