Growth of Old Field Herbs on a Nitrogen Gradient

Abstract

Eight species of herbs that reach peak abundance at different times during the first 50 years of secondary succession on a nitrogen impoverished sand plain were grown at both low and high density for one and two field seasons along an experimental nitrogen gradient. The data demonstrate that intraspecific competition is strong even on extremely nitrogen poor soils. The dependence of relative growth rates (RGR), biomass per plant, yield per pot, root:shoot ratios and seed:shoot ratios on soil total nitrogen were determined. These nitrogen-dependent characteristics were compared with the known successional status of the species. Species most abundant on early successional, nitrogen poor soils tend to have higher RGRmaX, lower root:shoot ratio, higher yield per pot and higher seed:shoot ratio than late successional species of more nitrogen rich soils. This suggests that early successional species may be inferior nitrogen competitors but have faster growth rates and better colonization abilities than later successional species. This supports the hypothesis that the successional dynamics of these herbs are the transient dynamics of colonization and competitive displacement, but tends to refute the equilibrial version of the resource ratio hypothesis of succession (i.e. succession caused by changing nitrogen to light ratios). Key-words: Successional herbs, nitrogen-limited growth, root:shoot, seed:shoot, succession on a sand plain, relative growth rates, transient dynamics, successional trade-offs