Growth of old field herbs on a nitrogen gradient

Eight species of herbs that reach peak abundance at different times during the first 50 yr of secondary succession on N impoverished sand plain were grown at both low and high density along an experimental N gradient. Intraspecific competition is strong even on extremely N 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 N were determined. Species most abundant on early successional, nitrogen poor soils tend to have higher RGR_Max, lower root:shoot ratio, higher yield per pot and higher seed:shoot ratio than late successional species of more N-rich soils, ie early successional species may be inferior N 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 (ie succession caused by changing nitrogen to light ratios). -from Authors