Density-dependent flowering phenology, outcrossing, and reproduction in Impatiens capensis

Summary

We investigated the effects of plant density on cleistogamous (CL) and chasmogamous (CH) flowering phenology and seed production in a natural Impatiens capensis population, by censusing individually marked plants at experimentally reduced and natural densities. CL flowering was earlier at natural density. This plastic density response may have resulted from a stress-related threshold for CL flowering; slower growing plants at natural density flowered earlier. Although apparently triggered by slow early growth, early CL flowering also involved an additional cost for later growth rate. In contrast, CH flowering was unrelated to relative growth rate, but apparently required a size threshold. Experimental density reduction resulted in earlier CH flowering and a dramatic increase in the percentage of plants producing CH flowers. Individual CL and CH flowering duration and flower production were greater at reduced density. These density-dependent effects caused differences between treatments in the shape and location of population flowering phenology curves. Moreover, the percentage of CH seeds produced per individual was much higher at reduced density. At natural density total seed production per plant was lower and more hierarchical than at lower density, suggesting that dominance and suppression shape jewelweed fitness distributions.