近30年青藏高原高寒草甸返青期变化综合评估及其机理研究

Studying on the spatio-temporal change in green-up date of alpine meadow in the Qinghai-Tibet Plateau and its driving mechanism can enhance our understanding about the response of vegetation to climate change. Three typical types of alpine meadow (i.e. Kobresia pygmaea alpine meadow, Kobresia pygmaea-Polygonum sphaerostachyum alpine meadow, Kobresia pygmaea-Stipa purpurea alpine meadow) will be selected as the research subjects in this proposed project. We first carry out ground investigation and remote sensing monitoring about green-up date of alpine meadow, and develop a scale transformation method between them. Then a unified phenology mechanism model is parameterized and validated to simulate the green-up date of alpine meadow. Later, we carry out a comprehensive analysis about the spatio-temporal change in green-up date of alpine meadow in the Qinghai-Tibet Plateau during recent 30 years based on the results from ground investigation, remote sensing monitoring and model simulation. In order to understand the driving mechanism about the spatio-temporal change in green-up date of alpine meadow, we further quantitatively analyze the contribution of chilling temperature and forcing temperature with the parameterized phenology model. This proposed project is characterized in two aspects. Firstly, distinguishing from traditonal visual observation for ground green-up investigation,this proposed project designs a new ground green-up investigation method based on optical digital camera photos. Secondly, distinguishing from a single monitoring means, such as ground investigation or remote sensing monitoring, this proposed project integrates three monitoring means (i.e. ground investigation, remote sensing monitoring and model simulation) to carry out a comprehensive analysis about the spatio-temporal change in green-up date of alpine meadow in the Qinghai-Tibet Plateau during recent 30 years, and further analyzes its driving mechanism.

研究青藏高原高寒草甸返青期时空变化及其机理，有利于深化认识植被对气候变化的响应规律。本项目以青藏高原三种主要的高寒草甸类型（小嵩草高寒草甸、小嵩草－圆穗蓼高寒草甸、小嵩草－紫花针茅高寒草甸）为研究对象，利用地面、遥感和模型模拟三种植物物候监测手段，设计基于光学数码相机的高寒草甸返青期地面观测方案，研究地面观测与遥感监测之间的尺度转换方法，开展通用物候机理模型的参数化及模型验证，综合评估近30年来青藏高原高寒草甸返青期的时空变化，并从机理上对其进行解释。本项目体现了两方面的特色：一是区别于传统基于目视观察的地面观测方法，本项目设计了一套基于光学数码相机的高寒草甸返青期地面观测方案；二是区别于以往基于地面或遥感的单一监测手段以及缺乏机理方面的解释，本项目综合地面、遥感和模型模拟三种监测手段，综合评估近30年来青藏高原高寒草甸返青期的时空变化，并从机理上对其进行解释。

研究气候变化对青藏高原草地春季物候的影响机制有助于明晰植被对全球气候变化的响应规律。本项目从基于温度的生长季始期、地面观测的草本植物春季物候和遥感监测的草地返青期三个角度综合评价了青藏高原草地春季物候的时空变化特征，并进一步通过分析物候变化影响因素，建立了具有一定机理性的植物物候模型，基于此分析了春季物候对气候变化的响应机制，获得了以下重要研究结果：（1）青藏高原不同温度阈值的生长季始期空间分异存在温度阈值效应，随着温度阈值的降低，基于温度的生长季始期空间分异的垂直地带性逐渐减弱，而纬度地带性逐渐增强。（2）青藏高原草地不同春夏季物候事件的提前速率存在差异，较早出现的物候事件具有更高的提前速率，这意味着植物的营养生长期有可能发生变化；进一步选择车前和蒲公英这两个广布物种开展分析，发现随着海拔降低（即温度升高），它们的营养生长期均在缩短，且车前缩短得更为厉害，物种间营养生长期的不均衡变化极可能引起物种变迁。（3）基于多源数据从不同角度和尺度的综合研究表明，青藏高原草地春季物候总体上呈提前趋势，但因不同物种、不同物候事件、不同局地气候环境而存在差异。（4）日最低气温是促使青藏高原大部分草地春季物候提前的主控因子；降水增多会使青藏高原南部部分干旱区草地的春季物候显著提前。（5）相对于当前已有的单气候要素模型，新构建的热时+降水双气候要素模型对青藏高原大部分草地春季物候的模拟效果更优；基于双温的通用物候模型模拟表明，在升温背景下，高寒草地返青期所受到的冷激作用趋于减弱而驱动作用趋于增强，但后者增强的影响明显大于前者减弱的影响，最终导致草地返青期提前。这些研究结果为研究和预测青藏高原草地物候对气候变化的响应规律以及物种变迁提供了新思路和理论依据。