利用香柏年轮揭示青藏高原中南部过去200年草地生长动态

In the south central Tibetan Plateau, the climate is characterized by typical semiarid conditions which the precipitation gradually decreases spanning the east to west gradient. The vegetation type is dominated by alpine grassland. Juniperus pingii var. wilsonii is one of the shrub species distributing in the grassland. Normalized Difference Vegetation Index (NDVI) could well represent the grassland growth and biomass variations in large spatial scale. However, the instrument period of measurement of NDVI series is too short to show vegetation growth in longer temporal scales. Thus, we will establish the shrub ring width chronologies in 5 sub-regions in the south central Tibetan Plateau. We will also examine the associations between ring width chronologies and growth season NDVI series. We then reconstruct the NDVI series of the past 200 years and compare the NDVI variations in different regions. We also assess the relationships between reconstructed NDVI and past climate change. Our effort contributes to use the shrub ring to indicate the grassland growth in the past 200 years in south central Tibetan Plateau. The result provides knowledge about sensitivity of the alpine grassland ecosystem to long term climate change.

青藏高原中南部属于半干旱气候类型，降水量至东向西逐渐减少。其植被类型以高寒草地为主，有零星的香柏灌木分布。归一化植被指数（NDVI）常作为指示该区域大空间尺度上草地生长和地上生物量变化的代用指标。然而，NDVI序列时间尺度相对较短，不能指示过去百年尺度上植被的生长变化。本项目拟在青藏高原中南部的降水梯度带上选择5个区域建立香柏灌木年轮宽度年表，揭示年轮宽度与生长季NDVI之间的关系，重建过去200年的草地NDVI变化，比较不同区域的草地生长变化及其对过去气候变化的响应。本研究旨在利用香柏年轮指示过去200年来青藏高原中南部大空间尺度上的草地生长动态，深入理解长期以来高寒草地生态系统对全球变化的敏感性。

青藏高原中南部地区的香柏灌木与草地的生长动态均受到春季水分条件的限制。与此同时，香柏灌木年龄较长，为重建草地生长动态、气候变化、灌丛更新动态提供了数据基础。本项目1）建立了300年以上的香柏灌木年轮宽度长年表；2）重建了青藏高原中南部地区的草地生长动态和气候干湿变化状况；3）分析了喜马拉雅山南坡滇藏方枝柏灌丛生长与气候的关系；4）揭示了气候变暖对青藏高原中南部和北极格陵兰地区灌丛更新的影响。结果显示：香柏灌丛具备建立300年以上年轮宽度年表的潜力；青藏高原中南部草地在近几十年（1989–2014）生长状态相对良好。在小冰期期间，青藏高原中南部存在1637–1683和1708–1785两个较长的干旱期。喜马拉雅山中段南坡灌木线上滇藏方枝柏灌木生长同样受到春季水分强烈限制；此外，气候变暖已经超过青藏高原中南部和北极格陵兰地区的灌木更新的最优生态阈值，近几十年变暖引起的干旱胁迫不利于灌木更新。