Model Simulation of Interspecific Competition between Two Plant Species in Relation to Light Environment

Abstract

Interspecific competition between plant species has long been interesting subjects among agriculturalists who have studied mixed culture of crops, range management, weed control and so on. But it is very difficult to explain the whole results about plant growth in mixed stands competing each other because of their complexity. Recently, however, it becomes able to analyze the dynamics of ecosystem by model simulation with electric computer owing to introduce the method of systems engineering into ecological research. This report deals with model simulation of plant gtowth in mixed stands competing each other in relation to light environment. In constructing a dynamic model of interspecific competition it was thought to be useful to apply the theory of system dynamics developed by J. W. Forrester, because it was simple and easy to understand. The direct object of the model construction in this study was limited to simulate the interspecific competition by simple factors and mathematics as possible, since it was easy to propose an alternative model after partly succes of a preliminary model. The model constructed has the following basic structures and functions. 1. Model plants grow in simple process, such as photosynthesis by leaves, respiration of plant organs and translocation of photosynthates (Fig. 1). Interspecific competition is assumed to play an important role only in relation to light environment of the canopy. 2. It is supposed that the competing force of the plants is realized by the following three subsystems; a. Decision of vertical distribution of leaf area in the canopy. b. Calculation of layers mean of radiation fluxes in the canopy. c. Calculation of gross photosynthesis in layers of the canopy. 3. Leaf area in each canopy layer is decided by top dry weight, plant height, canopy height, LAI and distribution pattern of leaf area density. 4. Radiation fluxes at the bottom of each layer are obtained by RAD(i)=RAD(i-1)EXP[-KC·LA(i)C-KW·LA(i)W], and layers mean of the flux densities are approximated by I(i)=[RAD(i-1)+RAD(i)]/2, where, RAD(0), RAD(i), solar flux density (constant value), and radiation flux density in the i-th layer, respectively; LA(i)C, LA(i)W, leaf area of crop and weed in the i-th layer; and KC, KW, extinction coefficient of crop and weed. 5. Photosynthetic rates in the i-th layer of crop is, then, approximated by PHS(i)C=[KC·BC·I(i)·LA(i)C]/[1+AC·I(i)·KC], where, AC, BC, parameters of light-photosynthesis curves. 6. Plants respire dark respiration which is proportional to its dry matter in day and night and light respiration which is proportional to the gross photosynthesis of the plant in daylight (12 hrs.). 7. It is assumed that the model plants have the flows of photosynthates from leaves to each organs after temporary store (one day). The following results were obtained by 100 day simulation of the interspecific competition model, supposing that crop was tipland rice and weed was crabgrass (Digitaria adsendens): 1. Comparison of simulated values obtained with parameters in Table 1 and initial values in Table 2 to observed values in field experiments reported earlier shows good agreement between both plant growth curves except only in the later growth stages of weed (Fig. (6). 2. The alternative values of the parameters, such as specific leaf area, respiration normals and growth normals (allocation ratio) of each organ, clearly show the dependence of growth curves of both plant on these parameters. It is evaluated that specific leaf area and growth normals have stronger effect than respiration normals (Figs. 7, 8 and 9). 3. Weeding experiments with the model simulations agree well to the field experiments obtained earlier, supposing a clean culture, weeding at early and middle growth stages of crop and no weeding with the use of the alternative values of initial dry matter of both model plants (Fig. 10).