overstory; canopy layer; tree stratum
Technical term in forestry and ecology for the uppermost layer of vegetation formed by tall trees.
The overstory of this forest is mainly composed of beech trees.
When the canopy layer develops, less light reaches the understory plants.
草本層 is the herbaceous layer on the forest floor, below the shrub and tree layers.
Compound of 高木 (こうぼく, 'tall tree') and 層 (そう, 'layer, stratum').