Disentangling Contributions of CO2 Concentration and Climate to Changes in Intrinsic Water-Use Efficiency in the Arid Boreal Forest in China’s Altay Mountains

Abstract

Intrinsic water-use efficiency (iWUE) is affected by the balance of photosynthetic rate, stomatal conductance, and climate, along with many other exogenous factors, such as the CO2 concentration in the atmosphere (CO2atm), nutrients, and water holding capacity of the soil. The relative contributions of CO2atm and climate to iWUE are still incompletely understood, particularly for boreal forests where the climate is undergoing unprecedented warming. We combined δ13C and δ18O in tree rings from the Siberian larch (Larix sibirica Ledeb.) in Northwestern China’s Altay Mountains, which receives 190 mm in annual precipitation, to detect the drivers of long-term iWUE changes and their time-varying contributions over the past 222 years. A climate optimization approach was used to isolate the influence of climate from CO2atm influence on iWUE. We found that iWUE increased about 33.6% from 1790 to 2011, and rising CO2atm contributed 48.8% to this iWUE increase. The contributions of CO2atm and climate (drought conditions) varied during the study period 1790–2011. From 1790 to 1876, the climate was the most important factor contributing to the changes in iWUE. From 1877 to 1972, CO2atm was the main contributor; however, after 1973, the climate was again the dominant contributor to the increase in iWUE, especially during 1996–2011. During the period 1996–2011, climate substantially (83%) contributed to the iWUE increase. Our findings imply that, in the boreal forest in Northwestern China’s arid region, iWUE experienced three changes: (1) the climate dominating from 1790 to 1876; (2) CO2atm dominating from 1877 to 1972, and (3) climate dominating again during the past four decades. We observed that the relationships between iWUE and tree-ring width shifted from positive to negative from 1996 onwards. These relationship changes indicate that CO2atm-mediated effects of increasing iWUE on tree growth are counteracted by climatic drought stress and iWUE increase cannot counter the stress from drought on tree growth in China’s arid boreal forest.

Carbon and Water Use Efficiencies: A Comparative Analysis of Ten Terrestrial Ecosystem Models under Changing Climate

“…In the arctic and boreal regions, models’ CUE and WUE are driven by atmospheric CO 2 levels and, to a lesser extent, by the changing climate (Figs 3 and 4). This contrast studies showing that climate is the dominant contributor to the increase in intrinsic WUE in boreal forest in the last five decades 21,40 . Also, there is a significant disagreement for the dominance environmental drivers for topical Asia with three models (LPJ-wsl, ORCHIDEE- LSCE, and VEGAS2.1) showing climate as the important driver, unlike the rest of the models (Fig.…”

“…The leaf ecophysiological properties indicate that CUE and WUE should increase due to increasing atmospheric CO 2 - known as “the CO 2 fertilization effect”. CO 2 enrichment experiments 3,17,18 , ecosystem models, isotope analysis 19–21 , and WUE model (WUE estimated independently from GPP and evapotranspiration) 22 , support an increase in plant WUE as atmospheric CO 2 concentration increases. However, the response of CUE to changes in atmospheric CO 2 levels is largely unknown leading to a debate about the role of nutrients limitations 10,19 .…”

Observed and modelled historical trends in the water‐use efficiency of plants and ecosystems

“…Another methodological issue when using stable carbon isotopes in tree rings is related to data preprocessing before assessing iWUE leaf trends. Two recent studies (Frank et al, 2015;Xu et al, 2018) performed an empirical adjustment of carbon isotopic series (11)…”

Age‐Related Climate Response of Tree‐Ring δ13C and δ18O From Spruce in Northwestern China, With Implications for Relative Humidity Reconstructions

“…In northwestern China, tree‐ring width data are only weakly ( r < 0.48 or not significant), if at all, sensitive to RH variability (Wang et al, 2007; Xu, Liu, Belmecheri, et al, 2018), and other proxies are therefore needed to reconstruct this climate parameter. RH, VPD, and temperature are important to plant photosynthesis, transpiration, and growth (Farquhar et al, 1982; Restaino et al, 2016).…”