SIMULATION OF FLUCTUATING POPULATIONS OF CAREX-BIGELOWII TILLERS CLASSIFIED BY TYPE, AGE AND SIZE

A projection matrix model was constructed using vital rates for a vegetatively reproducing tiller population of Carex bigelowii monitored for four years. Tillers were divided according to their age, size and type, as different morphological forms of tiller exist. Two matrices were used: one based on 1985 to 1986 transition probabilities when a massive flowering event occurred, resulting in a large decrease in population numbers, and one for the combined probabilities of the 1984 to 1985, and 1986 to 1987 transitions with very low flowering rates. When used separately, the two matrices led to rapid population growth, or extinction, respectively. When used in combination, fluctuations in population size occurred, similar to those found in the field. A long-term population growth rate close to stability (lambda = 1.001) was obtained when the cyclicity of flowering was set to every three years on average. An elasticity analysis revealed the large dependence of population growth on the two youngest age classes of the foraging (long-rhizome) tiller type. The ratio between tiller types is sensitive to environmental conditions such as soil fertility, and population growth rate will, therefore, respond markedly to changes in environment.