overstory

Overstory is the larger, taller trees of growth which occupies a forest area and shades young trees, hardwoods, brush, and other deciduous varieties which are growing beneath the larger trees (i.e., understory). ... (09 Oct 1997) ...
Found on http://www.encyclo.co.uk/local/20973

overstory

Trees composing the canopy layer of a forest; the highest trees.
Found on http://www.neonaturalist.com/nature/nature_glossary.html

Overstory

Trees that provide the uppermost layer of foliage in a forest with more that one roughly horizontal layer of foliage.
Found on http://www.encyclo.co.uk/local/21804

overstory

the level of forest canopy that includes the crowns of dominant, codominant, and intermediate trees.
Found on http://www.dnr.state.md.us/forests/gloss.html
No exact match found