The Resource-Ratio Hypothesis of Plant Succession

The resource-ratio hypothesis assumes that each plant species is a superior competitor for a particular proportion of the limiting resources and predicts that community composition should change whenever the relative availability of two or more limiting resources changes. It is suggested that (1) the major limiting resources for mesic terrestrial habitats are often a soil resource, especially nitrogen, and light; (2) these resources are naturally inversely related, the habitats with poor soils having high-light availability and the habitats with rich soils having low-light availability; (3) the life history of a plant should depend on the point along the soil-resource: light gradient at which the plant is a superior competitor; and (4) primary succession and secondary succession on poor soils result from a temporal gradient in the relative availabilities of a limiting soil resource and light. If, as hypothesized, plants specialized on low-nutrient habitats are relatively short in height, short-lived, and fast growing, and if they reproduce early in life compared to plants dominant when light at the soil surface is limiting, these life history differences could explain the similarity of secondary succession on rich soils to primary succession and secondary succession on poor soils.