生态系统尺度水汽通量分配——以寒温带兴安落叶松林为例

Partitioning of water vapor flux on the ecosystem scale:a case study on larch boreal forests

【目的】蒸散发(ET)包括蒸发(E)和蒸腾(T),是生态系统降雨(P)返回大气的最主要形式,在气候变化背景下,了解大兴安岭北部多年冻土区的寒温带兴安落叶松林的ET特征及其分配状况,有助于进一步理解北方森林对气候变化的响应模式。【方法】在2015年7月10日至8月10日期间,利用模型与野外实测的方法对寒温带兴安落叶松林蒸发(E)、蒸腾(T)及蒸散发(ET)进行研究。E包括林地蒸发(E_f)和林冠截留(E_c),而林分蒸腾总量(Ttot)则为优势木(T_d)、中等木(T_i)、劣势木(T_s)蒸腾量之和。分析非降水和降水日的ET及其组分特征和分配,探讨水汽通量对气象因子的响应。【结果】非降雨和降雨日的ET及其组分的日变化均呈单峰格局,且非降雨日曲线的日峰值均高于降雨日。非降雨日,E_f、T_d、T_i、T_s和ET分别为10.3、25.6、15.2、10.8和66.3 mm;降雨日,E_f、E_c、T_d、T_i、T_s和ET则分别为2.2、24.3、11.2、5.1、3.8和47.8 mm。非降雨日,E_f/ET为15.5%,而Ttot/ET为78.0%,其中T_d/ET、T_i/ET和T_s/ET分别贡献38.7%、23.0%和16.4%;降雨日,E_f/ET低至4.6%,E_c/ET则可以达到50.9%,而Ttot/ET降低至42.2%,其中T_d/ET、T_i/ET和T_s/ET分别为23.5%、10.6%和8.0%;表明非降雨日ET以T为主(具体为T_d),降雨日则以E(具体为E_c)为主。观测期间94.7%的P主要以ET形式返回大气,其中由T贡献57%,E贡献38%。总体上,无论降雨与否,ET与23 m处净辐射(Rn)的相关性均高于其与水汽压亏缺(VPD)的相关性,Ttot与二者的相关性则差异不大,而E_f的表现则与ET相反,说明Rn是生态系统能量循环和物质交换的最主要驱动力,Ttot同时主要受到Rn和VPD的约束,而E_f优先受VPD的限制。【结论】兴安落叶松优势木的蒸腾能力强于中等木和劣势木,以往研究多采用T_d(或包括较大径级的T_i)为林分尺度上推计算过程的基准值的方法会高估林分整体的蒸腾能力,实际误差的大小取决于林分的分化程度以及是否降雨等因素。非降雨日的气象条件更有利于植被-大气界面的水汽交换,降雨的发生会影响生态系统ET的分配模式。

[ Objective] Evapotranspiration （ET） including evaporation （E） and transpiration （T） , was the main way that ecosystem precipitation （P） returned to the atmosphere. Under the background of climate change, understanding the characteristics and distribution of ET in a cold-temperate Larix gmelinii natural forest at permafrost region of northern part of Daxing＇an Mountains, northeastern China could help to further comprehend the response modes of the boreal forest ecosystem to the climate change. [ Method] Model simulation and field experiments were applied for estimating and measuring ecosystem scale evaporation （E）, transpiration （T） , and evapotranspiration （ET） in a boreal larch forest. E includes forest floor evaporation （Ef） and canopy interception （Ec） which only occurred during rainfall events. Total forest transpiration （ Ttot） in this study is the sum of dominant tree transpiration （ Td） , intermediate tree transpiration （ Ti） and suppressed tree transpiration （ Ts）. Furthermore, we analyzed the variations and allocation proportions of ET and components under conditions of non-rainy or rainy. Then we discussed the responses of Ef, Ttot and ET to the net radiation （Rn ） and the vapor pressure deficit （VPD） under different water input conditions. [ Result ] Diurnal variations of ET and its components all performed as single peak patterns in both non-rainy and rainy days, and peak value in each curve at non-rainy day grouping was higher than in rainy day grouping. Ef, Ta, Ti, T and ET in non- rainy days were 10. 3, 25.6, 15.2, 10. 8 and 66. 3 mm, respectively. Meanwhile, Ef, Ec, Td , T~, T and ET in rainy days were 2. 2, 24. 3, 11.2, 5. 1, 3.8 and 47.8 mm, respectively. In non-rainy days, Ef/ET was 15.5% , and Tto,/ET was 78.0% , while TJET, Ti/ET and Ts/ET contributed 38.7% , 23.0% and 16. 4% , respectively. In rainy days, E/ET could go low to 4. 6% , Eo/ET reached 50. 9% , and T^ot/ET decreased to 42. 2% , in which TJET, T/ET and Ts/ET were 23.5% , 10. 6% and 8.0% , respectively. These results represented that ET was dominated by T （specifically for Td ） in non-rainy days. However, E （ specifically for Ec ） contributed the highest proportion of ET in rainy days. 94.7% of P returned to the atmosphere through ET during whole observation days, there into, T and E accounted for nearly 57% and 38%, respectively. Overall, without regard to rainfall events, ET had better correlation with net radiation at 23 m height （ Rn ） than with VPD and 7＇,o, owned similar correlations with both R, and VPD, but Ef expressed opposite result with ET. This result demonstrated that Ro was obviously the main driver for ecosystem＇s energy cycle and material exchange, T^ot restrained by Rn and VPD at the same time, and Ef was more susceptible to VPD compared to Rn. [ Conclusion] The transpiration capacity of dominant trees of Larix gemelinii species was stronger than intermediate and suppressed trees. So that, forest transpiration upscale from the individual tree could be overestimated if this method only considers in situ measured Td （even including Ti with larger DBH） and ignores T. The actual error depends on the degree of forest differentiation and moisture income conditions. The meteorological condition of non-rainfall days is more propitious to the water-vapor exchange at vegetation-atmosphere interface, and the occurrence of rainfall will affect the distribution pattern of ecosystem ET.