Historically, recurring, low to moderate severity, fire played an important role in shaping spatial patterns and species compositions in western ponderosa pine forests. However, the absence of fire has resulted in an increase in tree density, accumulation of fuels, and decline in disturbance-tolerant hardwood species – such as oaks – in many of these forests. Here, we studied how forest structure - density, composition, and spatial patterns - changes after a restorative wildfire in the Beaver Creek Pinery, an old growth ponderosa pine stand in Northern California. Particular attention is given to the relative performance of California black oak and ponderosa pine since black oak provides habitat and forage for wildlife, holds cultural importance to Native American tribes, and can influence fire dynamics in western conifer forests. Trees, regeneration (seedlings and saplings), and understory vegetation in 107 vegetation plots were first inventoried in 1998 and then re-measured in 2016. Vegetation was mapped in 6 large (~1 ha) plots in 2000 and 2016. We hypothesize that vegetation structure will converge and become more homogenous at multiple scales. To test this hypothesis at the landscape level, a hierarchical cluster analysis was conducted; at the stand level, Moran's I was conducted to identify the scale and intensity of clustering between entities (trees) with similar characteristics (age & dbh).
Results/Conclusions
Across the pinery, a significant increase in tree density was observed (p<0.05) with an ~100-fold increase in frequency of small diameter trees (5-15 cm dbh). In 1998, pine saplings outnumbered oak saplings 4:1; by 2016, pine saplings outnumbered oak saplings 27:1. From 1998 to 2016, ponderosa pine seedling density increased by 57% while black oak density increased by 21%. From 1998 to 2016, the number of distinct structural groups decreased by 50%; regeneration and small trees were large contributors to this observed homogenization. At the stand level, larger groups and increased intensity of clustering was observed in five out of six plots. Overall, this research broadens our understanding on how forests respond to fire and, in turn, how fire can be used to restore and maintain heterogeneity and resiliency in ponderosa pine forests where wildfires have been suppressed for nearly a century. Further, this research contributes to larger body of scholarship on interactions between disturbances, biodiversity, and species co-existence.