Germination and Establishment in Impatiens Capensis: Adaptations to Interspecific Competition.

Impatiens capensis, an annual herb, persists in particular localities in spite of competition from two perennial species, Urtica dioica and Eupatorium maculatum. Reported are experimental studies of mechanisms of germination and establishment of Impatiens that enable it to compensate for lack of overwintering vegetative tissue, which in established perennials allows rapid growth and development of above-ground parts early in the season. Impatiens seeds germinate earlier in the spring than do the seeds of Urtica and Eupatorium. Under a shadecloth designed to mimic the less variable microclimate found under litter, germination of Impatiens was earlier than the other species. Germination experiments in growth chambers showed that Impatiens' seeds can germinate at colder temperatures than seeds of Urtica or Eupatorium. Rapid growth rates for seedlings early in the season permit Impatiens to comm and an early height advantage. In growth chamber experiments, growth in height of Impatiens seedlings at a temperature regime similar to natural conditions during the period of establishment was no less than that at a warmer regime. Growth in height of Urtica and Eupatorium seedlings was significantly less at the cool regime. Biomass accumulation of Impatiens was less suppressed at the cool regime than that of Eupatorium but not of Urtica. Natural populations of Impatiens are often very dense; such populations form a canopy over other seedlings by early June. In a field experiment where seeds, seedlings and rhizomes of Urtica were placed among Impatiens seedlings, growth of seedling-derived Urtica plants was much suppressed. Only rhizome-derived plants exhibited much growth, and only a few of these flowered. Where Urtica seedlings and rhizomes were placed among other local species, virtually all rhizome-derived plants set seed. Impatiens growing in very high densities produce no more fruits per unit area than do Impatiens growing in lower densities, but the advantage of suppressing the establishment of perennial seedlings suggests an explanation for high density of Impatiens, and for the very unusual practice of Impatiens among annuals of maintaining no soil bank of ungerminated seeds.Ph.D.BotanyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/158368/1/8116357.pd