西藏干旱半干旱区变色锦鸡儿灌丛树轮宽度对生产力长期变化的指示

It is possible to predict the variation of net primary productivity (NPP) for regional vegetations based on ring width chronology. However, the mechanism of the relationship between ring widths and NPP is complex. Based on the study on an evergreen alpine shrub in humid regions in southeast Tibet, we found that the ring width chronology could indicate the long-term variation of shrub NPP. However, we are not aware if it can be applied in alpine deciduous shrubs in arid and semi-arid regions. In this proposal, we plan to conduct an investiagation on an alpine deciduous shrub Caragana versicolor along a rainfall gradient in arid and semiarid regions in western Tibet. We plan to measure the ring width chronology, relative growth rate, leaf nitrogen concentration (Nmass), carbon isotope ratio (δ13C) and related traits of the shrubs, as well as environmental factors like air and soil temperature, soil moisture and rainfall along the transect. We first aim to disclose the variations in ring widths, relative growth rate and simulated NPP based on Nmass and δ13C measurements along the rainfall gradient. We then explore the relaionships between current ring widths and simulated NPP of current or/and last year. And the varation of NPP during recent 30 years can be modelled according to the ring width chronology. The NDVI time series will be employed to validate the changes of simulated NPP. These will supply rationales for understanding the long-term variation of NPP and its response to climate change for shrub ecosystems in arid and semi-arid areas.

树轮宽度序列为反演区域植被生产力的长期变化提供了可能，然而树轮宽度与生产力关系的内在机制非常复杂。基于前期对湿润区常绿高山灌丛的研究，我们发现树轮宽度可以用来指示灌丛生产力的长期变化，但目前并不清楚这一结果是否存在于干旱半干旱地区的高山落叶灌丛中。本申请项目拟以西藏干旱半干旱区广泛分布的高山落叶灌丛变色锦鸡儿(Caragana versicolor)为研究对象，沿降水梯度(50-450mm)开展样带调查和定位观测，采样测定树轮宽度、相对生长速率、叶氮含量和碳同位素比值等性状以及监测气温、降水、土壤温度和水分等环境要素，揭示变色锦鸡儿灌丛树轮宽度和光合生产力随降水梯度的变化规律，建立树轮宽度与当年或（和）前一年冠层光合生产力的关系模型，基于树轮宽度序列反演过去近30年灌丛植被净初级生产力的变化趋势，为理解干旱半干旱区灌丛植被生产力的长期变化趋势及其对气候变化的响应提供科学依据。

树轮宽度序列为反演区域植被生产力的长期变化提供了可能，然而树轮宽度与生产力的内在关系机制十分复杂。前期对湿润区高山常绿灌丛的研究表明，灌木树轮宽度可以指示灌丛生产力的长期变化，但湿润区的研究结论是否也存在于主要受水分控制的高山干旱的落叶灌丛？为回答这一问题，我们通过对西藏干旱半干旱区的高山落叶灌丛变色锦鸡儿(Caragana versicolor)沿降水梯度(50-450mm)的样带调查和定位观测，结合对另一个生物地理替代种——砂生槐（Sophora moorcroftiana，豆科灌丛）的连续3年的调查采样，测定了两个物种叶氮含量和碳同位素比值等性状，监测了几个定位观测点的土壤温度和水分等微环境要素，初步揭示了变色锦鸡儿叶功能性状及生态系统功能的空间格局，发现温度比降水对5个采样点模拟NPP的影响更大。基于遥感植被指数反演了变色锦鸡儿灌丛近20年的生物量变化趋势，总体呈现先降低后增加的格局。进一步综合豆科灌丛砂生槐的树轮宽度和叶性状测定数据，分析了光合生产力与树轮宽度的关系，发现该物种近3年的光合生产力对当年的树轮宽度均有明显影响，综合3年的光合生产力能解释当年树轮宽度变化的50%。研究通过揭示植被指数与生物量的关系以及不同年份生物生产力对树轮宽度的影响机制，为理解气候变化下干旱半干旱区灌丛植被生物生产力近20年的变化趋势及机制提供科学依据，同时拓展了旱区灌丛树轮生态学的潜在应用价值。