Climate drivers of Pinus ponderosa tree development on volcanic tephra deposits in the Southwestern USA: Insights from radial increment and wood density variations

Understanding the complex dynamics of past tree growth-climate interactions is essential for predicting forest ecosystem responses to current climate change. Here, we explore the climate drivers of long-term growth dynamics in 400-year-old Pinus ponderosa trees at Sunset Crater Volcano in northern Arizona, including recent responses to unprecedented warming. To evaluate multiple climate factors potentially limiting montane trees on porous lava at 2450 m elevation, we employed several tree-ring proxies, including total ring width (TRW), earlywood width (EWW), latewood width (LWW), earlywood minimum density (minD), and latewood maximum density (maxD). We used static and moving correlations to assess how variations in previous and current year temperatures, precipitation, Standardised Precipitation-Evapotranspiration Index (SPEI), El Niño 3.4, and Pacific Decadal Oscillation (PDO) indices impact overall growth and density and their seasonal pattern. Our analyses revealed a seasonal shift in climate drivers, from the positive influence of winter and spring precipitation on EWW and minD to the negative effect of high summer temperatures and drought on LWW and maxD. This supports the hypothesis that tree growth in semi-arid regions results from a complex interplay between soil water content and evaporative forcing. Diminished precipitation and increased temperatures reduced EWW (constituting ∼60 % of total TRW), notably in the years 1925–1950 and 1990–2010, while the most favorable periods for growth were during cooler, wetter years 1900–1925 and 1960–1980, resulting in large EWW with low minD. During the warmer and drier years of 1930–1960 and 1990–2016, warmer Pacific waters, indicated by positive PDO and El Niño 3.4 indices, promoted wider earlywood with larger lumen size and thus lower minD, likely due to increased moisture and reduced spring drought. There was no marked growth decline in the last three warmest decades due to relatively stable precipitation. However, since the 1980s, climate drivers have shifted from winter and spring to summer, possibly contributing to extremely low growth years and fire events in the region due to summer heatwaves and droughts. These findings contribute to a better understanding of the complex relationship between climate change and tree growth dynamics in vulnerable semi-arid mountain forests.