不同径级马占相思(Acacia mangium)整树蒸腾的湿、干季变化

Variations of whole-tree transpiration at different diameter-classes in Acacia mangium during dry and wet seasons

利用Granier树干液流测定系统对广东鹤山丘陵地马占相思（Acacia mangium）林进行长期监测,并同步监测环境因子（空气温度、相对湿度、光合有效辐射、土壤体积含水量）,选择胸径具有代表性的样树,结合马占相思的形态学参数,计算马占相思的整树蒸腾。通过对光合有效辐射（PAR）分级,建立不同辐射强度等级的湿季（土壤水分θ≥33%）整树蒸腾与水汽压亏缺（VPD）的相关方程,以干季（θ≤24%）的VPD代入对应PAR等级的湿季的拟合方程,求出干季的潜在蒸腾,以潜在蒸腾和干季实际蒸腾之差分析不同径级整树蒸腾在不同季节的实际变化。在所有的PAR分级内,干季整树蒸腾显著低于湿季,仅占湿季蒸腾的10%-20%,就蒸腾减少的绝对量而言,优势木〉中间木〉劣势木。土壤水分下降缩小了不同径级树木之间液流密度的差异,土壤水分亏缺限制了树木的蒸腾,对马占相思的生长造成一定程度的水分胁迫。

Sap flow in a typical hilly Acacia mangium forest stand of the Southern China was measured continuously using the Granier＇s thermal dissipation probe method over a long period. Environmental factors （including air temperature, relative humidity, photosynthetically active radiation, soil moisture） were measured simultaneously. Sap flux density of sample trees representing the whole stand were selected for calculating the individual tree transpiration combining with morphological characters. Photosynthetically active radiation （PAR） was grouped into following six classes： 100 -2001μmol·m^-2·s^-1,200-400μmol·m^-2·s^-1,400-600μmol·m^-2·s^-1,600-800μmol·m^-2·s^-1,800-1000μmol·m^-2·s^-1, ≥ 1000μmol·m^-2·s^-1, and the relationship between whole-tree transpiration （E） and VPD in the wet season were analyzed for each RAR class. Potential transpiration in the dry season was then estimated based on the fitted regressions with VPD of the wet period. The results showed that the transpiration in the dry season was significantly lower than and was comprised only 10%--20% of that in the wet season. Concerning the reduction of transpiration at different diameter-classes, it was ranked as ： dominant trees 〉 intermediate trees 〉 suppressed trees. Compared to those in the wet season, the sap flow of different trees was lower as a result of deficit of soil water content in the dry season. Soil water deficit limited transpiration to certain extent, and the growth of A. mangium forest was subjected to water stress during dry period.