Abstract
Tree-level intrinsic water-use efficiency (iWUE) is derived from the tree-ring 13C isotope composition (δ13C) and is an important indicator of the adaptability for trees to climate change. However, there is still uncertainty regarding the relationship between long-term forest ecosystem carbon sequestration capacity and iWUE. To determine whether elevated atmospheric CO2 concentration (Ca) increase iWUE and tree growth (basal area increment, BAI), dendrochronological methods and stable isotope analyses were used to examine annual changes in the tree-ring width and carbon isotope composition (δ13C) of Platycladus orientalis in northern China. The iWUE derived from δ13C has increased significantly (p < 0.01). Long-term iWUE trend was largely and positively driven by the elevated atmospheric CO2 concentration and temperature. We observed a general increase in averaged BAI, which had significant positive correlation with iWUE (R2 = 0.3186, p < 0.01). Increases in iWUE indeed translated into enhanced P. orientalis growth in semi-arid areas of northern China. Elevated atmospheric CO2 concentration significantly (p < 0.01) stimulated P. orientalis biomass accumulation when Ca was less than approximately 320 ppm in the early phase; however, this effect was not pronounced when Ca exceeded 320 ppm.
Highlights
Many studies have shown that rising atmospheric CO2 concentrations can speed up tree growth by stimulating photosynthesis in a process known as “CO2 fertilization”; most of these studies were conducted www.nature.com/scientificreports/
Platycladus orientalis, an evergreen coniferous tree species that is endemic to China, is the main tree species used for afforestation in the semi-arid areas of northern China
Plant tissue δ13C is significantly lower than that found in the atmosphere because atmospheric CO2 contains 98.9% of light-stable carbon isotope and 1.1% heavy-stable carbon isotope, and the light-stable carbon isotope is preferentially absorbed during photosynthesis, resulting in carbon isotope fractionation, which is known as the “carbon isotope effect”[39]
Summary
Many studies have shown that rising atmospheric CO2 concentrations can speed up tree growth by stimulating photosynthesis in a process known as “CO2 fertilization”; most of these studies were conducted www.nature.com/scientificreports/. Studies of tree-ring stable carbon isotopes at a global scale have found variable tree responses to climate change, reflecting the complex relationship between environmental variability and tree growth, and suggest that adaptation strategies vary widely across species and sites[22,23,24,25]. The Yanshan Mountains of northern China, between the Mongolia Plateau and the North China Plain, are forested and dominated by P. orientalis. This area plays a significant role in the global carbon cycle due to its typical monsoon climate. The specific goals of the study were to (1) determine whether tree-level iWUE increased over time (i.e., across subsequent calendar years) and whether that translated into tree growth and (2) assess how iWUE and tree growth (as measured by the basal area increment [BAI]) responded to changes in atmospheric CO2 concentrations, annual temperature, and precipitation
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