黄土高原南北样带尺度人工刺槐林对气候响应的敏感性分析

根据黄土高原南北样带尺度的人工刺槐林(Robinia pseudoacacia)的年轮宽度资料，分析了该地区刺槐树木生长趋势，以及刺槐年表对气候响应随降雨梯度变化规律。研究结果表明延安以北的刺槐样点(绥德、神木)年轮指数近期趋于下降，树木有生长衰退现象；而延安以南刺槐样点(延安、富县、宜君、永寿)年轮指数近期趋于上升，树木无生长衰退现象。气候响应结果表明，刺槐年表对气候响应均以延安样点最为敏感，表现年表与温度的负相关关系，以及年表与降雨和干旱指数的正相关关系，而延安以北和以南刺槐样点对气候响应敏感性均较低。黄土高原中部延安地区地处森林草原过渡带，刺槐生长对外界环境变化最为敏感，年表中气候信号也较强；延安以南地区地处森林植被带，气候条件较为适宜刺槐林生长，因而年表中气候信号较弱；延安以北地区地处草原植被带，气候条件比较恶劣，刺槐生长对干旱气候已有一定适应性特征，因而年表中气候信号也较弱。

Climate sensitivity of black locust plantations along the precipitation gradient in the Loess Plateau of China

A network of six tree-ring width chronologies for black locust (Robinia pseudoacacia) plantation on the Loess Plateau along the precipitation gradients (from 420 mm to 600 mm) was constructed. The dendroclimatological techniques were used to assess the impact of climatic factors on radial tree growth of black locust trees exposed to the increasing drought stress. The results indicated that tree-ring indices of black locust plantations in the north of Yan''an (annual total precipitation about 500 mm) had experienced severe growth reductions since the 1990s, and the signal of tree growth decline was evident. In contrast, black locust plantation in the south of Yan''an (annual total precipitation more than 500 mm) have experienced overall positive growth trend since the 1990s, and no signal of tree growth decline was found. Correlation analysis between tree-ring chronologies and climatic data (monthly and seasonally) indicated that radial growth of black locust trees in Yan''an site had the higher climatic sensitivity, representing significantly negative correlation with temperature and positive correlations with precipitation and Palmer Drought Severity Index. However, the radial growth of black locust trees sampled from north and south of Yan''an site both had lower climatic sensitivity, and no determined associated patterns between tree rings and climate variables were found. Multiple linear regression models between tree-ring data and the climatic variables most influential for radial growth indicated that the explained variance of climate for radial growth of black locust trees was highest in Yan''an site and reached the significant level. In contrast, the explained variance for sites northern and southern of Yan''an sites were comparatively lower and did not reached the significant level. Yan''an site is located in the forest-steppe transition zone in the middle area of the Loess Plateau, where tree annual growth is most sensitive to fluctuations of external environment factors and climatic signal in tree rings is strong. The sample sites south of Yan''an site is located in the typical forest vegetation zone, where climatic conditions are suitable for radial growth of black locust plantations and climatic signal in tree rings is weak. The sample sites north of Yan''an site are located in the typical steppe vegetation zone, where climatic conditions are harsh and black locust trees maybe has developed adaptable strategy dealing with dry climatic condition. Consequently, the climatic signals of tree rings in those site were also relatively weak. Our results should benefit the understanding of site-specific growth-climate patterns for black locust plantations under drought stress conditions on the Loess Plateau, China.