水分利用效率是深入研究森林生态系统水碳循环耦合关系的重要节点之一.北京山区生态系统是北京市的天然生态屏障,研究该地区植被水分利用效率动态及其对气候变化的响应,对于评估区域碳水耦合关系及研究植物对全球气候变化的响应具有重要意义.利用北京市密云县东部山区红门川流域的油松树轮δ13C序列,分析了长期水分利用效率WUE的年际变化.结合密云站及上甸子站的气象数据资料分析结果表明:(1)自1952年至2014年,北京山区红门川流域油松树轮δ13C值序列呈现上升趋势,变动区间为-23.41‰—-27.63‰,平均为-25.56‰;油松WUE的年际值呈现波动下降趋势,变动区间为5.77-16.53,平均值为9.6,平均每年下降0.175,20世纪80年代左右下降趋势最为显著,且之后维持在相对较低的水平,最低值(5.76)出现在1994年,最高值(16.53)出现在1976年,1964年至1980年期间WUE为研究时段内最高,平均值为13.0.由此可见,在过去几十年中,红门川流域油松林的水分利用效率持续降低,固碳能力下降.(2)油松WUE对气温变化响应较好,总体呈现显著负相关,其中与年均气温相关性指数为r2=0.8248,P＜0.01,与生长季平均气温相关性指数r2=0.6952.平均气温每升高0.1℃,油松WUE下降0.205.且平均气温较高的年份油松WUE下降率比低温年份的WUE升高率大,由此推断,气温上升对油松林生态系统水碳循环及耦合关系影响更为显著.(3)油松WUE随着降水量增加而提高,与降水存在一定的正相关关系,但并不显著;在降水量突然减少之后,油松的WUE值会随之上升,持续一段时期后有回落现象,说明WUE值具有一定保守性.(4)WUE对温度变化的响应较降水变化的响应更加敏感.温度的升高及降水的减少导致植物叶片气孔导度降低,进而影响了植物的固碳速率.%Water-use efficiency (WUE) is fundamental in understanding the coupling relationships of the water-carbon cycle of forest ecosystems.WUE is defined as the ratio of the photosynthetic uptake of CO2 to the simultaneous transpiration loss of water vapor through the stomata.The stable carbon isotope ratio (δ13C) of tree-rings can be used to trace long-term,integrated effects of elevated CO2 and climate change on WUE and tree growth,which provides insights into how naturally growing trees respond to climate change.In the mountainous area of Beijing,northern China,forest ecosystems play an important role in preventing soil erosion and water loss,regulating the climate,and maintaining ecological stability,acting as a natural ecological barrier for Beijing city.It is of great significance to study the dynamic response of WUE and its response to climate change in this area.The present study adopted dendrochronological methods and interannual δ13C measurements to analyze the inter-annual variation in WUE of Pinus tabulaeformis between 1952 and 2014 in the Hongmenchuan watershed,Beijing.Moreover,we analyzed the correlation between WUE and environment factors,combined with temperature and precipitation data supported by the nearest meteorological stations.The results showed that the δ13C values of the P.tabulaeformis tree-ring varied in a range of-23.41‰ to-27.63‰,with an average value of -25.56‰,and the δ13C value reduced by 0.04‰ per year.With the meteorological data provided by the weather stations of Miyun and Shangdianzi,the WUE curve showed a quadratic function trend,and changed in a range of 5.77 to 16.53,with an average value of 9.6 and decrease of 0.175.The advantage interval,where P.tabulaeformis WUE was higher than that in other years,was when the precipitation was between 550 and 600 mm,and the average annual temperature was between 11 and 11.5℃,and the water use efficiency in the early growth stage was higher than that in later stages.The lowest value (5.76) appeared in 1994,and the highest value (16.53) appeared in 1976.From 1964 to 1980,the WUE maintained at a higher level,with an average of 13.Thus,in the past decades,WUE of P.tabulaeformis in the Hongmenchuan watershed continuously decreased,similar to the carbon sequestration capacity of forest ecosystems.WUE showed an overall significant negative correlation with annual temperature (r2 =-0.82,P＜0.01) and temperature of growth season (r2 =0.6952,P＜ 0.01),a certain but not significant correlation with precipitation.The correlation of WUE and annual temperature was stronger in the growth season,which also reflected the influence of non-growth season temperature.WUE increased slightly when temperature decreased,but declined sharply when temperature increased by 0.205 along with per 0.1 ℃ temperature increase.After the sudden reduction in precipitation,WUE increased temporarily and then decreased,which showed that WUE may have been conservative.This indicates that the response of WUE is more sensitive to temperature change than to precipitation.Increased temperature and decreased precipitation caused a decrease in stomatal conductance of plant leaves that affected the carbon fixation rate.
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