INTEGRATION OF ECOLOGICAL LEVELS - INDIVIDUAL PLANT-GROWTH, POPULATION MORTALITY AND ECOSYSTEM PROCESSES

The relationships between exclusive crown area, crown-area increase and density of plants are used to derive rates of population thinning in even-aged stands. The theory incorporates initial density to determine the time to reach a "temporal self-thinning line' prescribed by density-dependent mortality and the relationship between individual plant growth and population mortality. The maximum rate of per-area increase in exclusive crown-area projection is equivalent to the maximum per-capital mortality rate. Self-thinning proceeds at a near-constant rate until plants approach maximum size, at which point the rate of thinning decreases. These results are used together with density-plant-weight relationships (-3/2 power law) and Michaelis-Menten uptake dynamics for light and below-ground resources to examine the influence of individual plant growth and population mortality on the timing of net primary production (NPP), the relationship between NPP and rate of individual plant weight gain, resource limitation, and below-ground resource pool size. NPP is maximized at intermediate stand ages, and this maximum occurs sooner than does the maximum in the rate of individual plant weight gain. Limitation by below-ground resources is most severe as NPP is maximized. This serves to reduce demand and thus the extent to which pool size is reduced through its reduction in growth, and hence in uptake. Increased resources decrease density, because increased growth increases thinning rates. Because of the low potential relative growth rate of large individuals, large old plants gain little advantage by increasing allocation to structures that increase resource uptake. Theory suggests that resource competition among large old-canopy individuals is minimal, because of increased nutrient availability, decreased demand and decreased sensitivity of growth rate to nutrient levels. -from Author