Photosynthesis in a tropical montane rainforest of Southeast Asia: Field measurements and model analysis

Tropical rain forests occupy about 12% of the terrestrial surface and are assumed to contain as much as 55% of the carbon stored in the world as terrestrial biomass. Of the three major blocks (South and Central American, African and Asian) of tropical rain forests in the world, the Asian block (centered on southeast Asia) is the least studied in terms of ecosystem functioning and of regulating the global climate through carbon and water cycles. Models of photosynthesis have been widely used for such characterization of plants and forests. The biochemically based leaf photosynthesis model of Farquhar et al. (1980) was parameterized for selected tree species of the tropical montane rainforest at central Sulawesi, Indonesia, to characterize the photosynthetic capacities and their regulations. The three key parameters of the photosynthesis model, the maximum carboxylation rate of Rubisco (Vcmax), the maximum electron transport capacity (Jmax) and the dark respiration rate (Rd) were estimated from the relationships of leaf photosynthesis (A) to intercellular carbon-dioxide concentration (Ci), irradiance (Q) and leaf temperature. The field study was conducted in 2005 within the interdisciplinary research project Stability of Rainforest Margins in Indonesia (STORMA) in the Lore Lindu National Park in Central Sulawesi, Indonesia. The objectives of the study were (i) to estimate the photosynthetic capacity and dark respiration (parameter values for Vcmax, Jmax and Rd) of major tree species, (ii) to quantify the variability in these parameters, (iii) to determine the sources of variability, (iv) to scale photosynthetic capacity and respiration from leaf level to canopy level and (v) to simulate gross primary productivity and foliage respiration of the forest, with a canopy photosynthesis model, and to compare the results of the model with the results of measurements. Data on photosynthetic capacity, respiration and leaf biochemical and morphological properties were collected around the meteorological tower built by STORMA. Photosynthetic capacity of the sampled tree species in general was found to be high, suggesting that the trees in that forest are strong assimilators. Vcmax25 values, i.e. Vcmax values at 25 °C, ranged between 20.0 and 61.0 µmol m-2 s-1, and Jmax25 values ranged between 47.0 and 107.0 µmol m-2 s-1. Foliage dark respiration (Rd) values were generally low and Rd25 ranged between 0.77 and 1.30 µmol m-2 s-1. Variations in the temperature dependence, that is activation and deactivation energies, of Vcmax, Jmax and Rd among species was much less compared to the variations in the values of Vcmax25, Jmax25 and Rd25. Measured similarity in the temperature dependence implies similarity in the physiological processes among leaves and species of the investigated forest. The sources of variation in photosynthetic capacity (Vcmax and Jmax) and respiration (Rd) among leaves and species could be well described by leaf chemical (nitrogen and phosphorus per unit area) and physical (leaf mass per unit area) properties. Leaf nitrogen concentration (Na, g m-2) described the variation of photosynthesis parameters best and overall there was a strong and significant linear relationship (P, ranging from 0.05 to 0.001 among species) between physiological model parameters and leaf nitrogen. Despite the species differences in photosynthetic capacities and respiration, it was possible to upscale leaf level measurements to canopy level. It was also possible to simulate the canopy photosynthesis with considerable agreement to the results of measured turbulent CO2 flux. Physiological parameters for a canopy photosynthesis model were derived by up-scaling leaf level parameters with Na and temperature dependencies of photosynthesis and respiration. A one-dimensional forest canopy was generated from the measured forest structure data to represent the investigated forest. The canopy photosynthesis model MAESTRA was used to simulate the carbon-dioxide exchange for the months of May, June and July, with the estimated parameters and measured meteorological data. Model outputs were sensitive to parameters related to crown structure and physiology. Simulated gross primary productivity (Pg) and canopy foliage respiration (Rf) rates were 7.15 g (C) m-2 day-1 and 3.18 g (C) m-2 day-1 respectively, in yearly terms that would be 2608 g (C) m-2 year-1 and 1161 g (C) m-2 year-1 respectively. The simulated Pg was only 4.0% less compared to the estimated value from turbulent flux measurements. Pg of the investigated forest was expected to be driven only by absorbed photosynthetically active radiation (Qabs), making a constant light use efficiency (EGPP). But that was not the case in the model investigations. It was influenced by vapour pressure saturation deficit (D) and EGPP of the investigated canopy was not constant. Pg was found to be saturated at higher light intensities, not only at half hourly time scales but also in both daily and monthly time scales. Pg saturation level of light (Qsat) was decreasing under high D. This saturation of simulated primary productivity is also seen in data of light response curves from eddy covariance flux measurements. These findings are important when developing models for CO2 fixation in tropical montane rain forests. Further studies are needed in this diverse forest to be able to explain seasonal variations in the Pg saturation level that are not connected to variations in D.