黄土高原人工刺槐林生长及对气候响应的时空变异研究

Artificial Locust (Robinia pseudoacacia) forest is the most important plantation type in the large scale afforestation program of the Loess Plateau of China, which provide essential ecological, societal and climatological benefits. However, the Locust forest in the Loess Plateau has showed signs of widespread growth recession triggered by the unreasonable ecological restoration actions and the combined effect of the recent warm and dry climate. Tree-ring materials act as the important indicators of forest growth, has been widely used in the evaluation of regional-scale forest growth and its climatic sensitivity. Comparably regional analyses based on tree-ring data, nevertheless has not attracted enough attentions in the Loess Plateau of China..In this study, we will construct the tree-ring database for the Locust forest in the Loess Plateau based on the investigation of the Locust forests along the gradients of rainfall and restoration age, then to determine the spatial and temporal variations of tree growth pattern and its climate response for the Locust forests in the Loess Plateau. We will also carry out the location monitoring experiments to get the information on the timing and dynamics of secondary wood growth of Locust forests, with the aim of linking the Locust growth process with the potential limiting climatic factor at the spatial and temporal scales. Based on the validation of tree-ring data, the process-based model, BIOME-BGC, will be used to simulate net primary productivity (NPP) of the Locust forests in the Loess Plateau and investigate the response of Locust forest NPP to the future climate change scenarios. This study will provide a scientific basis for the sustainable development of the large-scale afforestation projects in the Loess Plateau of China.

人工刺槐林是黄土高原最为重要的人工林生态恢复林型，由于不合理的经营模式和暖干化气候趋势的共同影响，大面积刺槐林出现生长衰退现象，所以对该地区刺槐林生长特征及对气候响应敏感性时空变异研究有重要的生态学意义。树木年轮材料作为指示森林生长的重要指标，也被广泛应用于区域尺度森林生长对气候响应敏感性研究当中，然而这方面研究在黄土高原地区还未引起足够的重视。.本项目以黄土高原人工刺槐林为研究对象，通过树木年轮学方法构建该地区沿降雨和恢复年限梯度的刺槐林树木年轮数据库，以反映刺槐林树轮生长特征及对气候响应的时空变异特征；通过定位监测实验来获取刺槐林木质部活动状态指标，从树木解剖学角度探讨人工刺槐林木质部活动特征及对气候响应的时空变异特征；结合树木年轮资料构建刺槐林生产力模型，来揭示刺槐林生产力及其限制性气候要素及未来变化趋势。本研究将为黄土高原地区人工林生态恢复建设的可持续发展提供科学依据。

人工刺槐林是黄土高原最为重要的人工林生态恢复林型，由于不合理的经营模式和暖干化气候趋势的共同影响，大面积刺槐林出现生长衰退问题；以往的研究主要是从土壤水分消耗探讨黄土高原刺槐林生长衰退问题，而从树木年轮气候学和生态学角度来探讨刺槐林生长衰退问题的研究在黄土高原还未受到人们的关注。本项目基于降雨梯度刺槐树木年轮数据库和多个定位监测点数据，从不同角度提供黄土高原人工刺槐林生长衰退的树木年轮学证据。研究结果主要有1）延安以南地区刺槐树轮指数近代有明显上升趋势，无生长衰退现象；延安以北地区刺槐树轮指数近代有明显下降趋势，生长衰退现象明显。2）刺槐树轮年表在降雨梯度上大致体现出干旱胁迫的气候信号，以位于林草交错带的延安羊圈沟刺槐年表干旱胁迫信号最为明显。3）延安羊圈沟年龄较小（15年、20年）刺槐年轮指数有下降趋势，对干旱胁迫响应敏感性较高，生长衰退风险较大；年龄较大刺槐（25年、35年）年轮指数无下降趋势，对干旱胁迫敏感性较低，生长衰退风险较小。4）在样带尺度上，延安羊圈沟刺槐年轮数据与BIOME-BGC模型模拟NPP数据吻合程度最高（R = 0.772, P < 0.01），并构建基于温度和降雨数据的刺槐林NPP多元线性回归方程，在RCP4.5情景下预测结果发现，在未来30年（2019−2050年）刺槐NPP具有不断下降的趋势特征。5）基于完整生长季木质部活动定位监测，发现延安羊圈沟刺槐林木质部活动从3月中旬开始，到9月中旬结束，木质部生长周期大约为180天左右，年轮生长90%以上是在4−6月形成的，与气候要素分析发现刺槐木质部细胞分裂和生长速率主要受水分胁迫限制。本研究从树木年轮学角度较为系统地阐述了黄土高原刺槐生长衰退特征及其限制性气候要素，将为黄土高原地区刺槐人工林生态恢复建设的可持续发展提供科学依据。