Increased intrinsic water-use efficiency during a period with persistent decreased tree radial growth in northwestern China: Causes and implications

To understand how tree growth responds to rising atmospheric CO2 concentration ([CO2]) and climate change in arid and semi-arid regions, it is important to determine how trees in natural forests adjust physiologically to the changing atmospheric environment. Here, we developed an annual-resolution tree-ring stable carbon isotope (delta C-13) chronology from the dominant natural spruce (Picea crassifolia) trees in the Xinglong Mountains, in the eastern part of northwestern China, from 1800 to 2009. The climate response recorded in the tree-ring delta C-13 residuals indicated that insufficient moisture during the previous autumn and the current growing season was the dominant climatic factor that controlled tree-ring carbon isotope discrimination in the semi-arid Xinglong Mountains. By combining the tree-ring delta C-13, the atmospheric [CO2] (C-a), and ring width data, and by calculating the variations in leaf intercellular [CO2] (C-i), their ratio (C-i/C-a), intrinsic water-use efficiency (iWUE), and basal area increment (BAI), we found that the C-i/C-a remained mostly constant during the last century whereas C-i and iWUE increased significantly. The increase in iWUE totaled about 40% by the end of the study period. However, after 1998, the trees switched from an active to a passive response to elevated atmospheric [CO2], leading to relatively stable iWUE. Since 1800, tree radial growth did not always parallel the increase in iWUE; the tree growth rate declined during drought periods, and especially during the severe droughts that occurred from 1923 to 1934 and recent decade. This response is a concern for future forest management and conservation under the current climate change scenario. (C) 2012 Elsevier B.V. All rights reserved.