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 ofI _{0, max} equal to or greater than 80 klux was assigned to the upper stratum, where the higher the value ofI _{0, max} , the greater the biomasses and the carbon fluxes at the steady state as a result of enhanced productivity. WhenI _{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 toI _{0, max} elevation may have a detrimental effect upon the survival of the middle stratum, since increasingI _{0, max} decreases the light energy available for the middle stratum even in absolute terms, resulting in disappearance of this stratum whenI _{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 ofI _{0, max} between 80 and 110 klux, light conditions which are much higher than the light compensation point for canopy photosynthesis This article is dedicated to Professor Toshiro Saeki, University of Tokyo, in appreciation of the sincere encouragement he has given to the author.