Implications of diameter caps on multiple forest resource responses in the context of the four forests restoration initiative: Results from the forest vegetation simulator

Meeting multiple resource objectives, such as increasing resilience to climate change, while simultaneously increasing watershed health, conserving biodiversity, protecting old-growth, reducing the risk of catastrophic wildfire, and promoting ecosystem health, is paramount to landscape restoration. Central to public land management efforts in the West is the widespread adoption of size-prohibited cutting of “large” trees, a limitation referred to as a “diameter cap.” In this study, we used the most commonly proposed prescription for the Four Forest Restoration Initiative in northern Arizona to explore the implications of diameter caps for multiple resource responses through the use of model simulations. We found that implementing progressively smaller caps in southwestern ponderosa pine may result in relatively similar live tree densities, canopy cover, and large snag densities but higher basal areas, mean tree size, torching indices, and scenic beauty with lower water yield and herbaceous production. When diameter cap scenarios are compared, tradeoffs exist, and no single metric is suited for overall scenario evaluation.