Studies on the dynamic properties of terrestrial ecosystems based on a simulation model I. Critical light conditions for stability of a tropical rainforest ecosystem

By employing a microcomputer model developed in a previous study (Oikawa, 1985), the stability of a tropical rainforest ecosystem composed of three strata was analyzed in relation to incident light flux density. Surplus production ( P s ), calculated as a function of the leaf area index and light attenuation coefficient, was remarkably affected by the maximum illuminance at noon ( I 0, max ). Simulation experiments for a period of 100 years demonstrated that the upper stratum was able to reach a steady state at about 50 years and thereafter, when a value of I 0, max equal to or greater than 80 klux was assigned to the upper stratum, where the higher the value of I 0, max , the greater the biomasses and the carbon fluxes at the steady state as a result of enhanced productivity. When I 0, max was assigned a value of 70 klux, on the other hand, this experiment predicted a failure of the upper stratum to maintain stability due to deficiency of surplus productivity. Moreover, it was also suggested that excessive luxuriance of the upper stratum due to I 0, max elevation may have a detrimental effect upon the survival of the middle stratum, since increasing I 0, max decreases the light energy available for the middle stratum even in absolute terms, resulting in disappearance of this stratum when I 0, max is equal to or greater than 120 klux. These simulation experiments suggested that a tropical rainforest ecosystem composed of three strata is able to exist within a narrow range of I 0, max between 80 and 110 klux, light conditions which are much higher than the light compensation point for canopy photosynthesis