Comparative species assessments of five-needle pines throughout the western United States

Five-needle white pine species provide important ecosystem services throughout the western U.S., and many of these species have become susceptible to stressors including warmer temperatures, insect epidemics, nonnative disease, and altered disturbance regimes. The objective of this study was to characterize recent broad-scale demographic patterns, including species abundance (i.e., numbers of individuals, tree density, size-class distributions, recruitment, growth rates, mortality rates, and causes of mortality, for the six species of five-needle pine that occur in the western US. We used the U.S. Forest Service's Forest Inventory and Analysis (FIA) dataset, spanning greater than 10 years, to quantify demographic status and trends for each species. FIA data were compiled from a probabilistic sample design and consistent analysis framework that included not only the dominant community types of five-needle pines, but also all other forest community types, which have previously been demonstrated to encompass abundant regeneration of five-needle pine species. Our analysis revealed similar trends for whitebark and limber pines: both species exhibited increased levels of mortality that are occurring faster than growth of surviving trees, as well as abundant regeneration in forest types that are not dominated by five-needle pines. Although limber pine has experienced lower mortality rates than whitebark pine, it nonetheless showed signs of decline that are comparable to broad-scale indicators exhibited by whitebark pine 10 years prior. In contrast to whitebark and limber pines, Great Basin bristlecone and foxtail pine mortality rates were relatively low, and their populations exhibited a flat diameter distribution except for restricted recruitment from seedling to sapling size-classes. Our findings suggest that five-needle white pine species would benefit not only from increased seedling recruitment, but also from enhanced recruitment among older and larger age and size classes. Thus, it may be important to apply a variety of management strategies – including artificial regeneration with disease-resistant seedlings, direct seeding, and early intervention to decrease competition that may allow natural regeneration to achieve recruitment inter large size-classes – and also apply silviculture techniques to forest types that may be dominated by other species. The consistent monitoring conducted by FIA can allow future assessment of the demographic trajectory of each five-needle pine species, and thus can help prioritize management and restoration priorities, both within communities dominated by five-needle pines and in other community types that may be important targets for silvicultural interventions or restoration treatments.