Alternative approaches to mixed conifer forest restoration: partitioning the competitive neighborhood

Forest restoration efforts in the intermountain west of North America generally seek to promote the continuation of pine dominance, enhance wildlife habitat, and decrease hazardous fuels, thereby mitigating catastrophic losses from various stressors and disturbances. We propose a method of focal tree release thinning that partitions the competitive neighborhood to provide alternative approaches to managers. Specifically, we sought to examine how competition index (CI) derived harvest simulations alter forest structure, composition, and variability, and evaluate the ecological implications and efficacy of achieving management recommendations. We used a tree inventory collected across 38 experimental plots in the mixed conifer forest of the Sierra Nevada and simulated post-harvest structure using common silvicultural prescriptions to the ownership and cover type. We calculated competition values for all trees using 10 CIs and simulated harvests from two defined integrals of each corresponding probability density function to compare with the standard marking scenario, for a total of 21 harvest simulations. We assessed post-harvest structure through tree density and diameter distributions, basal area by species, and a structural diversity index. Post-harvest conditions exhibited differences in levels of structural diversity and species dominance; however we did not detect any influence on tree density across diameter classes. Most simulations resulted in a decline of non-pine species basal area relative to the default, while only 3 thinning scenarios showed concomitant increases in pine. Every simulation resulted in greater variance of structural diversity than the default marking guidelines. Review of this method highlighted the variability in tree ranked competitive status across indices. We emphasize that no harvest simulation by CI was clearly superior in all aspects of achieving desired objectives, and there was no clear benefit to incorporating inter-tree distances to calculate CI. We consistently observed favorable outcomes in harvest simulations derived from an open-grown crown width parameter which averts the need for tree distance measurements. This approach can be tailored across multiple individual plots, each maintaining unique competitive environments that reflect local tree neighborhoods.