Multiple Elevation-Dependent Climate Signals From Quantitative Wood Anatomical Measurements of Rocky Mountain Bristlecone Pine

Southwestern North America has experienced significant temperature increases over the last century, leading to intensified droughts that significantly affect montane forests. Although tree-ring data have provided long-term context for this recent drought severity, the varying physiological responses of trees to climate variability make it challenging to disentangle the combined influence of temperature and soil moisture. Here we investigate complex climate-growth relationships in Rocky Mountain bristlecone pine (Pinus aristata) at a low-elevation and a high-elevation site using quantitative wood anatomy (QWA). Significant correlations with climate were found for low-elevation tree-ring width (TRW) and earlywood chronologies, including positive correlations with spring and early summer precipitation and drought indices and negative correlations with spring and early summer maximum temperatures. At high elevations, TRW and earlywood chronologies show positive responses to summer moisture, whereas latewood chronologies correlate positively with August and September maximum temperatures and negatively with August precipitation. We leverage this differing seasonality of moisture and temperature signals and compare the QWA data to known droughts. The earlywood lumen area is found to be highly responsive to drought because of its physiological reliance on water availability for maintaining turgor pressure during cell enlargement. We also observed a decline in temperature sensitivity at the high elevation site, suggesting shifts in the dominance of limiting factors. Integrating QWA with traditional dendrochronology improves interpretations of tree-ring data for use in climate reconstruction, offering detailed insights into tree physiological responses and the mix of environmental and developmental controls on cell growth.