Glossary entry

1.
Gradual and continuous change in environmental conditions of an ecosystem or community.

2.
A gradient of ecosystems along an environmental gradient, including both the gradient of natural communities and the complex gradient of environmental conditions.

3.
A geographical gradient of vegetation structure associated with one or more environmental variables.

Sources:
ad 1. Citation of definition from http://filaman.ifm-geomar.de/Glossary/Gl...

ad 2. Citation of definition from http://dir.gardenweb.com/glossary/ecocli...

ad 3. Citation of definition from http://animals.about.com/od/e/g/ecocline... according to
Rickleffs, R.E. 'Ecology (3rd Edition)'. W. H. Freeman & Co Ltd. New York. 1990.