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Recent evidence has revealed the emergence of a megadrought in southwestern North America since 2000. Megadroughts extend for at least 2 decades, making it challenging to identify such events until they are well established. Here, we examined tree-ring growth and stable isotope ratios in Pinus ponderosa at its driest niche edge to investigate whether trees growing near their aridity limit were sensitive to the megadrought climatic pre-conditions, and were capable of informing predictive efforts. During the decade before the megadrought, trees in four populations revealed increases in the cellulose δ13C content of earlywood, latewood, and false latewood, which, based on past studies are correlated with increased intrinsic water-use efficiency. However, radial growth and cellulose δ18O were not sensitive to pre-megadrought conditions. During the 2 decades preceding the megadrought, at all four sites, the changes in δ13C were caused by the high sensitivity of needle carbon and water exchange to drought trends in key winter months, and for three of the four sites during crucial summer months. Such pre-megadrought physiological sensitivity appears to be unique for trees near their arid range limit, as similar patterns were not observed in trees in ten reference sites located along a latitudinal gradient in the same megadrought domain, despite similar drying trends. Our results reveal the utility of tree-ring δ13C to reconstruct spatiotemporal patterns during the organizational phase of a megadrought, demonstrating that trees near the arid boundaries of a species’ distribution might be useful in the early detection of long-lasting droughts. 

Across forests, photosynthesis and woody growth respond to different climate cues. 

Multiple lines of evidence suggest that plant water-use efficiency (WUE)—the ratio of carbon assimilation to water loss—has increased in recent decades. Although rising atmospheric CO 2 has been proposed as the principal cause, the underlying physiological mechanisms are still being debated, and implications for the global water cycle remain uncertain. Here, we addressed this gap using 30-y tree ring records of carbon and oxygen isotope measurements and basal area increment from 12 species in 8 North American mature temperate forests. Our goal was to separate the contributions of enhanced photosynthesis and reduced stomatal conductance to WUE trends and to assess consistency between multiple commonly used methods for estimating WUE. Our results show that tree ring-derived estimates of increases in WUE are consistent with estimates from atmospheric measurements and predictions based on an optimal balancing of carbon gains and water costs, but are lower than those based on ecosystem-scale flux observations. Although both physiological mechanisms contributed to rising WUE, enhanced photosynthesis was widespread, while reductions in stomatal conductance were modest and restricted to species that experienced moisture limitations. This finding challenges the hypothesis that rising WUE in forests is primarily the result of widespread, CO 2 -induced reductions in stomatal conductance. 

Abstract Tree‐ring carbon and oxygen isotope ratios have been used to understand past dynamics in forest carbon and water cycling. Recently, this has been possible for different parts of single growing seasons by isolating anatomical sections within individual annual rings. Uncertainties in this approach are associated with correlated climate legacies that can occur at a higher frequency, such as across successive seasons, or a lower frequency, such as across years. The objective of this study was to gain insight into how legacies affect cross‐correlation in the δ¹³C and δ¹⁸O isotope ratios in the earlywood (EW) and latewood (LW) fractions ofPinus ponderosatrees at thirteen sites across a latitudinal gradient influenced by the North American Monsoon (NAM) climate system. We observed that δ¹³C from EW and LW has significant positive cross‐correlations at most sites, whereas EW and LW δ¹⁸O values were cross‐correlated at about half the sites. Using combined statistical and mechanistic models, we show that cross‐correlations in both δ¹³C and δ¹⁸O can be largely explained by a low‐frequency (multiple‐year) mode that may be associated with long‐term climate change. We isolated, and statistically removed, the low‐frequency correlation, which resulted in greater geographical differentiation of the EW and LW isotope signals. The remaining higher‐frequency (seasonal) cross‐correlations between EW and LW isotope ratios were explored using a mechanistic isotope fractionation–climate model. This showed that lower atmospheric vapor pressure deficits associated with monsoon rain increase the EW‐LW differentiation for both δ¹³C and δ¹⁸O at southern sites, compared to northern sites. Our results support the hypothesis that dominantly unimodal precipitation regimes, such as near the northern boundary of the NAM, are more likely to foster cross‐correlations in the isotope signals of EW and LW, potentially due to greater sharing of common carbohydrate and soil water resource pools, compared to southerly sites with bimodal precipitation regimes.