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1. Stable isotopes of tree rings reveal seasonal-to-decadal patterns during the emergence of a megadrought in the Southwestern US

   https://doi.org/10.1007/s00442-021-04916-9  

   Szejner, Paul ; Belmecheri, Soumaya ; Babst, Flurin ; Wright, William E. ; Frank, David C. ; Hu, Jia ; Monson, Russell K. ( April 2021 , Oecologia)

   null (Ed.)

   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. 

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2. Drought increase since the mid-20th century in the northern South American Altiplano revealed by a 389-year precipitation record

   https://doi.org/10.5194/cp-19-457-2023  

   Morales, Mariano S. ; Crispín-DelaCruz, Doris B. ; Álvarez, Claudio ; Christie, Duncan A. ; Ferrero, M. Eugenia ; Andreu-Hayles, Laia ; Villalba, Ricardo ; Guerra, Anthony ; Ticse-Otarola, Ginette ; Rodríguez-Ramírez, Ernesto C. ; et al ( January 2023 , Climate of the Past)

   Abstract. Given the short span of instrumental precipitationrecords in the South American Altiplano, longer-term hydroclimatic recordsare needed to understand the nature of climate variability and to improvethe predictability of precipitation, a key natural resource for thesocioeconomic development in the Altiplano and adjacent arid lowlands. Inthis region grows Polylepis tarapacana, a long-lived tree species that is very sensitive tohydroclimatic changes and has been widely used for tree-ring studies in thecentral and southern Altiplano. However, in the northern sector of thePeruvian and Chilean Altiplano (16–19∘ S)still exists a gap of high-resolution hydroclimatic data based on tree-ringrecords. Our study provides an overview of the temporal evolution of thelate-spring–mid-summer precipitation for the period 1625–2013 CE at thenorthern South American Altiplano, allowing for the identification of wet ordry periods based on a regional reconstruction from three P. tarapacana chronologies. Anincrease in the occurrence of extreme dry events, together with a decreasingtrend in the reconstructed precipitation, has been recorded since the 1970sin the northern Altiplano within the context of the last ∼4 centuries. The average precipitation over the last 17 years stands outas the driest in our 389-year reconstruction. We reveal a temporal andspatial synchrony across the Altiplano region of dry conditions since themid-1970s. Independent tree-ring-based hydroclimate reconstructions andseveral paleoclimatic records based on other proxies available for thetropical Andes record this synchrony. The influence of El Niño–SouthernOscillation (ENSO) on the northern Altiplano precipitation was detected byour rainfall reconstruction that showed past drier conditions in our studyregion associated with ENSO warm events. The spectral properties of therainfall reconstruction showed strong imprints of ENSO variability atdecadal, sub-decadal, and inter-annual timescales, in particular from thePacific NIÑO 3 sector. Overall, the recent reduction in precipitation incomparison with previous centuries, the increase in extreme dry events andthe coupling between precipitation and ENSO variability reported by thiswork is essential information in the context of the growing demand for waterresources in the Altiplano. This study will contribute to a betterunderstanding of the vulnerability and resilience of the region to theprojected evapotranspiration increase for the 21st century associated withglobal warming. 

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3. Correcting tree-ring δ13C time series for tree-size effects in eight temperate tree species

   https://doi.org/10.1093/treephys/tpz138  

   Vadeboncoeur, Matthew A ; Jennings, Katie A ; Ouimette, Andrew P ; Asbjornsen, Heidi ( January 2020 , Tree Physiology)

   Cernusak, Lucas (Ed.)

   Abstract Stable carbon isotope ratios (δ13C) in tree rings have been widely used to study changes in intrinsic water-use efficiency (iWUE), sometimes with limited consideration of how C-isotope discrimination is affected by tree height and canopy position. Our goals were to quantify the relationships between tree size or tree microenvironment and wood δ13C for eight functionally diverse temperate tree species in northern New England and to better understand the physical and physiological mechanisms underlying these differences. We collected short increment cores in closed-canopy stands and analyzed δ13C in the most recent 5 years of growth. We also sampled saplings in both shaded and sun-exposed environments. In closed-canopy stands, we found strong tree-size effects on δ13C, with 3.7–7.2‰ of difference explained by linear regression vs height (0.11–0.28‰ m−1), which in some cases is substantially stronger than the effect reported in previous studies. However, open-grown saplings were often isotopically more similar to large codominant trees than to shade-grown saplings, indicating that light exposure contributes more to the physiological and isotopic differences between small and large trees than does height. We found that in closed-canopy forests, δ13C correlations with diameter at breast height were nonlinear but also strong, allowing a straightforward procedure to correct tree- or stand-scale δ13C-based iWUE chronologies for changing tree size. We demonstrate how to use such data to correct and interpret multi-decadal composite isotope chronologies in both shade-regenerated and open-grown tree cohorts, and we highlight the importance of understanding site history when interpreting δ13C time series. 

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4. Hydroclimate and ENSO Variability Recorded by Oxygen Isotopes From Tree Rings in the South American Altiplano

   https://doi.org/10.1029/2021GL095883  

   Rodriguez‐Caton, Milagros ; Andreu‐Hayles, Laia ; Daux, Valérie ; Vuille, Mathias ; Varuolo‐Clarke, Arianna M. ; Oelkers, Rose ; Christie, Duncan A. ; D’Arrigo, Rosanne ; Morales, Mariano S. ; Palat Rao, Mukund ; et al ( February 2022 , Geophysical Research Letters)

   Hydroclimate variability in tropical South America is strongly regulated by the South American Summer Monsoon (SASM). However, past precipitation changes are poorly constrained due to limited observations and high‐resolution paleoproxies. We found that summer precipitation and the El Niño‐Southern Oscillation (ENSO) variability are well registered in tree‐ring stable oxygen isotopes (δ¹⁸O_TR) ofPolylepis tarapacanain the Chilean and Bolivian Altiplano in the Central Andes (18–22°S, ∼4,500 m a.s.l.) with the northern forests having the strongest climate signal. More enrichedδ¹⁸O_TRvalues were found at the southern sites likely due to the increasing aridity toward the southwest of the Altiplano. The climate signal ofP. tarapacana δ¹⁸O_TRis the combined result of moisture transported from the Amazon Basin, modulated by the SASM, ENSO, and local evaporation, and emerges as a novel tree‐ring climate proxy for the southern tropical Andes.

    

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5. Global tree intrinsic water use efficiency is enhanced by increased atmospheric CO 2 and modulated by climate and plant functional types

   https://doi.org/10.1073/pnas.2014286118  

   Mathias, Justin M. ; Thomas, Richard B. ( February 2021 , Proceedings of the National Academy of Sciences)

   We conducted a meta-analysis of carbon and oxygen isotopes from tree ring chronologies representing 34 species across 10 biomes to better understand the environmental drivers and physiological mechanisms leading to historical changes in tree intrinsic water use efficiency (iWUE), or the ratio of net photosynthesis (A_net) to stomatal conductance (g_s), over the last century. We show a ∼40% increase in tree iWUE globally since 1901, coinciding with a ∼34% increase in atmospheric CO₂(C_a), although mean iWUE, and the rates of increase, varied across biomes and leaf and wood functional types. While C_awas a dominant environmental driver of iWUE, the effects of increasing C_awere modulated either positively or negatively by climate, including vapor pressure deficit (VPD), temperature, and precipitation, and by leaf and wood functional types. A dual carbon–oxygen isotope approach revealed that increases inA_netdominated the observed increased iWUE in ∼83% of examined cases, supporting recent reports of global increases inA_net, whereas reductions ing_soccurred in the remaining ∼17%. This meta-analysis provides a strong process-based framework for predicting changes in tree carbon gain and water loss across biomes and across wood and leaf functional types, and the interactions between C_aand other environmental factors have important implications for the coupled carbon–hydrologic cycles under future climate. Our results furthermore challenge the idea of widespread reductions ing_sas the major driver of increasing tree iWUE and will better inform Earth system models regarding the role of trees in the global carbon and water cycles.

    

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