RADIATION USE EFFICIENCY AND ABOVE-GROUND BIOMASS PRODUCTION OF SELECTED FOREST TREE SPECIES

Radiation use efficiency (RUE). defined as biomass produced per unit ofradiation intercepted. is considered a genotype-specific constant. Ifthe amountof radiation intercepted is known. RUE can be used to predict the potentialbiomass production o]'a given tree species. A comparative growth analysis wasdone on some selected species, with the objective of determining their lightinterception characteristics, biomass production, and RUE. Similar-agedseedlings of six forest tree species: Acacia mangium, Eucalyptus grandis,Leucaena leucoccphala, Swietenia macrophylla, Azadirachta indica, andTectono grandis, were planted at the university farm, Peradeniya, from July1995 to June 1996. Regular sampling was done to determine above-groundbiomass and leal area. Canopy radiation interception was estimated fromsimultaneous measurements of incident and transmitted radiation. by tubesolarimeters. In a/I six species, above-ground biomass production was found tvbe linearly related to cumulative intercepted radiation. The slopes of therelationships, which indicate R liE. showed significant inter-species variation.The greatest RUE was observed in E. gram/is (3.05 gA1fI) and the least inAzadarachta indica (0.296 gUfI). The differences in RUE were primarilyresponsible for the differences between species in above-ground biomassproduction. Radiation interception by the canopies ofthe different species wascharacterized by the light extinction coefficient, which showed significant interspeciesvariation. These parameters offer a promising approach Jill' modellingand prediction ofbiomass production by forest tree species

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

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. 

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

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.

Capacity for Carbon Sequestration and Climate Change Mitigation in Different Ecologically-Distinct Zones of Sri Lanka

Vegetation has the capacity to mitigate greenhouse gas (GHG) induced climate change byabsorbing and sequestering carbon dioxide, the principal GHG, in plant biomass. Sri Lanka,an island located in the humid tropical South Asia, has a considerable range of ecologicallydistinctzones (EDZs) as a result of the spatial and temporal variation of its climate. TheseEDZs are characterised by different dominant vegetation types and ecosystems with varyingground cover. Hence, the carbon sequestration capacity which determines the strength of the„land carbon sink‟ is likely to vary in the different EDZs. Analysis of long-term climatic datahas shown that trends of climate change (i.e., increasing atmospheric temperature andpotential evapotranspiration and decreasing precipitation and soil water availability) of thedifferent climatic zones of Sri Lanka reflect the established global trends. These trends inclimate change are likely to modify the carbon sequestration capacity of different EDZs overtime. Therefore, the objective of this work is to estimate the carbon sequestration and climatechange mitigation capacity of different EDZs of Sri Lanka and its historical variation todetermine the possible impacts of climate change.Simulations from nine dynamic global vegetation models (DGVMs) were used to estimatecarbon balance parameters such as net primary productivity (NPP), heterotrophic respiration(Rh) and net biome productivity (NBP) for eight 1o(latitude)×1o(longitude) grid cellscovering Sri Lanka. Models were run over the period from 1900 to 2009 using the climateforcing data from CRU-NCEP, which were validated using data from the MeteorologyDepartment of Sri Lanka. Carbon balance parameters were calculated for six ecologicallydistinctzones of Sri Lanka (i.e., south-west, central highlands, eastern coastal plain, northwest,north-east and north) that were defined based on 1o×1o grid cells. A validation check ofthe model outputs was done by comparing simulated NPP with actual NPP for selectedvegetation types. An initial analysis of all nine DGVMs, which included models running atdifferent resolutions (3.75o×2.5o, 2.5°×2.5o and 0.5o×0.5o) showed substantial within-zonevariation and did not clearly distinguish carbon sequestration capacities of different EDZs.This was probably because of spatial averaging of outputs from coarse resolution modelsacross different EDZs. A second analysis with the four finer resolution DGVMs showedsubstantially improved results. Subsequent simulations running the fine resolution modelJULES on a finer grid of 0.5o×0.5o allowed estimation of carbon balance parameters in thirtyfive0.5o×0.5o cells, which substantially-improved the spatial resolution of estimated carbonsequestration capacities of different EDZs of Sri Lanka. Temporal and spatial trends of theestimated carbon balance parameters will be presented along with analyses of theirunderlying causes and climatic drivers.Keywords: Terrestrial carbon balance, Net primary productivity, Climate change, Sri Lank

Estimation of Canopy Leaf Area Index of Tropical Rainforests of Sri Lanka from the Normalized Difference Vegetation Index along a Wide Altitudinal Gradient

The leaf area index (LAI) of a forest is a key determinant of its primary productivity. Estimation of LAI  via remote sensing of forest canopies offers an alternative to direct measurement, which is difficult in  tropical rainforests (TRFs). Our objectives were to determine the variation of LAI and Normalized Difference Vegetation Index (NDVI) of a range of TRFs of Sri Lanka across a wide altitudinal range (117-2,132 m above sea level) and develop a predictive model to estimate LAI from NDVI and canopy  architectural properties. Ten permanent sampling plots (PSPs) of one hectare each were established in forest reserves of Kanneliya (117-174 m), Sinharaja-Pitadeniya (509-618 m), Sinharaja-Enasalwatte (1,042-1,065 m), Rilagala (1,668 m), Hakgala (1,804 m), Piduruthalagala (2,080 m) and Horton Plains (2,132 m). Canopy LAI and its architectural properties (i.e., Mean Leaf Angle (MLA) and leaf angle distribution) were computed by analysis of ‘fish-eye’ images obtained from September 2019 to July 2020, using hemispherical photography and HemiView software. Satellite images for March-April, 2020 were downloaded from Landsat 8 OLI/TIRS C2L1. NDVI (NDVI=λNIR-λRED/λNIR+λRED) was calculated from ENVI software, where λNIR and λRED are reflectances of near-infrared and red wavebands. ENVI software computed the maximum NDVI (NDVIMax) and minimum NDVI (NDVIMin) values among 30 m×30 m pixels within each PSP. Mean NDVI (NDVIMean) was computed by taking the mean of NDVI values of all pixels within a PSP. Canopy LAI ranged from 1.94 (Pidurutalagala) to 3.38 (Pitadeniya). The corresponding ranges for NDVIMax, NDVIMean and NDVIMin were 0.620-0.767, 0.594-0.764 and 0.429-0.747 respectively. Canopy LAI, NDVIMax and NDVIMean showed significant (p<0.05) linear decreasing trends with increasing altitude. For every 1,000 m increase in altitude, LAI, NDVIMax and NDVIMean decreased by 0.396 (Adjusted-R2=0.407, AIC=-19.92), 0.066 (0.698,-66.72) and 0.058 (0.493,-61.27). In contrast, NDVIMin did not show a significant trend with altitude. Second-order polynomial functions showed greater explanatory power than the linear functions, in terms of adjusted-R2and AIC, in fitting the variation of LAI (Adj.-R2=0.492, AIC=-20.77) and NDVIMax with altitude (0.755,-68.15). The estimated maximum LAI and NDVIMax were at 637 and 329 m above sea level respectively. From among a range of multiple linear regression models using different combinations of NDVI, canopy architectural properties and altitude, the following two models were selected for predicting LAI, based on their adj-R2 and AIC values: LAI=1.269+[3.567.NDVIMax]–[1.598.NDVIMin]–[0.000215.Altitude] (Adj[1]R2=0.544, AIC=-21.41); LAI=-0.673+[5.887.NDVIMax]–[1.490.NDVIMin]–[0.000452.MLA] (0.489, -20.27). Keywords: Normalized Difference Vegetation Index, Leaf Area Index, Tropical rainforests, Altitude, Canopy architectur