Succession as Model – design

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NATURAL SUCCESSION AS A MODEL FOR THE DESIGN OF SUSTAINABLE AGROECOSYSTEMS

The experiment at CATIE contained six replications of each of four treatments: natural succession, successional mimic, enriched successional vegetation, and monoculture

Plots on which succession proceeded without investigator influence contained more than 100 species at any given time, and several hundred species came and departed during the five year study. At the end of the research the successional vegetation was a reasonably well developed forest, dominated by tree species.

The successional mimic consisted of an ecosystem whose species composition was guided by the life forms observed in the natural succession, but controlled by the investigators. Thus, if at a particular time the successional system contained four vine species, the mimic contained four vine species as well. Some of those species substitutions were domesticated plants, and others were wild species; the only criteria were that they had to be the same life form as a species in the natural succession and they had to be exogenous to the site. Species richness in the mimic tended to run about 10 per cent lower than that of the natural succession.

The enriched succession consisted of natural successional vegetation continuously enriched by an investigator supplied rain of seeds. Seeds of what? Anything that did not occur naturally at the site was fair game, whether wild or domesticated, and regardless of its biogeographic origin. (Anything suspected of being potentially invasive was killed at the end of the experiment.) During the five-year experiment, seeds of hundred of species were added, some of which became established (and influenced ecosystem functioning), while most did not colonize. Species richness in the enriched succession was typically about 10 per cent higher than that of the natural succession throughout the experiment.

The monoculture changed in composition as the life-form dominance in the natural succession developed: during the first year it was a large grass (two rotations of maize), then it was a shrub (cassava), and for the last three years it was a tree (Cordia alliodora).