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Exploring Whitebark Pine Tree Mortality in the Greater Yellowstone Area Using Tree-Ring Carbon Isotopes

Authors: Clay Tucker*, Louisiana State University, Adam Csank, University of Nevada, Reno, Justin DeRose, USDA Forest Service, Forest Inventory and Analysis
Topics: Biogeography, Paleoenvironmental Change, Climatology and Meteorology
Keywords: dendrochronology, isotopes, Wyoming, NADEF
Session Type: Paper
Presentation File: No File Uploaded

Since 2009, whitebark pine (Pinus albicaulis) in the Greater Yellowstone region have seen substantial dieback concurrent with mountain pine beetle (Dendroctonus ponderosae) outbreaks. Recent research has aimed at understanding the fragile ecosystem shifts in alpine regions inherent to global temperature changes. Tree-ring research can provide annually resolved information on centuries of growth changes in trees and can help explain the possible factors affecting growth and mortality over the tree’s life. However, because tree growth is influenced by many factors (e.g., water, temperature, insects), studies based only on measurements of the ring-width variability are limited in scope. Analysis of stable isotopes in tree rings can help narrow that scope through the identification of processes that modulate growth response to climatic conditions. For example, carbon isotope ratios measured in wood are driven strongly by stomatal conductance that is related to water availability. Here, we explore the use of carbon isotopes to identify putative drivers of mortality for whitebark pine that succumbed to a recent mountain pine beetle outbreak. Δ13C ratios were positively correlated with temperature (R2 = 0.23) and negatively correlated with precipitation (R2 = 0.12). The strongest correlations are with late summer temperature when evaporation is highest, whereas precipitation correlations are strongest earlier in the growing season. Δ13C ratios recorded in whitebark pine tree rings indicated that a strong positive relationship with moisture stress immediately preceded tree death. Δ13C ratios indicate that whitebark pine die-off associated with a mountain pine beetle outbreak was likely enhanced by multiple years of moisture stress.

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