Elevation-dependent variations of tree growth and intrinsic water-use efficiency in Schrenk spruce (Picea schrenkiana) in the western Tianshan Mountains, China.

Guoju Wu,Xiaomin Zeng,Wenzhi Wang,Tuo Chen,Guobao Xu,Xiaohong Liu,Xuanwen Zhang

doi:10.3389/fpls.2015.00309

Abstract

Rising atmospheric CO2 concentration (Ca) is expected to accelerate tree growth by enhancing photosynthesis and increasing intrinsic water-use efficiency (iWUE). However, the extent of this effect on long-term iWUE and its interactions with climate remains unclear in trees along an elevation gradient. Therefore, we investigated the variation in the radial growth and iWUE of mature Picea schrenkiana trees located in the upper tree-line (A1: 2700 m a.s.l.), middle elevation (A2: 2400 m a.s.l.), and lower forest limit (A3: 2200 m a.s.l.), in relation to the rising Ca and changing climate in the Wusun Mountains of northwestern China, based on the basal area increment (BAI) and tree-ring δ13C chronologies from 1960 to 2010. We used the CRU TS3.22 dataset to analyze the general response of tree growth to interannual variability of regional climate, and found that BAI and δ13C are less sensitive to climate at A1 than at A2 and A3. The temporal trends of iWUE were calculated under three theoretical scenarios, as a baseline for interpreting the observed gas exchange at increasing Ca. We found that iWUE increased by 12–32% from A1 to A3 over the last 50 years, and showed an elevation-dependent variation in physiological response. The significant negative relationship between BAI and iWUE at A2 and A3 showed that tree growth has been decreasing despite long-term increases in iWUE. However, BAI remained largely stable throughout the study period despite the strongest iWUE increase [at constant intercellular CO2 concentration (Ci) before 1980] at A1. Our results indicate a drought-induced limitation of tree growth response to rising CO2 at lower elevations, and no apparent change in tree growth and diminished iWUE improvement since 1980 in the upper tree-line. This study may contradict the expectation that combined effects of elevated Ca and rising temperatures have increased forest productivity, especially in high-elevation forests.

Highlights

Atmospheric changes, and the recent humaninduced increases in carbon dioxide (CO2) concentrations and temperature, significantly affect tree growth (Martinez-Vilalta et al, 2008; Allen et al, 2010; Koutavas, 2013)
Ci and basal area increment (BAI) were significantly lower and intrinsic water-use efficiency (iWUE) was significantly higher at the lower elevations (ANOVA, p < 0.01; Figures 3F–H)
We found a warmth-induced drought limitation on tree growth accompanied by increasing iWUE at the lowerelevation sites in the spruce forest

Summary

Introduction

Atmospheric changes, and the recent humaninduced increases in carbon dioxide (CO2) concentrations and temperature, significantly affect tree growth (Martinez-Vilalta et al, 2008; Allen et al, 2010; Koutavas, 2013). Rapid changes in the atmospheric CO2 concentration (Ca) can dramatically affect short-term plant physiology and growth (Martinez-Vilalta et al, 2008), and research has shown that plants increase their intrinsic water-use efficiency (iWUE) in response to increasing CO2 (Morison, 1993; Morgan et al, 2004). Compare to these short-term experimental results, the long-term variations on the physiological responses to increased Ca, obtained from the carbon isotope discrimination ( 13C) of tree-ring series, give insight into how trees respond to increasing atmospheric CO2 concentrations under naturally growing conditions (Peñuelas et al, 2008, 2011). There have been insufficient studies on the simultaneous elevationdependent effects on tree growth in response to increasing Ca and climate warming

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Conclusion