荷木(Schima superba)水力导度的干湿季变化及个体差异

Wet-dry seasonal patterns and inter-tree variation of hydraulic conductance of Schima superba

通过对不同个体水力导度(KL)的研究,揭示水力导度在树木水分利用中的关键作用,可为深入研究森林水分循环提供理论依据.采用热消散探针液流测定系统连续监测广州地区6棵不同大小荷木(Schima superba)样树的液流密度(JS),结合树木的叶片水势(ΨL)、土壤湿度、树高、边材面积(AS)、冠幅等参数,分析整树水力导度的日变化和干湿季变化,揭示不同大小样树水力导度的变化规律.结果显示:不同胸径大小的6棵样树液流密度差别明显,液流密度最大值未出现在胸径最大的样树中,而是在胸径略小的树木中.通过测定6棵样树的液流密度值可以进一步求算出样树的整树水力导度,总体上讲,6棵树的日均水力导度表现出明显的起伏波动;其中干季水力导度(0.017 g m-2s-1 M Pa-1)略低于湿季(0.019 g m-2 s-1 M Pa-1),但差异并不明显.样树土壤/叶片水势差与平均液流密度显著相关(P<0.0 01),而样树树形特征和水力导度之间关系表现为水力导度与树高呈现明显相关性(P=0.016),与胸径、冠幅相关性均不显著.上述结果表明,荷木水力导度表现出明显的日变化,其大小在一定程度上受到树形指标的影响.

Our study aimed at revealing the diurnal and seasonal patterns of hydraulic conductance （KL） and the relationshipbetween Kt. and individual characteristics, in order to understand the roles of KL in individual tree and to offer a theoretical basis for further researches on water circulation in forests. Sap flux densities （Js） in six trees of different size were continually monitored using Granier＇s thermal dissipation probes （TDP） in a Schima superba plantation in southern China. Combined with the measurements of leaf water potential （ΨL）, soil moisture, tree height, sapwood area （As） and crown size, we analyzed the daily variation and seasonal changes of the ΨL among individual trees. The results showed significant difference in the diurnal course of Js among individuals. The maximum Js was observed in individual with the second largest （rather than the largest） diameter at breast height （DBH）. Determined with the measured Js and ΨL, KL of individual trees all showed fluctuations in a day, slightly higher in wet season （0.017 g m-2 s-1 MPa-1） than in dry season （0.019 g m-2 s-1 MPa-1）. Besides, Js were significantly correlated with soil-leaf water potential gradient （Ψs.0 in both wet and dry seasons （P 〈 0.001）. Partial regression analysis showed that K-1. was significantly and positively correlated with tree height （P = 0.016）, but not with DBH or crown size. Our results indicated the significant diurnal patterns in ΨL that is to some extent closely related to individual characteristics.