Effects of environmental factors on sap flow in Acacia mangium

Acacia mangium is one of the pioneer tree species of re-vegetation in degraded hilly lands in South China. Whole-tree sap flow in an A. mangium forest was measured continuously on 14 trees using Granier's thermal dissipation probe method over a period of four months. Environmental factors including air temperature, air relative humidity, soil relative humidity, photosynthetically active radiation (PAR) and global radiation were measured simultaneously with sap flow. Sapwood depth was highly correlated with tree diameter at breast height. Maximum values of sap flux density of each trees had marked linear correlations with sapwood area. Within one tree there were marked differences in sap flux density at four orientations on the trunk and sap flux density on one side was found to be highly correlated with the others. Even when the sap flux density in any one orientations for five trees was combined, there were still good correlations between the four orientations. Time lag between sap flow and PAR as well as vapor pressure deficit (VPD) in both a wet month(September)and a dry month (December) are discussed. PAR exhibited diurnal peak values earlier than sap flow by 40～50min (Sep.) to 100～150 (Dec.) but VPD lagged behind sap flow by 10～60 (Dec.) to 90～110min (Sep.). Differences in time lags among individuals were not obvious in the wet month but significant in the dry month. Mean values and maximum values of sap flow as well as the total sap flow in the wet month (Sep.) were significantly higher than in the dry month (Dec.). There were marked correlations between sap flow and ambient environmental factors (air temperature, air relative humidity, PAR, radiation and VPD), the sequence of correlated degree was: PARradiationVPDair relative humidityair temperature.