Drought conditions, via increased atmospheric moisture demand and/or altered timing and amount of precipitation, are projected to become more common with climate change in the southwestern US (“Southwest”). In this region, such changes are exacerbated by La Niña anomalies that tend to lead to recurring drought conditions, interspersed with El Niño events associated with above average winter precipitation. This study aims to understand how annual pinyon pine (Pinus edulis) growth (ring-width chronologies) may differentially respond to climate following La Niña versus El Niño years in the Southwest. To address this, we evaluated annual growth responses to monthly precipitation, temperature, and drought indices at varying time-scales. To better understand the physiological controls on tree growth during these periods, we conducted a large-scale field study of pinyon, complemented by data on co-occurring Utah juniper (Juniperus osteosperma). In two campaigns (before and after the 2016 summer monsoon), we revisited 11 sites across AZ, NM, UT, and CO, spanning an unusually spatially variable 2015-2016 El Niño precipitation gradient. We quantified a suite of ecophysiological variables, and we focus on a subset of variables expected to underlie annual growth responses, including non-structural carbohydrates (NSCs) and predawn water potential (ѰPD).
Compared to other years, pinyon ring widths were more coupled to current year’s February and June precipitation and to previous year’s October and November precipitation during La Niña years. Pinyon responds more negatively to current year’s temperature in March, April, and June during these periods. Large differences in sensitivities to June precipitation and July temperature suggest precipitation received during early/mid-summer (before monsoon arrival) and the monsoon arrival date (lowers daytime temperatures; July 4th average arrival) are important during drought years. NSC strategies likely underlie differential growth responses during El Niño and La Niña periods. For example, NSCs align with water stress such that leaf sugar and stem starch in pinyon and leaf starch in juniper were positively related to ѰPD (r = 0.447, 0.323, and 0.693, respectively). Stem starch in juniper could be extremely small (~0.2%), offering little potential to use NSCs under stress. Differences in NSC storage may be related to drought-induced photosynthetic inhibition as growing season starch depletion was evident in pinyon (stems: 0.012±0.002% vs. 0.003±0.005% [pre- vs. post-monsoon]) and juniper (leaves: 0.035±0.005% vs. 0.015±0.013%). Starch draw-down following the recent El Niño suggests NSC reserves are used for growth and maintenance across a range of moisture conditions.