Precipitation in the southwestern U.S. (“Southwest”) comes primarily through winter Pacific frontal systems and summer monsoon storms. Spatial patterns of monsoon and winter precipitation result in a northwest – southeast gradient from winter to summer moisture dominance in the Southwest; upwards of 70% of annual precipitation is contributed by the monsoon in some areas. Annual tree growth is strongly tied to seasonal (e.g., winter, summer) and yearly climate, but less is known about the time-scales over which climate influences growth (i.e., the “climatic memory” of tree growth). Thus, we applied a Bayesian model to evaluate the influence of antecedent exogenous (temperature and precipitation) and endogenous (age and prior growth) factors on Pinus ponderosa annual growth. In particular, we analyzed ring widths from 128 trees growing in 11 sites across the Southwest, along with 119 years of monthly precipitation and temperature data from PRISM. The sites spanned a gradient in monsoon precipitation contribution covering Utah, Arizona, and New Mexico. This analysis allowed us to address the following questions: Over what times-scales (past months and years) do precipitation and temperature influence annual tree growth? And, does the relationship between antecedent climate and tree growth vary across the Southwest’s seasonal (monsoon) precipitation gradient?
Results/Conclusions
Annual tree growth was affected by precipitation and temperature that occurred up to four years prior to growth, indicating that trees have long-term “memory” of climatic conditions. While growth was positively associated with antecedent precipitation at 10 out of 11 sites, there was a >100-fold difference in the growth sensitivity to precipitation among sites. The drier the site, the greater the influence of precipitation on growth; site-level growth sensitivities were negatively correlated with mean annual, winter (Dec, Jan, Feb, March), and summer (July, Aug, Sept) precipitation amounts (Pearson’s r = -0.88, -0.77, and -0.55, respectively). The temperature sensitivity of annual tree growth was variable across sites, with negative correlations (3 sites), positive correlations (4 sites), and no correlation (4 sites). At all sites, annual growth decreased with tree age and was positively correlated with the previous year’s growth. The effects of age and prior growth varied widely among trees within sites. The effects of antecedent precipitation and temperature were fairly uniform within sites, but varied greatly among sites, reflecting different site-specific climate conditions. Any perturbations to the precipitation regime in the Southwest will have multi-year implications for tree growth that will not be uniform across the monsoon gradient.