Stand changes brought on by fire exclusion have contributed to reduced resilience to wildfire in ponderosa pine forests throughout the western US. Growing recognition of how structural attributes influence resilience has led to interest in restoring more heterogeneous conditions once common in these forests, but key information about interactions between stand and fuel development in such stands is currently incomplete or lacking. Few contemporary examples of structurally restored old-growth ponderosa pine forest exist. We re-measured plots in the Beaver Creek Pinery (BCP), a remote site in the Ishi Wilderness on the Lassen NF in California, that were installed following a 1994 wildfire, to better understand forest and fuel succession over time. The BCP experienced four wildfires since 1900 that restored the structure to one believed similar to historical ponderosa pine forest. Stand-scale change in overstory and understory vegetation were quantified in 2016 by remeasuring and remapping six one hectare plots that were initially mapped in 2000, and landscape-scale change was evaluated by remeasuring circular plots systematically arranged across the BCP in 1998. Tree recruitment, mortality, and growth were measured and changes in tree group and gap size and structure were calculated. We also examined the relative performance of California black oak, a declining but important species valued by tribes for food and wildlife for habitat, to better understand how fire interval and severity maintain the conifer and oak mixture. Using data from the re-measurement, we modeled stand and fuel development over the next 30 years using the Forest Vegetation Simulator (FVS), in order to predict the type of fire and return interval that would be necessary to maintain the desired heterogeneous structure over time.

At the stand scale, evaluated by remeasuring six one ha plots (Taylor plots) overstory tree density and basal area did not change significantly between 2000 and 2016. However, abundant ingrowth resulted in far higher densities of seedlings and saplings in 2016, and the numbers of single trees declined, while average tree group size increased (groups averaged 6 trees (range 2–38) in 2000 and 9 trees (range 2–240) in 2016). Wildfires initially promoted California black oak regeneration via sprouting, but the number of oak stems declined while the number of ponderosa pine stems increased between the two time periods. On average, the rate of gap infilling from tree regeneration (1.44% yr−1) and crown extension into gaps (1.05% yr−1) exceeded rates of new gap creation (0.58% yr−1). At the landscape scale (evaluated by remeasuring 107 0.08 ha circular plots established on a grid over the entire Pinery – PSW plots), plots remained somewhat less dense on average, with lower average basal area than the Taylor plots, but the same increase in smaller diameter ponderosa pine trees and pine regeneration overwhelming that of black oak was found. Over the two time periods, grass cover declined from 33% of 3%, shrub cover increased from 16% to 31%, and tree canopy cover increased from 14% to 34%. The forest became somewhat more homogeneous, with most structural diversity metrics declining. Still, both the 1998/2000 forest and the 2016 forest resembled reference conditions for pre-fire suppression frequent-fire forests in the western United States. Despite the 22 years that had transpired since the last fire and abundant ingrowth since then, potential fire behavior was predicted to be a surface fire with low overstory tree mortality at the stand scale, and mainly a surface fire with some mostly passive crown fire behavior at the landscape scale, suggesting the current forest would be resilient to a wildfire under most weather and fuel moisture conditions. However, modeling stand growth of forest conditions into the future suggested that a threshold may be crossed within the next ten years, with crown fire becoming the predominant behavior. This would likely result in loss of key structural attributes. Burning under controlled conditions and in the near future will be necessary to reduce the demographic pressure of ponderosa pine, promote black oak, and to maintain and create future spatial heterogeneity.