Background
Many factors, including
the availabilities of limiting resources, the past history of a habitat, the
physical characteristics of a habitat, herbivory, mutualism, and disturbance
can influence the structure and dynamics of plant communities. E002 is an experimental
study designed to determine the effects on successlonal vegetation of two of
these factors: the rate of supply of a limiting soil resource and the past history
of the vegetation.
Both observational and theoretical studies have suggested that nutrient supply
rates can control the course of succession and the abundances of species in
communities (e.g., Olson 1958, Snaydon 1962, Pigott and Taylor 1964, Tilman
1982, 1985). However, only experimental studies can determine how the species
composition, diversity, and dynamics of successional vegetation depend on the
supply rate of a limiting soil nutrient. If resource supply rates do structure
natural communities, then the responses of species to experimental nutrient
gradients should be similar to the correlational patterns observed in natural
vegetation. However, it has also been argued that plant communities have considerable
inertia. Thus, it is thought that once an individual plant has become established
at a site, it may be able to prevent the establishment of other individuals
of the same or different species because the new individuals would have to invade
the area as seedlings or shoots, and could be shaded out or outcompeted for
soil resources by the established plants. This could lead communities to have
multiple stable equilibria (Lewontin 1969, Strobeck 1973, May 1977, 1979), i.e.,
to have the state of a community at equilibrium depend on both interspecific
interactions and on past history , with the particular equilibrium attained
determined by the initial densities and the order of colonization of the species.
Experiment E002 is performed at Cedar Creek Natural History Area, Minnesota
where nitrogen is the major limiting nutrient (Tilman 1983, 1984). Applying
experimental nitrogen gradients on newly disturbed (disked) soil in three fields of different successional ages allows the effects of
nitrogen supply rates on species diversity, species dominance, and population
dynamics during secondary succession to be studied. Because disking killed
most of the existing perennial plants and favored plants that started from seed
or small rhizome fragments, there were great differences in the initial sizes
and life history stages of individuals and in the initial densities of species
between this disked experiment and the undisked experiment E001. This experiments
can thus help determine the extent to which the responses of plant species to
different rates of supply of nitrogen depend on the initial abundances and life
history stages of the species.
[Modified from: Tilman, D. 1987. Secondary succession and the pattern of plant dominance along experimental nitrogen gradients. Ecological Monographs 57:189-214.]