Monitoring Tropical Rain Forest Microclimate
Total Page：16
File Type：pdf, Size：1020Kb
PEER.BEYIE}YED ARIICTE MonitoringTropical Rain Forest Mictoclimate JanetE, Nichol Abstract TheStudy Area Singapore's original tropical rain forest cover has largely The study area comprisesthe central portion of Singaporeis- been removed, leaving only a total of 2700 hectares, or 5 Iand containing the CatchmentArea Nature Reservesand is percent of the island, under primary and secondary forest. bounded on all sides by major trunk roads (Figure 1)' This This area in the center of the island is protected as water area totals 60 sq km of forest,reservoirs, and some urban catchment and nature reservesin spite of intense pressure and suburban development (Figure 2). The most serious en- for development. croachmentinto the forestedarea is from golf courses,from The paper investigates the application of Landsat TM bungalow style residential developmentin the southeast,and thermal data to surface temperature mapping in the vicinity from an expressway,constructed in the mid-1980s,which of Singapore's forested central catchment area as an indica- has effectively divided Singapore'smain primary rain forest, tion of the relative degree of disturbance to the original rain- Bukit Timah Nature Reserve,from the rest of the Central forest microclimate. This will assistin the designation of CatchmentArea. The latter is largely 70- to BO-yearsucces- priority consewation status to areas which remain structur- sion secondaryforest, though significant areasof mature for- ally and microclimatically viable as primary rain forest. est, including Nee Soon Swamp in the northeast,remain' Apparent temperatures were coftected for emissivity dif- Terrain is locally steep,and elevationsof over 160 m are ferences of different land-cover types. Pixel data were then found in Bukit Timah Nature Reserve,which is classedas input into a vector GIs and overlaid with existing coverages Hill DipterocarpForest. representing High Forest, Water, Urban Areas, and Elevation Concern has been expressedabout the future regenera- in order to examine the thermol charocteristics of each land- tion statusof this reservedue to its small size and isolated cover type. Distinct spatial variations in thermal characteris- state,though this is difficult to assessdue to the longevity of tics are observed to corrcspond to land-cover differences and rain forest trees in comparison with the rate of environmen- to parameters of forest conservation value. tal changes. The study area is a mixture of lowland and hill diptero- Introduction carp forest,and secondaryforest (Corlett, 19BB),of which the gronpr Shorea,Hopea, Dipterocarpus,and Dryobanalopsare The differential spatial distribution of sreen biomass in Sin- "microclimatic welllepresented. These forest generarequire cool and humid gaporegives rise to highly contrasting condi- conditions for successfulregeneration. Trials have shown tions which can readily be observedon daytime LandsatTM humidity must be above 95 percent to keep dip- thermal imagery, and can be related to other variablessuch that relative terocarp seedsviable with over 20 percent moisture content as topography,land cover, and population concentrations. (Sasaki,1980) because seed deterioiation depends on mois- The accuracyof the satellite derived data is, however, ture content and temperature.Tamari (f gzo) found that the affectedby the complexity of interactionsbetween electro- maximum temperaturetolerances for seedviability in species magnetic radiation and the earth surfaceand atmosphere of and Hopea were 34oCand 35"C,respectively, while within the field of view of the sensor.The most important Shorea the optimum temperaturefor germination of Hopea halferi interactions are the absorptionand emittancepropeities of was between 25'C and 30'C. When the forest at Bukit Timah water vapour and other atmosphericconstituents and differ- was replacedby secondaryforest and golf courses,tempera- ential emissivity (e) of land-covertypes, tures increasedand humidities and soil moisture were re- Thus, although satellite derived radiance values can duced (Hill, 1966),thus exceedingthe tolerancelevels for readily be converted to Apparent Temperature(Black Body tree regeneration. Equivalent Temperature)using Planck's law (Malaret ef 41., Whitmore's observation(rsa+) of the death of B0 Shorea 1985),this usually underestimatesSurface Temperature (Ts) curtsii trees at SenalingInas, Malaysia, in 1977 following a if correctionsfor e are not carried out. The resulting Ts, dry period also suggeststhe importance of maintaining a hu- moreover, can only be consideredaccurate in clear, dry at- mid microclimate, and Richards (t952) notes the requirement mospheres,and a further correction using atmosphericdata for a moist atmosphereat critical growth phasesof rain for- should be made if absolutetemperatures are required. The est plants, such as germination or flowering. presentsl.udy assumed that atmbsphericwaterjapor was Aoki ef a|. (1.s78),working in PasohForest Reserve, Ma- uniform over the study site. Becauseemissivity differences laysia, approximately 200 km north of Singapore,show 8:45 are not significant for vegetatedsurfaces, correction was only Ru to be the approximatethermal crossovertime between carried out for broad land-cover classes. The main oblective of the study was to evaluateLandsat TM thermal data for monitoring the distribution of tempera- PhotogrammetricEngineering & RemoteSensing, tures in and around Singapore'sRain ForestNature Reserves, Vol. 61,No. 9, September1995, pp. 1159-1165. including analysis of the interior forest microclimate. oo9}jt't 1.2I S5l6109-1 1 59$3.00/0 Division of Geography, NanyangTechnological University, @ 1395 American Society for Photogrammetry 469 Bukit Timah Road, Singapore1025. ald Remote Sensing PE&RS PEER.REVIEWED ANTICTE ttGtfi0 El ronrsr |:1 UREAN ! weren I sr-nunaaru/uxro F:ll GoLFcorRsE --- NAIUEERESER1r'ES BOUNDARY - EXPBESSWAY tukit Timah 0t23km Sourco, LANDSA"I'IM, 24.5.89 Figure2. Landcover in and aroundthe CentralCatch- ment Area. Figure1. ThematicMapper thermal waveband of Singa- pore lsland (coolestareas with darkertone). pixel data, is usually a point measurement, and not spatially averaged. temperatures above the canopy and at ground level. The time of the overpasswas 2 hours later (10:40Ru solar time) and air temperatures in Singapore (mean value for six air bases) EnelgyBudget had reached 30"C, with minimum and maximum values for Spatial variations in canopy temperature may be examined that day of 25.5"Cand 32.2'C, respectively.Thus, land-cover with reference to the energy balance for a plant canopy related differences in surface temperature should be evident. (Equation 1) for which the physical and biological storage The date of image aquisition, 24May 1989, corresponds components(G) can be considerednegligible during daytime to the end of the February to May period which has the low- (Oke,1987): i.e., est rainfall and highest sunshine amounts. Climate data at (1) Macritchie Reservoir(Singapore Meteorological Service, Qn:LE+H+G L990) indicate a warm dry period during the eight days prior where Qn is the net all wave radiation flux, LE is the latent to image aquisition, with a total of only B mm (mean heat flux, and H is the sensible heat flux. rainfall for May is 173 mm). On the six individual monthly Horizontal temperaturevariations in a closed canopy for- days prior to 24 May,24-hourly mean temperaturesin Singa- est are, thus, related to the amount of net radiation (Qn) and pore mean values for the month (27.3'C),and mean exceeded the partition between H and LE.The partitioning depends ielative humidity was lower than the monthly mean value of both on moisture availability and on the heat regulating 86.6 percent. On 24 May, 10.4 hours of sunshine,the highest mechanism of leavesby stomatal closure,generally referred for month, were recorded;relative humidity reachedan the to as canopy resistance(Rb). This is likely to be greaterdur- exceptional low of 55 percent, and these warm and dry con- ing moisture stress.Under normal conditions, if Rb is small, ditions culminated in a thunderstorm during the night of 24- canopy temperatureis closely coupled to air temperature 25 May. Because24 May also recorded the highest wind (Oke, 1987),as surplus canopy heat is used for evapotran- speedsfor the month (2.4 m sec-1,with a monthly mean of spiration, thus maintaining a balancebetween biomassand 1.3 m sec-'), optimum atmosphericconditions prevailed for air temperature. evaptranspirationand, thus, latent heat loss from the forest In such conditions, mature closed canopy forest may be canopy. expected to be cooler than immature forest with an open canopy due to a high leaf areaindex and efficient water up- Thelmportance ofthe Rain Forest Canopy and the take from a well develooedroot svstem in a mature soil. thus Significanceof Forest Canopy Temperatute. promoting LE.However,^in conditions of soil moisture stress, The relevanceof satellite derived Ts for studies of forest mi- an increasein Rb is likely to result in increasedcanopy tem- croclimate depends largely on the relationship between tem- peraturesfor all forest types. If wind speedsalso happen to perature at the canopy and that within the forest. Satellite be low, both types of convectiveheat loss are restricted,re- derived temperaturescorrespond to SurfaceTemperature sulting in a lower relationship between canopy and air tem- which, in a forest zone, is representedby the temperatureat perature, the top of the canopy. However, the relationship between re- During occasionaldry periods