The leaf photosynthesis models The rate of photosynthesis in a leaf is determined by the rates of carboxylation and regeneration of ribulose-1,5-bisphosphate (RuBP) catalyzed by the enzyme RUBISCO (ribulose-1,5- bisphosphate carboxylase-oxygenase). The net leaf photosynthesis (An) is limited by the minimum of these two limiting processes (Farquhar et al., 1980; von Caemmerer and Farquhar, 1981): An = min (Ac, Aj)-Rd
Ac is the rate of photosynthesis when Rubisco activity is limiting and Aj is the rate when ribulose1,5-bisphosphate (RuBP)-regeneration is limiting. Rd is the rate of mitochondrial respiration.
Scaling of photosynthesis and respiration in a forest with leaf properties and temperature dependencies Among the most important components of global biogeo-chemical cycling are the processes that mediate the fluxes of carbon, water and energy between biosphere and atmosphere. The need for a clear understanding of the role of the terrestrial biosphere in global climate change generates a requirement for assessments of processes such as photosynthesis and respiration at large scales. A major difficulty in improving our understanding of the functioning of the biosphere-atmosphere system lies in the problem of effectively scaling measurements of the key processes, such as photosynthesis, respiration and evapotranspiration, to generate regional estimates of these fluxes. The photosynthetic capacities of leaves in canopies acclimate to the light environment in which the leaves are growing (Meir et al., 2002). Most canopy trees experience diverse light conditions during their lifetime, starting as seedlings on the poorly lit forest floor but gaining access to the well-lit canopy layer at maturity. Many tree species have different maximum photosynthetic capacities, i.e. photosynthesis rates at light saturation according to growth stage or light conditions, or both, as a result of differences in leaf morphological and biochemical properties (Larcher, 2003). It is well known that sun leaves, i.e. leaves of the sun crown, have higher leaf nitrogen and leaf mass per unit area, corresponding to higher photosynthetic capacities, than shade leaves, i.e. leaves of the shade crown. Shade leaves have higher leaf chlorophyll content and are thinner and thus have a lower dark respiration rate and light compensation point than sun leaves (Lambers et al., 1998). To guide development of models of carbon dioxide fixation there is a need for a detailed understanding of the changes in the photosynthetic capacities and respiration with the leaf chemical and morphological characteristics. The dark respiration of leaves plays a key role in the carbon economy of plants, but it is poorly understood in comparison to photosynthesis. Leaf respiration in forest canopies may consume 9-22% of gross primary production, and comprise 50-70% of above-ground (autotrophic) respiration. A linear relationship between dark respiration and leaf chemical (nitrogen and phosphorus) and physical (leaf mass per unit area) properties of forest trees has been shown in many studies.
Simulation of CO2 exchange of the Sundarbans It is recognized that the world’s forests contribute significantly to the global carbon (C) balance, and that changes in forest C uptake may act as an important feedback to the current increase in atmospheric carbon dioxide (Malhi et al., 1999). The interannual and interdecadal variability in climate, and other changes in the environment, like rising atmospheric CO2 concentration and large scale changes in land cover, have motivated several studies about the behavior of ecosystems in a changing environment. Such studies lead to the development of several numerical models to understand the effects of these changes on the carbon, water and energy fluxes between the ecosystems and the atmosphere. Models of carbon (C), water and energy fluxes play an important role in the quantitative understanding of both the functioning of forests and their impacts on the atmospheric C cycle (Jarvis, 1989; Sellers et al., 1997). Gross canopy photosynthesis (Pg) and foliage respiration (Rf) can be simulated with canopy photosynthesis models or retrieved from turbulent CO2 flux measurements above the forest canopy. Pg and Rf of the Sundarbans forest could be simulated with the forest canopy model MAESTRA. Biophysical parameters for the model simulation can be estimated from gas exchange measurements at leaf level. Meteorological data for the model simulation can be taken from a measurement tower established in the forest. The Sundarbans forest could be one of the important ecosystems in terms of regional and global carbon cycling; nevertheless, the impact of environmental factors on this ecosystem CO2 flux remains barely understood. Three-dimensional multilayer biosphere-atmosphere models such as MAESTRA (Wang and Jarvis, 1990; Medlyn, 2004) are promising tools for understanding how interactions between environmental factors and leaf-level physiological parameters might impact canopy-level CO2 exchange in the Sundarbans forest.