RESULT FRONTIER Newsletter No.11 July.2000

Why is CO2 gas balance important for the water balance between vegetation and the atmosphere?

Katsunori Tanaka (Hydrological Cycle Research Program)

A vegetation model is used to explain why it is important to consider the CO2 gas balance when considering the water balance between vegetation and the atmosphere.

Fig. 1 shows the simulated evapotranspiration and CO2 absorption in a tropical monsoon forest and how the total leaf area per square meter (LAI) influences both. Within the model, the evapotranspiration consists of transpiration from stoma and evaporation of rainfall and condensation from wet leaves, while CO2 absorption in vegetation consists of net photosynthesis by leaves and CO2 release from the ground (Fig. 2). The amounts of evapotranspiration and CO2 absorption are calculated within the model using biological, optical, and physical characteristics of leaves, the vertical LAI profile, and so on.

The amount of evapotranspiration increases with the LAI and reaches a maximum (Fig. 1). On the other hand, the amount of CO2 absorption increases with the LAI initially, until the LAI reaches a value of 3, and then decreases until the vegetation releases CO2 gas into the atmosphere. The simulated vertical profile of CO2 absorption shows that leaves in lower layers release CO2 gas and are useless in CO2 assimilation when the LAI exceeds 5. This is mainly because these lower leaves do not receive the full amount of short wave radiation needed for photosynthetic activity (Figs. 1 & 2).

Since plants tend to assimilate CO2 gas while they retain moisture, a canopy structure that results in a small amount of CO2 absorption and a large amount of evapotranspiration is improbable. In this simulation, the optimal LAI of vegetation is approximately 3, so the amount of evapotranspiration is likely the amount that occurs when the LAI is about3.

I believe that the amount of evapotranspiration over different vegetation surfaces can be judged by calculating the amount of CO2 absorption simultaneously.

a) The relationship between the simulated evapotranspiration
and CO2 absorption in a tropical monsoon forest and LAI.
b) The vertical profile of CO2 absorption.

CO2 Absorption
Water balance and CO2 gas balance in vegetation,
a) when the LAI is 2-4 and b) when the LAI is 5-7.

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