Tibetan Plateau (TP) is known as the "Water Tower" in Asia, yet it is unclear whether or not water supply to the north and south of the plateau is the same. Dr. Qi-Bin Zhang`s research group at the State Key Laboratory of Vegetation and Environmental Change, Institute of Botany of the Chinese Academy of Sciences, answered this question from studies of tree rings. They found that May-June moisture condition exists difference in the north and south of the plateau during the past five and a half centuries. Particularly, a "wet south and dry north" condition occurred in 1463-1502 and 1693-1734 CE. On August 21, Nature Communications reports their findings in an article entitled "Moisture Dipole over the Tibetan Plateau during the past five and a half centuries".
(http://www.nature.com/naturecommunications).
Historical hydroclimate records on the TP is of importance for understanding driving factors causing changes in water resources. Data of the past few decades can be obtained from observational stations, and pre-instrumental records relies on climate proxies such as tree rings. However, it is not an easy job to obtain such tree-ring records on the TP. On the one hand, old growth forests are unevenly distributed at remote mountain areas of the plateau, making it difficult to collect tree-ring samples that is old in age and dense in spatial coverage within a short time. On the other hand, the growth of tree rings is affected by micro-climate due to mountainous topography, thus bringing difficulties in extracting regional hydroclimate signals from tree rings.
Dr. Zhang`s group has conducted TP tree-ring research for more than 10 years. They have collected a large number of samples over large areas of the TP and established a tree-ring databank following dendrochonological technuqie of crossdating. From this databank, they were able to select 23 tree-ring chronologies that are of the same genus in Juniperus and also all sensitive to May-June moisture condition (represented by PDSI index). They found that 9 chronologies north of the 33 degree north latitude and 14 chronologies south of the latitude have the same cluster characteristics, respectively. Chronologies combined for the north and for the south contain strong May-June PDSI regional signals. Accordingly, north and south tree-ring chronologies were used to reconstruct May-June PDSI history for the past five and a half centuries. Comparisons of the north and south moisture history reveal two significant and prolonged "wet south and dry north" intervals.
The results obtained in this study is useful for further understanding how the TP water resources is influenced by interaction of the Westerlies and the South Asian Monsoon. The data also help studying spatiotemperal patterns in forest ecological processes on the TP.
The study is supported by key and regular projects from the National Natural Science Foundation of China. Co-authors of the article are Dr. Michael Evans from the University of Maryland, USA and Dr. Lixin Lyu from the Institute of Botany, Chinese Academy of Sciences.