When we talk about woodland structure we specifically mean the layers of a woodland, these are the canopy layers, Under storey, Field and Ground.
Canopy Layer - Canopy layers will be older trees and the most tallest. The species will be typically be Oak, Ash, Beech and Birch.
The leaves of these trees are effectively patch worked and angled to catch the maximum amount of light.
Ground Layer - The ground layers will have Moss ( Bryophyta) which grows in dark and damp areas and grows usually on deadwood. Ivy, Lichen and Fungi are also found on the ground layer.
Field Layers - Ferns and grasses are found on the field layers. The ferns reproduce via spores and are carboniferous. The grasses include Sedges and Herb and many grow in heathlands.
Image source:
https://www.slideshare.net/anandcharvin/canopy-management-practices-information