Rapid development of human society has triggered unprecedented global environmental changes, including the well documented atmospheric carbon dioxide (CO₂) concentration elevation. Plants may buffer increasing atmospheric CO₂ concentrations through enhanced growth, but the question whether other essential resources availability (water, nitrogen, etc) constrains the magnitude of this ecosystem service remains unresolved.  Previous research in controlled environments supports idea that plant productivity, particularly in trees, is stimulated by increased amounts of carbon dioxide. Yet in most natural forest ecosystems, the rising CO₂ concentrations has seldom translated into increased tree growth, instead recent declines in productivity and even high mortality rates have been observed across biomes. Nevertheless, Qinghai-Tibetan Plateau Alpine ecosystems seem to be an exception, where the combined effect of climate warming and rising carbon dioxide levels has resulted in growth acceleration of trees, coincident with the expansion of forests into grasslands that have dominated the region for millennia. 

　　 Seeking to investigate the mechanisms speeding up the growth of trees in Alpine Tibetan forests, an international collaborative team from Chengdu Institute of Biology and University of California, Davis, supervised by Prof. Sun Geng, explored plant-soil-atmosphere interactions and their effects on tree growth and resource use, combining dendrochronological and isotopic measurements. At Hongyuan County (33°03.858′N, 102°48.847′E, 3500 a.sl.), they sampled the dominant Abies faxoniana trees from a vegetation gradient in spanning forests, forest patches, and isolated trees growing in open grasslands.  

　　 Dendrochronological analyses show a steady increase in tree growth since the early 1900s, which was intensified during the 1930s and 1960s, and have reached unprecedented levels since 1760. Moreover, trees have grown roughly 60% faster in forest patches and on the forest border, compared with trees located in the forest interior. And, small/young trees in the grasslands have grown slightly faster than large/old trees in the forest interior. These differences in the timing and magnitude of tree growth acceleration in different habitats may be due to differences in resource availability. Measurements of stable isotopes (carbon, oxygen, and nitrogen) in tree rings indicate that the boost of tree growth has been stimulated by the synergistic effect of rising atmospheric CO₂ and a warming-induced increase in water and nutrient availability from thawing permafrost, which enhanced the physiological performance of dominant trees leading to forest expansion. 

　　 The findings illustrate the importance of considering soil-plant- atmosphere interactions to understand current and anticipate future changes in productivity and distribution of forest ecosystems. Our results show that long-term positive responses in forest productivity due to changes in climate and atmospheric composition are possible and can be predicted based on edaphic properties. Our study will incur great enthusiasm by researchers especially form working in Tibetan Plateau, and supply comprehensive framework to understand water-soil-air-ecosystem interactions over heterogeneous landscapes and scientific evaluation for the environment and resource effects under climate changes. 

　　 Our study entitled “Tree growth acceleration and expansion of Tibetan forests are caused by the synergistic effect of climatic and edaphic change” has been online published in Science Advances. 

Contact:

Sun Geng

Chengdu Institute of Biology