Disentangling seasonal and interannual legacies from inferred patterns of forest water and carbon cycling using tree‐ring stable isotopes

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 δ C and δ O isotope ratios in the earlywood (EW) and latewood (LW) fractions of Pinus ponderosa trees at thirteen sites across a latitudinal gradient influenced by the North American Monsoon (NAM) climate system. We observed that δ C from EW and LW has significant positive cross-correlations at most sites, whereas EW and LW δ O values were cross-correlated at about half the sites. Using combined statistical and mechanistic models, we show that cross-correlations in both δ C and δ O 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 δ C and δ O 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.

Legacies of La Niña: North American monsoon can rescue trees from winter drought

“…Pairs of annual weights that do not differ between OY and LNP periods are colored gray. There is a continuing need for experimental approaches to understanding the physiology of drought legacies and to disentangle the processes integrating physiological and climatic effects into whole ring width over a growing season (e.g., Szejner et al, 2018). Shifts in temperature-related-but not precipitation-effects also occurred for P. ponderosa, and changes in temperature effects on growth of P. ponderosa are consistent with previous analyses showing shorter recovery times of precipitation sensitivity, while temperature sensitivity recovery times can be as long as~5 years for this species (Peltier et al, 2016).…”

“…Recent studies are also implementing a more direct approach to understanding sub-annual controls on sub-annual ring width processes in this region (Belmecheri, Wright, Szejner, Morino, & Monson, 2018;Szejner et al, 2018). Recent studies are also implementing a more direct approach to understanding sub-annual controls on sub-annual ring width processes in this region (Belmecheri, Wright, Szejner, Morino, & Monson, 2018;Szejner et al, 2018).…”

“…are alsõ 5 years (Peltier et al, 2016), we speculate that decreases in AR1 effects, but not climate sensitivities, could suggest climate legacies in this species are related to physiological traits such as non-structural carbohydrate (NSC) dynamics, rather than changes in precipitation sensitivity. There is a continuing need for experimental approaches to understanding the physiology of drought legacies and to disentangle the processes integrating physiological and climatic effects into whole ring width over a growing season (e.g., Szejner et al, 2018).…”

“…Here, we take a statistical approach to understand how fine-scale (monthly) climate affects variability in annual growth (ring widths) under different climate conditions. Recent studies are also implementing a more direct approach to understanding sub-annual controls on sub-annual ring width processes in this region (Belmecheri, Wright, Szejner, Morino, & Monson, 2018;Szejner et al, 2018).…”

Fire deficits have increased drought sensitivity in dry conifer forests: Fire frequency and tree‐ring carbon isotope evidence from Central Oregon

“…Alternatively, other tree-ring carbon isotope studies of Western US forests used pentadal or decadal sampling units that preclude determinations of sensitivity to meteorological drought on a year-by-year basis (Feng, 1998;Knapp & Soulé, 2011;Knapp, Soulé, & Maxwell, 2013;Leavitt & Long, 1988;Reed, Ballantyne, Cooper, & Sala, 2018;Soulé & Knapp, 2015). Additional ponderosa pine treering carbon isotope records with interannual resolution have been developed but offer no insight on responses to competition since the sites chosen had widely spaced trees to help ensure the stability of climate responses (Szejner et al, 2016;Szejner et al, 2018).…”