NOAA/WDS Paleoclimatology - Southwest Alaska Tree Ring Stable Isotope Data

Increasing temperatures have resulted in reduced growth and increased tree mortality across large areas of western North American forests. Here we use tree-ring isotope chronologies (d13C and d18O) from live and dead trees from four locations in south-central Alaska 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 d13C and d18O 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 d13C in live and dead trees was related to winter/spring precipitation, with dead trees showing stronger correlations between d13C and precipitation, again suggesting increased water stress in dead trees. At all sites where d18O was measured, d18O chronologies showed the greatest difference in climate response between live and dead groups, with d18O 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.