any climatic condition in a relatively small area, within a few metres or less above and below the Earth's surface and within canopies of vegetation. The strongest gradients and cyclical patterns of temperature and humidity occur just above and below the terrestrial surface. Complexities of microclimate are necessary for the existence of a variety of life forms because, although any single species may tolerate only a limited range of bioclimate, strongly contrasting microclimates in close proximity provide a total environment in which many species of flora and fauna can coexist and interact.
Source: Taken from Encyclopaedia Britannica 2002