Pinus edulis (pinyon pine) is a foundation tree species in the southwestern United States that has experienced high levels of mortality in a drought extending from the mid-1990s to the present.  Despite high mortality levels, radial growth of pinyon pine in northern Arizona over the past century shows an increasing trend not found in four centuries of preindustrial growth.  Atmospheric carbon dioxide concentration ([CO₂]) shows a strong correlation with recent radial growth, and a simple linear AR(1) model with only [CO₂] as a covariate explains average yearly growth well.  No climate variable except [CO₂] has a consistent long-term trend that accounts for increasing radial growth, but growing season and precipitation are responsible for interannual variation in radial growth.  Our goal is to understand how [CO₂] and other climate variables, including drought severity, influence growth and survival of pinyon pine.

Results/Conclusions

To model and predict growth for individual pinyon pine trees, we adapted a hierarchical Bayesian state-space model from Clark et al. (2007) that incorporates both tree ring and diameter data.  We included [CO₂], water year precipitation, and an index of growing season length based on temperatures early and late in the growing season as covariates while adjusting for known size-related growth trends.  Elevated [CO₂], longer growing season, and wetter years are positively correlated with growth (parameter values and standard errors of 0.010 ±  0.000039, 0.0096 ± 0.000082, and 0.010 ± 0.000035, respectively).  These trends are consistent with recent studies of growth patterns in other long-lived tree species in the western U.S.  Ongoing analyses are evaluating interactions between [CO₂] and precipitation to test the idea that rising CO₂ may ameliorate growth reductions due to water stress.