COS 277-4 - Geographic Variation of Lodgepole Pine Drought Resilience in Western U.S. Forests

Abstract: Climate change is expected to exacerbate drought conditions in upper montane forests of the western U.S., potentially affecting recruitment, growth, and survival. To mitigate these effects, a better understanding of the interaction between climatic conditions and seed zones within a species is necessary. There is a growing body of research suggesting that certain subspecies of lodgepole pine (Pinus contorta var. latifolia) have the capacity to respond positively to assisted migration and drought conditions. However, most efforts have focused on the species’ northernmost extent in British Columbia, with little research in the Rocky Mountains of the western U.S., where lodgepole pine is widespread. In this southern region, drought conditions are already linked to increasing species bark beetle susceptibility. There is a critical need to increase drought resistance and resilience of lodgepole pine forests by promoting drought-tolerant subspecies. Here, we make use of a provenance study established in 1981 to analyze the relationship between variation in seed zone and drought tolerance in lodgepole pine and its capacity for assisted migration and planting efforts in drought-affected forests. The study site, located at the Fraser Experimental Forest in Fraser, Colorado, was planted with 1,347 lodgepole pine (Pinus contorta var. latifolia) collected along a latitudinal (37.6 N to 48.3 N) and elevational gradient from 34 populations (16 Colorado, 12 Montana, 6 Wyoming). In 2022, we collected tree status (live/dead), diameter at breast height (DBH), and height data for a subset of trees to determine growth. Additionally, tree cores were collected from the subset to evaluate annual radial growth in response to climatic conditions (drought, average, and wet). Across the 34 populations, we found clear winners and losers in terms of overall growth and size accumulation. This research will shed light on the relationship between seed zone variation and drought tolerance of lodgepole pine to inform management decisions surrounding climate adaptation strategies for upper montane and subalpine forested areas in the western U.S. More broadly, insights gained from this study will help to guide future efforts in other forest types across the U.S. to create resistant and resilient forest communities in the face of our current climate crisis.