The aridity influence on oxygen isotopes recorded in tree rings.

This article investigates how the apparent proportion of oxygen isotope exchange (apex) during cellulose synthesis influences the recording of source water and atmospheric aridity signals in the stable oxygen isotopes (δ¹⁸O) of wood cellulose (δ¹⁸O_cell) from Ponderosa pine (Pinus ponderosa) stands at the northern boundary of the North American Monsoon (NAM) climate system. By analyzing δ¹⁸O of xylem water, leaf water, and cellulose from seven forest sites with varying aridity, the study finds that apex varies with site aridity—higher apex values occur under drier conditions with higher vapor pressure deficit (VPD), leading to under-representation of VPD signals in δ¹⁸O_cell, while wetter sites with lower apex over-represent VPD signals. The research highlights the importance of considering seasonal growth periods (earlywood vs. latewood) and variable apex values to improve isotope-based reconstructions of past hydroclimate and plant-water relations, especially in semi-arid to arid regions with bimodal precipitation regimes like the NAM area.