Integrating forest health conditions and species adaptive capacities to infer future trajectories of the high elevation five-needle white pines

Abstract

Tree mortality rates have been increasing globally with mountainous regions experiencing higher temperatures and impacts from the expansion and intensification of pests and invasion by non-native agents. Western North American high-elevation forests exemplify these trends, and they often include one or more species of five-needle white pines (High-5 hereafter). These species share many characteristics critical to defining the structure and function of many subalpine forests. The main threats to High-5 populations include the non-native pathogen Cronartium ribicola, which causes the disease white pine blister rust, climate-driven drought stress, episodic and high mortality from mountain pine beetle (Dendroctonus ponderosae), and wildfires of increasing frequency, size, and intensity. The six High-5 species occurring in western North America (whitebark pine, Pinus albicaulis; limber pine, P. flexilis; southwestern white pine, P. strobiformis; Rocky Mountain bristlecone pine, P. aristata; Great Basin bristlecone pine, P. longaeva; and foxtail pine, P. balfouriana) differ in their health status and threat level. The convergence of threats impacting the rapidly declining species could portend future declines in the species and populations currently less impacted by recent disturbances. Differences in the innate adaptive capacities of the species affect their population trajectories under these novel combinations of stressors. We evaluate the status and outlook for each species and address the following questions: (1) Is the environment changing too fast and the intensity of stressors too great for the species to adapt and recover? (2) Do the species have the heritable traits necessary to sustain fitness under C. ribicola and climatic stresses? (3) Are other mortality factors increasing to the degree that they reduce the populations further and delay or preclude adaptation and population recovery? (4) Can the species escape the stressors through migration? Insights related to these questions provide guidance for forest management to facilitate adaptation and increase the resilience of these species into the future.