大连4种城市绿化乔木树种夜间液流活动特征

夜间液流有助于树木物质运输及其体内水分的补充(water recharge),它不仅对植物的生长发育具有重要的生理生态学意义,而且对大尺度植物蒸腾耗水的估算可能产生重要影响。2008年6月1日至8月31日,以热扩散探针(thermal dissipation probe,TDP)技术对大连市劳动公园内的雪松(Cedrus deodara)、大叶榉(Zelkova schneideriana)、丝棉木(Euonymus bungeanus)和水杉(Metasequoia glyptostroboides)4种乔木树种的不同径阶样木树干边材液流进行了测定,并结合同步土壤水分与小气候观测结果分析了树木夜间(18:00至次日5:00)液流特征。实验结果表明,树木普遍存在可感夜间液流,夜间液流总量占观测期液流总量的比例在样木个体间呈现显著差异,其变化范围为0.44%–75.96%;观测期雨天夜间液流波动活跃,显著高于晴天,其单日夜间液流总量可持平,甚至高于日间液流。相关分析表明:水汽压亏缺(vaporpressuredeficit,VPD)和风速的变化与夜间蒸腾显著相关,它们能够较好地解释液流变化(R20.6);树木夜间液流主要用于夜间蒸腾和自身水分补充,夜间液流现象主要发生在前半夜,后半夜液流平稳且极接近0,夜间液流量与相应的日间流量(R2=0.356,p=0.00)及胸径(R2Spearman0.80)显著相关,说明植物本身的结构和生理特点也是影响树木夜间液流的重要因子。单株样木夜间液流占全天总蒸腾量的比例低于14.4%,如不考虑夜间液流的影响,根据日间液流通过尺度扩展推算的森林生态系统年蒸腾量可能偏低。

Nocturnal sap flow of four urban greening tree species in Dalian, Liaoning Province, China

Aims Nocturnal sap flow is a driving force not only for chemical transport and nutrient movement within trees, but also for water recharge caused by diurnal transpiration loss. Therefore, it is both ecologically and physiologi-cally significant for estimating tree and stand transpiration at larger spatial and longer temporal scales. Our objec-tive was to analyze nocturnal sap flow (18:00-5:00 next morning) and its biophysical controls in four tree species (Cedrus deodara, Zelkova schneideriana, Euonymus bungeanus and Metasequoia glyptostroboides) in Laodong Park in Dalian, China.Methods During June 1-August 31,2008, we measured sap flow of twelve urban trees of four species and three diameter at breast height (DBH) classes of each species continuously by thermal dissipation probes, soil moisture by ECH2O soil moisture probe and microclimate data by an automatic weather station. SPSS 13.0 was used for statistical analysis of sap flow and environmental factors.Important findings Nocturnal sap flow was measured in all sampled trees, but varied from 0.44% to 75.96% of total daily water consumption. Nocturnal flow was larger and more active on rainy days than that on sunny ones, to the extent that it equaled or exceeded the diurnal volume. Vapor pressure deficit and wind speed were signifi-cantly correlated with nocturnal water movement (R~2 > 0.6), which could provide an explanation for nocturnal transpiration. Nocturnal sap flow mainly contributed to nocturnal transpiration and water recharge. It peaked be-fore midnight and dropped to nearly zero afterwards. Furthermore, nocturnal sap flow was remarkably correlated with diurnal sap flow (R~2 = 0.356, p = 0.00) and DBH (R~2_(Spearman)> 0.80), substantiating the important role of spe-cific structural and physiological characteristics. Nocturnal sap flow of individuals accounted for an average of < 14.4% of the total transpiration volume. Total water consumption of a forest ecosystem scaled up by daytime sap flux measurements can be underestimated without integration of nocturnal sap flux.