Estimating Vegetation Function from Satellite Data
in Seasonal Tropical Environments
Data and Methods
Thailand has remaining areas of moderately natural forest as well as larger areas of substitute landscapes covering the full range from evergreen to deciduous and from forest to open savanna (e.g. ~ Ogawa et al. 1961, Santisuk 1988) .The composition, structure "~"~ and productivity of the relatively natural vegetation have been ~ studied in the field for some time (e.g. Kira et al.1967, Smitinand 1980) .The main natural landscapes and their phenology are shown schematically in Figure 1 (from Fujiwara 1993)
Figure 1. Schematic Zonation of Natural and some Substitute Landscapes (Vegetation) of Thailand.
Net primary production (NPP) is gross primary production (GPP, essentially photosynthesis) minus respiration (R) .GPP increases with warmth and water availability (other factors not limiting, Lieth & box 1977) , while respiration increases at least quasiexponentially (per unit biomass) with increasing temperature (e.g. Kira 1975) .NPP thus also increases with warmth and wetness (Lieth & Box 1972) but becomes a smaller fraction of GPP under warmer conditions (high respiration losses) and can become negative if GPP is low or zero, as in, an extreme dry season. Litterfall (dead leaves and other detritus) is also related to climatic conditions (Meentemeyer et al. 1982) ; detrital decomposition (D) also increases with warmth and wetness (Meentemeyer 1985) , and completes.
the equation for overall net ecosystem production:
net CO2 flux = GPP -R- D (1)
each component of which can be expressed in units of dry biomass or carbon equivalent. All components of this vegetation-detritus energy budget, as well as atmosphere-biosphere water fluxes, can be simulated at monthly intervals by the model MONTHLYC (Box 1988) , which is based on individual process models calibrated globally to climatic data and now NDVI from field-measured annual metabolic data. In the absence of field data for monthly metabolism, monthly climatic GPP is driven by AET (actual evapo- transpiration) , its closest apparent correlate (cf. Box et al. 1989) .Respiration is a function of temperature and is simulated therefrom using the usual Q1O value of two. For comparison purposes, field-measured values for natural and artificial primary production in Southeast Asia are shown in Table 1.
Satellite data for Southeast Asia, especially NDVI data, have been received, converted into imagery, and interpreted by various groups, including the Asian Institute of Technology in Bangkok, the Institute of Industrial Science at Tokyo University, and the NASA Goddard Space Flight Center in Washington. For this study, however, individual NDVI pixel values are used, since it is at pixels corresponding to field measurement sites that comparisons, calibrations and validations must ultimately be done.
Table 1. Field-Measured Primary Production in Tropical Asia.
| Standing Production |
| Location |
Vegetation |
Biomass |
Gross |
Net |
Source |
| Khao Chong |
rainforest |
32.6 |
12320 |
2860 |
(4) |
| Ping Kong |
monsoon forest |
29.1 |
7000 |
1190 |
(6) |
| savanna |
woodland |
7.8 |
3200 |
760 |
(6) |
| Pasoh |
rainforest |
52.5 |
8190 |
2740 |
(5) |
| Cheko |
rainforest |
41.5 |
11700 |
-- |
(3) |
| Substitute/successional vegetation: |
| Gorakhpur |
teak plantation |
74.3 |
-- |
2665 |
(1) |
| Varanasi |
succ. dry forest |
3.5 |
-- |
301 |
(1) |
| subst. grassland |
1.0 |
2166 |
1177 |
(2) |
Biomass values are in units of kg/m
2, production values in units .of g/m
2/year, each for dried biomass. Net production was
generally estimated by harvest methods, whereas gross production
! involved gas-exchange, micrometeorological and other methods.
Sources: 1 = Cannell 1982, 2 = Dwivedi 1971, 3 = Hozumi et al.
1969, 4 = Kira et al. 1967, 5 = Kira 1978, 6 = Yoda 1967.
Results
Parallel monthly values of NDVI, climate and correspondingly t driven simulated landscape metabolism are compared in Table 2 for
Ping Kong, a site in northern Thailand {near Chiang Rai) where
vegetation metabolism has been field-measured {Yoda 1967) .The !
r potential natural vegetation at Ping Kong is mixed deciduous ~ "monsoon" forest, but the actual landscape is evidently partly r evergreen, as seen from the NDVI values, which never drop near r zero. NDVI does parallel to some extent the seasonal course of
precipitation and AET, all rising sharply in May. The monthly patterns of climate-driven and NDVI-driven GPP are quite different, however, and thus also the resulting monthly patterns of NPP and overall metabolic balance {equation 1) .Even with significant NDVI values and NDVI-simulated GPP during the dry season, however, the corresponding NPP still falls negative in April and the overall C02 balance is negative for five months {with a time lag due to the smaller amplitude of the GPP curve for the more evergreen actual landscape) .
