基于遥感植被指数的青藏高原生长季开始时间分析和模拟

Analysis and Simulation of the Start of Growing Season on the Qinghai-Xizang Plateau Based on Remote Sensing Vegetation Index

青藏高原是重要的牧草产地、生态屏障和水源涵养区,其植被生态过程可以直接牵动中国乃至东亚的变化。随着全球变暖,青藏高原植被物候期不断改变,通过碳循环、水热循环等影响着气候和生态系统,物候变化及其影响因素研究成为关键问题,构建可以实现未来物候预测的模型具有重要科学意义。本文基于2000-2020年卫星获取的归一化差值植被指数(MODIS NDVI),利用动态阈值法提取青藏高原植被生长季开始时间(SOS),并结合植被类型分析其时空变化规律,构建SOS与气温、土壤水分的多种物候模型,探究不同区域、种类植被开始生长所需水热条件。结果表明:(1)2000-2020年,青藏高原SOS整体呈提前趋势,东部最为显著,提前速率超过10d·(10a)^(-1),针叶林、灌丛、草甸和高山植被覆盖区SOS提前比例较高,草原有约50%的轻微延后区域;(2)青藏高原东部和北部区域有明显的暖湿化趋势,年均温上升速率约为0.36℃·(10a)^(-1),年均土壤水分增加速率约为每年3.8×10^(-4) m^(3)·m^(-3)(p<0.01);(3)四个物候模型参数率定结果均显示,青藏高原东部和南部植被开始生长对水热条件要求更高,南部植被SOS的主控因子为气温,而北部则为土壤水分,不同植被类型生长季开始的气温、土壤水分阈值和主控因子也与其空间分布位置密切相关;(4)文中构建的积温-累积土壤水分阈值模型对于青藏高原主要植被类型(草原、草甸和高山植被)模拟效果最好,均方根误差仅在8天左右,对于青藏高原未来SOS预测、物候与气候之间的相互作用机制等相关研究具有参考意义。

The Qinghai-Xizang Plateau(QXP)is an important herbage producing area,ecological barrier and water conservation area,the vegetation ecological process on which can directly affect the changes of China and even East Asia.With global warming,the phenological period of vegetation on the QXP is constantly changing,affecting climate and ecosystem through carbon cycle and hydrothermal cycle,etc.The study of phenological change and its influencing factors has become a key issue,and the construction of models that can realize future phenological prediction is of great scientific significance.In this paper,based on the Normalized Difference Vegetation Index acquired by satellites during 2000-2020(MODIS NDVI),the dynamic threshold method was used to extract the start of growing season(SOS)on the QXP,and its spatiotemporal variation was analyzed in combination with vegetation types,so as to construct multiple phenological models of SOS,air temperature and soil moisture,exploring the hydrothermal conditions required for different regions and types of vegetation to start growing.The results showed that:(1)From 2000 to 2020,the overall SOS advance trend of the QXP was most significant in the eastern part of the region,where the SOS advance rate exceeded 10 d·(10a)^(-1).Coniferous forests,scrub,meadows,and alpine vegetation cover areas had a high percentage of SOS advance,and grasslands had about 50%of slightly delayed areas.(2)The eastern and northern regions of the QXP showed an obvious warming and humidification trend.The average annual temperature rise rate was about 0.36℃·(10a)^(-1),and the average annual soil moisture increase rate was about 3.8×10^(-4) m^(3)·m^(-3)(p<0.01).(3)The parameters of the four phenological models showed that the vegetation growth in the eastern and southern QXP required higher hydrothermal conditions.The main controlling factor for vegetation SOS in the south was air temperature,while in the north it was soil moisture.The temperature and soil moisture thresholds and main controlling factor of different vegetation types were also closely related to their spatial distribution locations.(4)The cumulative temperature and cumulative soil moisture threshold model established in this paper has the best simulation effect for the main vegetation types(grassland,meadow and alpine vegetation)on the QXP,and the root-mean-square error is only about 8 days,which has reference significance for the future SOS prediction and the interaction mechanism between phenology and climate on the QXP.