Nonstationary temporal response of mountain hemlock growth to climatic variability
Kailey Marcinkowski, David L. Peterson, and Gregory J. Ettl
A stationary response of tree radial growth to climatic variables is assumed as a basis for climatic reconstructions and future growth projections in response to climate change. Mountain hemlock (Tsuga mertensiana (Bong.) Carri.re) trees on the western slopes of the North Cascade Range (Washington, USA) were examined for stability in growth response to climatic influences at a small spatial scale. Moving correlation functions demonstrate that climate–growth interactions are nonstationary over time, alternating between periods of significant and nonsignificant responses. Correlations between growth and winter
precipitation have weakened, becoming statistically insignificant in the last decade, but correlations with spring temperature and previous-year summer temperature have strengthened, becoming statistically significant. The Pacific Decadal Oscillation
influences patterns in climate–growth correlations but does not seem to account for the most recent changes in correlation strength. At an interannual scale, growth differs between El Ni.o Southern Oscillation phases, specifically between El Ni.o and
La Ni.a years and between La Ni.a and neutral phase years. The variability in growth response to climate at interannual and interdecadal time frames, especially with the climate changes emerging in recent decades, will challenge the reliability and accuracy of reconstruction and predictive models.