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

Guobao Xu,Xiaohong Liu,Bo Wang,Soumaya Belmecheri,Xiaomin Zeng,Wenzhi Wang,Tuo Chen,Guoju Wu

doi:10.3390/f9100642

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.

Highlights

The global carbon and hydrological cycles are coupled through leaf gas exchange [1,2]
A negative relationship between vapor pressure deficit (VPD) and gs was observed for Larix [43], and gs was affected by VPD and drought stress [8,42,44,45]
Tree-ring width showed a weak negative relationship with percent change of intrinsic water-use efficiency (iWUE) during 1963–1972 in the 11-year consecutive interval periods for medium-aged trees (Figure S5, Supplementary Materials). These results suggest that iWUE had a weak effect on tree-ring growth and did not fully confirm the enhancement of tree-ring growth

Summary

Introduction

The global carbon and hydrological cycles are coupled through leaf gas exchange [1,2]. Water-use efficiency (WUE), the ratio of CO2 assimilation to water loss, is a critical link between carbon and the terrestrial water cycle. WUE has been identified as an effective indicator to assess ecosystem and forest response to climate change and the rising CO2 concentration in the atmosphere (CO2atm ) [3,4,5]. The stable carbon isotope ratios (δ13 C) in tree rings can be used to estimate the intrinsic water-use efficiency (iWUE) [6,7] and the plant’s physiological response to CO2atm , often explored in strategies for stomatal regulation of leaf gas exchange [8,9,10]. Variability of gs is related to relative humidity, vapor pressure deficit (VPD) and other climatic variables [7,11]

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Conclusion