Although herbs typically contribute only a minor fraction of the total biomass or production of a forest, they may be critically important in regulating forest composition and dynamics through competition with tree seedlings. However, because forests are extremely variable in herb abundance, the impact of herbs on tree establishment is not consistent. Resource competition theory leads to the prediction that under conditions of abundant soil nutrients and moisture but low light, herbaceous plants will win in competition with woody plants, while higher light or lower soil resource levels should shift the competitive interaction to favor woody plants. Experimental manipulations of light and soil resources will be used the hypothesis that herbs on high-resource sites influence tree composition by reducing growth rates, and increasing mortality rates of tree seedlings. Observation made along herb-cover gradients will be used to test the general hypothesis that the light:soil resource supply ratio determines the outcome of competition between woody and herbaceous plants in understories. If this hypothesis is true, then canopy gaps, herb removal areas, and other low resource microsites could play a critical role by shifting the competitive balance to be more favorable for tree seedlings.
