COMPARISON OF LONG·TERM. TRANSPLRAnON OF DIFFERENT TREE SPECIES IN SEPARATE CANOPY LAYERS IN A KANDVAN FOREST GARDEN

Abstract

Kandyan Forest Gardens (KFG s) are a specific vegetation type covering a considerableland area of the Central Province of Sri Lanka. Therefore, water use by KFG s form animportant component of the catchment water balance of this area. KFG s consist of amixture of tree species whose canopies are arranged in different vertical layers.

The study was conducted in a typical Kandyan Forest Garden located at Pilapitiya,Pilimathalawa within the agroecological region ~IJ.·hA vegetation survey was carried outin this garden and it showed 201 trees belonging to 24 different species. Out of these, twotree species were selected to represent upper and middle canopy layers. They were jak(Artocarpus heterophyllus) and toona (Cedrella laona). Transpiration of these trees wasmeasured as the sapflow in their trunk using thermal dissipation probes. Measurements ofthe weather parameters were also taken. The incident solar radiation was measured usingthe tube solari meters. Relative humidity in the open and inside the KFG was measured bytwo solid state sensors. The soil moisture content al five soil depths (i.e. 20, 40, 60, 80 and100 em) was measured by gravimetric sampling. All the data except soil moisture weretaken at 30 second time intervals integrated over five minutes and stored in the data logger.Tile measurements that were taken during the period from 25.12.2001 to 20.02.2002 wereanalyzed.

Both species showed a similar pattern of daily variation of sapflow. However, the sapflowwas substantially greater in jak than in toona. The total water use of jak for theexperimental period was 3881.25 kg/tree. The corresponding value of toona was 462.83kg/tree. The upper canopy jak received a greater irradiance than middle canopy toona.There was a linear relationship between the radiation incident on jak and toona canopies.

Daily water use of the upper canopy jak showed a clear linear relationship with dailyirradiance on its canopy. However, the water use of toona did not show such a relationshipIn both species, there were significant negative linear relationships between daily sapflowand daily mean relative humidity. However, air temperature did not have a significanteffect on sapflow of both jak and toona trees. The canopy leaf area of jak remained more orless constant during the experimental period. But in toona, the canopy leaf area increasedduring the latter parr of the experimental period. Water use of both jak and toona did notshow a significant relationship with the variation of soil moisture content of the top l m ofthe soil profile. This indicated that the trees were extracting water from the deeper layers ofthe soil profile. It can be concluded from results of the present study that water use of aKFG is dominated by large upper canopy trees and that it is largely determined by theincident solar radiation. This study also showed that large trees in this ecosystem maintaintheir high levels of water use even during dry periods of the year by extracting water fromdeeper layers of the soil profile.