降雨和非降雨日兴安落叶松天然林蒸腾及蒸散发特征

摘要

We continuously measured the transpiration of different tree differentiation classes and ecosystem evapotranspiration using the sap flow method and eddy covariance technique in a natural Larix gmelinii-dominated boreal forest located in the Northern Great Hinggan Mountains.The sapwood area of stem was used for scale expansion from single stand to forest.We examined variations in total transpiration of the forest (Ttot) and its three components,namely,the transpiration of dominant (Td),intermediate (Ti),and suppressed (Ts) trees,and measured ecosystem evapotranspiration (ET).We also analyzed the regression correlations between Ttot or ET and vapor pressure deficit (VPD) and net radiation (Rn).The results showed that daily variation in sap flow velocity in different tree differentiation classes exhibited a unimodel pattern on both rainy and non-rainy days.Tot on rainy days (24.7 mm) was lower than that on non-rainy days (50.6 mm),and Td was higher than Ti and Ts on both rainy and non-rainy days.ET on rainy days (24.7 mm) was lower than that on non-rainy days (50.6 mm);however,the ratio of latent heat to Rnon rainy days (31％) was higher than that on non-rainy days (25.1％),indicating that environmental conditions on non-rainy days were more conducive to water vapor exchange over the interface between the atmosphere and plant stoma.The values of Ttot/ET,Td/ET,Ti/ET,and T/ET on rainy days (38.1％,27.2％,8.5％,and 2.4％,respectively) were lower than those in non-rainy days (65.0％,45.5％,15.3％,and 4.2％,respectively).This finding demonstrates that evaporation represented the principal water loss in ET on rainy days,whereas on non-rainy days,ET was dominated by transpiration.This,however,would overestimate the forests' transpiration capacity when scaled up considering only dominant tree transpiration consumption.In general,it was found that the correlation between Ttot and environmental variables (i.e.,VPD and Rn) is better than that between ET and these variables,which indicates that Ttot iS more sensitive to environmental variations.Furthermore,the correlation between Rn and water vapor fluxes (i.e.,Ttot and ET) is better than that between VPD and water vapor fluxes,indicating that Rn is the primary driver of an ecosystem's water vapor fluxes.%综合利用树干液流法和涡动相关技术,对大兴安岭北部寒温带兴安落叶松(Larix gmelinii)天然林的林木蒸腾(T)和生态系统蒸散发(ET)进行连续监测;采用边材面积对单木蒸腾耗水进行尺度扩展,分析降雨和非降雨日林木总蒸腾(Tot)及其蒸腾组分(优势木蒸腾Td、中等木蒸腾Ti和劣势木蒸腾Ts)与生态系统ET的变化特征,探讨Ttot与ET对水汽压亏缺(VPD)和净辐射(Rn)变化的响应.结果表明:降雨和非降雨日各分化等级林木液流速率的日变化均呈典型单峰格局,且降雨日的Ttot(9.7mm)低于非降雨日(31.4 mm),同时Td在降雨和非降雨日均明显高于Ti和Ts.降雨日的ET(24.7 mm)同样低于非降雨日(50.6mm),而潜热通量与同期Rn之比(31％)则高于非降雨日(25.1％),表明非降雨日的环境条件较有利于植物-大气界面的水汽通量交换.降雨日Ttot/ET、Td/ET、Ti/ET和TsET(分别为38.1％、27.2％、8.5％和2.4％)均低于非降雨日(分别为65.0％、45.5％、15.3％和4.2％),说明降雨日的ET以自由水蒸发为主,而非降雨日时则以Td占优;同时,仅以优势木蒸腾耗水作为平均水平进行尺度上推易高估林分的蒸腾能力.总体上,Tlot与VPD、Rn的相关性均较ET的高,即Ttot对环境因子的响应略敏感;同时Rn与Tot、ET的相关性均较VPD高,说明Rn是驱动生态系统水汽通量的首要条件.