Effects of physiological and environmental variations on size-structure dynamics in plant populations

Sensitivity analysis was conducted, based on the canopy photosynthesis and continuity equation models which were developed in a previous paper (Yokozawa and Hara, 1992), to investigate effects of variation in physiological parameters (maximal photosynthetic rate per unit leaf area, respiration rate per unit leaf area, maintenance respiration rate per unit weight, growth respiration rate per unit weight, light extinction coefficient of the canopy, etc.) on the size-structure dynamics in plant populations. As the degree of asymmetry in competition between individuals increased, effects of variation in physiological parameters diminished. Therefore, a population undergoing one-sided competition (most asymmetric competition) is a stable system, little affected by temporal and spatial variations in the environmental conditions which lead to variation in physiological parameters, whereas a population undergoing symmetric two-sided competition is sensitive to these fluctuations. It was also shown by simulation that the degree of asymmetry in competition decreases (through effects on canopy photosynthesis) as nutrient level in the soil is reduced. It is suggested that symmetric two-sided competition is associated with non-transitivity of competition between species (i.e. frequent reversals of rank order of species), and hence with species diversity. Several other ecological phenomena are discussed in relation to allometry (i.e. allocation-growth pattern) and the degree of asymmetry in competition.