TRANSPIRATION OF TREE SPECIES IN DIFFERENT VERTICAL LAYERS OF A KANDYAN FOREST GARDEN

Abstract

Water use of a forest ecosystem is: nn important determinant of the water balance of a wholewatershed. Kandyan Forest Gardens (KFGs) contain a mixture of different tree species. Thedifferent tree species are of different sizes and are arranged into distinct vertical canopy layers.The total water use of the KFG is determined by the transpiration rates of individual trees, whichform these different vertical layers.

The study was conducted in a KGF at Pilapitiya, Pilimathalawa in the Mid-Country Wet Zone(WM3) during the period from Marci to July 2001. A vegetation survey showed that there were56 tree species in this KFG. Out cf these, three tree species representing three vertical canopylayers were selected for continuous measurement of transpiration. These were Jak (ArtocarpusheterophyIlus), toona (Toona ciliata) and Mahogany (Swietenia macrophylla) which representedupper, middle and lower canopy layers respectively. Transpiration of all trees was measured asthe sap flow in their trunks using two thermal methods. In jak (DBH = 40.5 ern) and toona(DBH=9cm), which had larger trunks, sap flow was measured using thermal dissipation probes.In mahogany, which had a smaller trunk (DBH=3cm), sap flow was measure using a dynagage.Solar radiation incident on the respe-ctive tree canopies were measured using tube solarimeters.Relative humidity in the open and inside the KFG was measured by two solid state humiditysensors. Soil moisture content at three soil depths (30, 60 and 120 em) was measured using thetaprobes. All measurements were done continuously at 30 second time intervals using a datalogger. The data were averaged every 5 minutes and stored in the logger.

Detailed continuous measurements taken during a n-hour period (i.e. from 0000 hours on23.06.2001 to 2400 hours 25.06.20)1) were analysed. All three-tree species showed similardiurnal variation patterns in their S8P flow rates with maximum rates occurring between 1300and 1400 hours. Sap flow rate of jak was significantly greater than those of toon a and mahogany,which did not differ significantly. However, the velocity of sap flow was highest in the smallesttree, i.e. mahogany, and decreased with increasing sap wood cross-sectional area. The dailytranspiration rates oftoona and mahogany ranged from 19% to 27%. The incident solar radiationwas highest on the upper canopy tree (i.e. jak), Both the middle canopy (toona) and lowercanopy (mahogany) trees received similar levels of radiation because of the incomplete uppercanopy cover in the KFG. There was a clear linear relationship between daily transpiration of alltree species and the solar radiation levels received by the respective trees. This relationship canbe used to predict the daily water use of a given tree when there is no significant soil waterdeficit.

The relative humidity (RH) under both open conditions and within the KFG showed a similarvariation pattern. However, RH within the KFG was slightly higher than that in the open.Transpiration rates of all tree species had negative linear relationships with the respective RHvalues experienced by them.