Tree-ring isotopes reveal drought sensitivity in trees killed by spruce beetle outbreaks in south-central Alaska

Increasing temperatures have resulted in reduced growth and increased tree mortality across large areas of western North American forests. We use tree-ring isotope chronologies (δ¹³ C and δ¹⁸ O) from live and dead trees from four locations in south-central Alaska, USA, to test whether white spruce trees killed by recent spruce beetle (Dendroctonus rufipennis Kirby) outbreaks showed evidence of drought stress prior to death. Trees that were killed were more sensitive to spring/summer temperature and/or precipitation than trees that survived. At two of our sites, we found greater correlations between the δ¹³ C and δ¹⁸ O chronologies and spring/summer temperatures in dead trees than in live trees, suggesting that trees that are more sensitive to temperature-induced drought stress are more likely to be killed. At one site, the difference between δ¹³ C in live and dead trees was related to winter/spring precipitation, with dead trees showing stronger correlations between δ¹³ C and precipitation, again suggesting increased water stress in dead trees. At all sites where δ¹⁸ O was measured, δ¹⁸ O chronologies showed the greatest difference in climate response between live and dead groups, with δ¹⁸ O in live trees correlating more strongly with late winter precipitation than dead trees. Our results indicate that sites where trees are already sensitive to warm or dry early growing-season conditions experienced the most beetle-kill, which has important implications for forecasting future mortality events in Alaska.