Experimental Ecology of Dryas Octopetala Ecotypes: II. A Demographic Model of Growth, Branching and Fecundity

Abstract

SUMMARY (1) A general model of plant growth, branching, and seed production is described. The model simulates plant growth by considering the plant to be an assemblage of repeated units (modules). A shoot module, defined as a meristem and associated live leaves, passes through growth, branching, and flowering stages governed by size-dependent transition probabilities. Lefkovitch matrices formed from these transition probabilities are used to project shoot-module population size, and therefore, plant growth. (2) Model parameters were calculated for two ecotypes of Dryas octopetala, a perennial dwarf shrub of circumpolar arctic-alpine distribution. Shoot-module size categories were defined by leaf area. Censuses of tagged shoots in 1979 and 1980 revealed size-dependent growth, branching, and flowering patterns for both ecotypes. (3) Rates of new shoot production increased with shoot size as did the proportion of flowering versus branching shoots. Contrasting patterns of allocation of materials for the two ecotypes maintained positive and comparable rates of vegetative growth for natural populations found in different habitats in the field. (4) A sensitivity analysis showed shoot population growth to be most sensitive to annual growth increments, especially of small shoots, and less sensitive to the rate at which shoots produced 'offspring' shoots. (5) Growth was relatively insensitive to the proportion of shoots which flowered (rather than branched), the number of offspring shoots produced per branching shoot and the number of offspring shoot3 produced per flowering shoot.