荷木整树蒸腾对干湿季土壤水分的水力响应

Hydraulic responses of whole tree transpiration of Schima superba to soil moisture in dry and wet seasons

降雨在时间上的非均匀分配导致森林土壤含水量呈现明显的干、湿季变化,并可能在干季形成水分胁迫,引起植物蒸腾变化。在监测环境因子的同时,利用Granier热消散探针连续监测荷木(Schima superba)的树干液流,以液流密度值计算整树蒸腾,并结合水力导度与叶片/土壤的水势差,探讨环境因子和水力导度对荷木整树蒸腾的协同控制。结果表明,华南地区的季节性降雨形成的干、湿季并未引起荷木蒸腾在季节上的显著差异,但对产生蒸腾的水力生理产生了显著影响。荷木蒸腾在干、湿季均与主要驱动环境因子(光合有效辐射PAR和水汽压亏缺VPD)呈显著正相关。在水热充足的湿季,荷木蒸腾主要受气孔导度调节;在干季,当空气水汽压亏缺达2.132 MPa时,水力导度与气孔导度协同控制蒸腾。整树水力导度对整树蒸腾的水力补偿出现在15:00—17:00,平均补偿值为0.08 g/s。利用蒸腾的估测值与实测值之间的差值量化荷木的水力补偿效应,是对水力导度与气孔导度协同控制树木蒸腾机理的深入探索。研究结果对于掌握季节性降雨不均背景下华南地区主要造林树种需水和耗水规律,有效发挥森林保水功能具有重要意义。

The seasonal uneven distributions of precipitation results in significant difference of soil water content in subtropical forests of southern China. This potentially induces water stress in the dry season, which in turn may affect plant transpiration. In the present study, we intended to explore effects of environmental variables and plant hydraulic conductance on the whole tree transpiration （ET） of Schima superba, a dominant tree species in subtropical forests, and paid a special attention to the mechanisms of hydraulic compensation for transpiration under water stress condition. ET was determined from simultaneously measured stem sap flow using Granier＇s thermal dissipation probes, and soil-leaf water potential difference （ΨL-S） was measured with the aid of a PMS pressure chamber on sunny days in dry （November 3 to 5） and wet seasons （August 21 and 22） of 2009. In addition, atmospheric evaporative demands and soil moisture conditions were monitored. The results showed that the rainfall seasonality brought about distinct soil moisture conditions between wet and dry seasons, but had no significant effect on the ET of S. superba. It turned out that ET of S. superba was significantly correlated to photosynthetically active radiation （PAR） and vapor pressure deficit （VPD） in both dry and wet seasons. However, in dry season ET was co-regulated by both stomatal and hydraulic conductance, whereas ET was mainly regulated by stomatal conductance in wet season possibly due to sufficient supply of water and radiation. Regression analyses of ET and soil-leaf water potential difference （ΨL-S） allowed the prediction of ET in both dry and wet seasons. A distinct discrepancy between predicted and measured ET （calculated from sap flow measurements） was found, i.e. the measured ET was 91.74% of the predicted ET in dry season, while in wet season measured ET was 1.33 times higher than the predicted one. This revealed a hydraulic compensation for ET that usually occurred in the afternoon of the dry season （13：00-17：00）, when VPD reached as high as 2.132 MPa, and the compensation value was calculated to be 0.08 g/s. The whole tree hydraulic conductance （k） decreased within the daytime in both dry and wet seasons; while it was generally lower in dry than wet season, especially in the morning （7：00-9：00） and afternoon （13：00-17：00）, indicating higher hydraulic resistance during that time in dry season. Taken together, the present results suggested that the lower and continuously decreasing k in dry season especially in the afternoon when VPD was high, strengthened the water stress on leaves and increased the sensitivity of stomata to drought, which thus balanced the water loss and heat absorption and promoted plant survival under drought conditions. The dynamic variation of k and subsequent hydraulic compensation may represent one of the reasons why subtropical broadleaf trees have potentially similar ET in dry and wet seasons. The quantification of hydraulic compensation may be useful for a better understanding the interactive regulations of stomatal and hydraulic conductance on ET. Our results would also be helpful to evaluate the ecological functions of subtropical forests in southern China, in terms of water conservation under the background of seasonal uneven precipitations.