Positive feedbacks among biotic and abiotic stressors mediate whitebark pine decline

Abstract

Interactions between biotic and abiotic stressors reduce forest health, particularly in high-elevation forests. Because trees are expected to resist biotic and abiotic stressors in unique ways, it remains unknown how specific processes, such as growth and defense, relate to tree survival in a compound disturbance environment. Comparing trees that either died or survived mountain pine beetle (MPB) provides a natural experiment to test how multiple stressors jointly mediate susceptibility to MPB. We cored whitebark pine in the Greater Yellowstone Ecosystem to measure growth, resin ducts, and xylem anatomy using standard quantitative wood anatomy techniques. Overall, anatomical measurements increased model accuracy by 7.1 % compared to ring widths alone. We found growth-defense tradeoffs for all trees; for resistant trees, however, resin duct density varied negatively with the pore space fraction (lumen) of tree rings – not the structural carbon fraction (tracheid walls). Despite showing less structural carbon and total growth at the time of the outbreak, susceptible trees displayed anatomical resistance in their early lives. Over time, susceptible trees had less positive growth responses during years of high snowpack, and more negative responses of lumen area and theoretical hydraulic conductivity during years of high temperatures and drought. Susceptible tree decline was catalyzed during the decade 1940, which coincides with first reports of the invasive white pine blister rust (WPBR) in the region. Together, our findings provide support that differential responses of whitebark to temperature and drought contributed to reduced structural carbon storage that, likely exacerbated by WPBR, predisposed trees to MPB mortality.