树轮记录的过去300年来青藏高原东南部积雪覆盖的变化

INTERDECADAL VARIATIONS OF THE SNOW COVER IN THE SOUTHEASTERN TIBETAN PLATEAU RECORDED IN TREE RINGS

由于高海拔和高降水，青藏高原东南部成为全球少有的积雪集中分布的亚热带地区。其积雪覆盖的变化受到亚洲夏季风的强烈影响，同时积雪覆盖又能通过改变青藏高原东南部的热效应，对亚洲夏季风的动态造成影响。然而当前器测记录的积雪覆盖面积只有几十年，限制了对高原积雪面积变化和亚洲季风动态关系的探讨，尤其是在年代际尺度上二者的关系。本文首次开展了基于树轮的青藏高原积雪覆盖重建的研究。通过收集了青藏高原东南部的25个树轮年表，发现其中9个树轮年表和积雪覆盖存在显著正相关；进一步研究发现这些树轮年表和积雪覆盖的关系主要表现在年代际波动上，在年际尺度上的波动两者的关系不显著。这9个树轮年表的年代际波动在历史时期也能很好的吻合，进一步说明这些树轮序列受到共同的积雪覆盖变化的影响，且这种影响在年代际尺度上比较稳定。积雪覆盖和干旱指数表现出显著正相关以及9个年表均表现出干旱响应，说明积雪覆盖可以通过调制土壤湿度来影响树轮生长。年际尺度上树轮生长主要受到区域气候条件的影响，而年代际尺度上，积雪覆盖通过调控区域气候和土壤湿度，对树轮生长起到决定性的影响。基于这些树轮年表，定量重建了青藏高原东南部过去300年来积雪面积的年代际波动，并讨论它与亚洲季风动态的关系。

The southeastern Tibetan Plateau has one of largest subtropical snow cover due to its low temperature and high precipitation. Changes of the snow cover in the southeastern Tibetan Plateau are not only influenced by the Asian summer monsoon but can also strongly feedback the dynamics of the monsoon by changing the thermal conditions of the plateau. The brevity of the instrumental snow cover data of a few decades has limited our ability to detect the interactions between snow cover and Asian summer monsoon, particularly for the interdecadal scale. This is a first tree-ring based snow cover reconstruction for the Tibetan Plateau. We collected 25 tree-ring chronologies in the southeastern Tibetan Plateau （28°~32°N, 92°~98°E） and found 9 of them are significantly correlated with the snow cover changes. These 9 chronologies concentrate in northern Yunnan Province in the rain shadow region of the mountain ranges between Myanmar and China. Tree rings in these relatively dry regions are generally sensitive to moisture. Further, we found that the good matches between snow cover and tree-ring variations are only for the interdecadal changes but not conspicuous for the interannual changes. The interdecadal variations of these tree-ring chronologies agree with each other well through their common period of the past 300 years. This suggests that these chronologies are likely controlled by one environmental factor of the snow cover, and the relationships between tree growth and snow cover at the interdecadal time scale are stable over time. The snow cover modulates the growth of tree rings via modulating the soil moisture as indicated by the high correlations between tree growth and the snow cover and the drought index. The influences of snow cover on tree growth outcompete the other factors at the interdecadal scale, while at the interannual scale, tree growth may be dominated by the local climate change as indicated by varying interannual variations among these 9 tree-ring chronologies with coherent interdecadal variations. Our study adds in evidences that different environmental factors may modulate tree growth at different time scales. This study reconstructed the snow cover changes in the southeastern Tibetan Plateau over the past 300 years using tree rings and explored its linkages with the Asian summer monsoon. The tree-ring based snow cover reconstructions agree well with the drought reconstructions from the local historical documets.