黄土高原多年生草本年轮生长特征及其生态学价值

Grassland is the most widely distributed vegetation type in the Loess Plateau and the grassland in this region had steadily increased after the initiative of the‘Grain to Green’ large-scale revegetation programme. Grassland ecosystem has made remarkable contribution to the improvements of ecological conditions in the Loess Plateau, such as the enlargement of vegetation recovery, the declining soil erosion rates and the increase of carbon sequestration efficiency. Recent studies have demonstrated that many perennial forbs at temperate zones develop growth rings in the secondary xylem of their roots that represent annual growth increments. Such anatomical patterns allow scanning for particularly wide or narrow rings has been used in the evaluation of regional-scale grassland growth and its climatic sensitivity. Comparably regional analyses based on annual rings of main roots of herbaceous species, nevertheless has not attracted enough attentions in the Loess Plateau of China..By use of the method of herb-chronology, the main objective of our study was to detect large-scale coherent ecological and climatic imprints on the growth of herbaceous plants by analyzing patterns in the annual rings of roots in the Loess Plateau. With this method we hoped to construct the annual-ring database of herbaceous species in the Loess Plateau based on the collection of forb’s main roots along the rainfall gradient, and then assess a proxy parameter for the growth response in the herbaceous vegetation to climatic fluctuations at the spatial and temporal scales. Based on the analyzed dynamic patterns of annual growth rings for the forbs and the related parameters for their life history strategies, we also want to determine the possible importance of changes in selection pressures across the rainfall gradient on life history strategies for the perennial forb species in the Loess Plateau and provide evidence of a consistent shift toward more conservative and strongly constrained life history strategies in the more stressful conditions. Furthermore, we will carry out the location monitoring experiments for the Medicago sativa (a widely planted herbaceous species in the Loess Plateau) to get the information of its physiological parameters and the anatomical characteristics of main roots, especially for xylem size and thickness, then quantitatively evaluate the water use efficiency along the planted ages and those potential constrained factors. Subsequently, we will develop a conceptual framework to understand the adapted features to the drought conditions for this high water-sensitivity plant grassland, and interpret the ecological mechanism of widespread growth recession of M. sativa at the later life stage triggered by the severe water-limited conditions. This study will provide a scientific basis for the sustainable development of the large-scale ecological restoration projects in the Loess Plateau of China.

草地是黄土高原面积最大的植被类型，草地生态系统对提高黄土高原植被覆被率、水土流失治理、改善生态环境和增加区域碳汇等方面都有重要作用。多年生草本根部年轮资料作为草地生长的重要指标，已被用于区域尺度草地生长动态及其对气候响应敏感性研究当中，然而这方面研究在黄土高原还未引起足够的重视。.本项目以黄土高原多年生草本为研究对象，通过草本年轮学的理论与方法构建该地区沿降雨梯度的草本根部年轮资料数据库，揭示多年生草本年际生长对气候响应的时空变异特征；结合草本根部年际生长动态和生活史策略指标，归纳黄土高原多年生草本生活史策略特征及其在降雨梯度上的适应和调整方式；依据定位生理生态学监测和根部年轮解剖结构分析，揭示在黄土高原广泛种植的紫苜蓿的导水能力及其限制性生态因素，探讨其对干旱环境的适应特征及其在生活史后期出现生长衰退的生态机制，本研究对黄土高原草地生态恢复建设的可持续发展提供科学依据。

草地是黄土高原面积最大的植被类型，草地生态系统对提高黄土高原植被覆被率、水土流失治理、改善生态环境和增加区域碳汇等方面都有重要作用。多年生草本根部年轮资料作为草地生长的重要指标，已被用于区域尺度草地生长动态及其对气候响应敏感性研究当中，然而这方面研究在黄土高原还未引起足够的重视。本项目基于黄土高原样带尺度上广泛采集多年生草本物种根部年轮材料，构建了区域草本年轮数据库，分析了黄土高原草本物种年际生长及导水特性变化趋势及对气候响应敏感性特征。研究结果主要有1）黄土高原草本根部年轮年际生长主要与水分和干旱指数关系密切，根部导管主要参量（包括导管分量、最大导管面积、总导管面积、平均导管面积、最大水分传导率、总体水分传导率、平均水分传导率、理论导水直径）在样带尺度均体现出与降雨因素的关系特征，这充分说明了水分条件限制是决定黄土高原草本物种根部年轮生长和导水效率的空间变异特征最为重要的外界环境因素；2）黄土高原多年生草本物种在降雨较少地区趋向于采取导水效率优先策略而形成较大导管，而在降雨较多地区趋向于采取导水安全优先策略而形成较小导管，表明黄土高原草本物种能够调整根部导水能力和耗水策略来适应不同干旱胁迫条件；3）黄土高原人工草本物种年轮宽度对气候敏感性较低，导水能力随年龄梯度出现先上升后下降趋势，生长衰退现象明显，而自然草本物种年轮宽度对气候响应敏感性较高，导水能力随年龄梯度呈线性上升趋势，未出现生长衰退现象。本研究从草本年轮学角度较为系统地阐述了黄土高原多年生草本物种年际径向生长和导水特征及其气候响应敏感性，将为黄土高原地区草地生态系统生态恢复建设的可持续发展提供科学依据。