Sometimes the term canopy is used to refer to the extent of the outer layer of leaves of an individual tree or group of trees. Shade trees normally have a dense canopy that blocks light from lower growing plants.
Canopy structure is the organization or spatial arrangement (three-dimensional geometry) of a plant canopy. Leaf area index (LAI), leaf area per unit ground area, is a key measure used to understand and compare plant canopies.
Dominant and co-dominant canopy trees form the uneven canopy layer. Canopy trees are able to photosynthesize relatively rapidly due to abundant light, so it supports the majority of primary productivity in forests. The canopy layer provides protection from strong winds and storms, while also intercepting sunlight and precipitation, leading to a relatively sparsely vegetated understory layer.
Forest canopies are home to unique flora and fauna not found in other layers of forests. The highest terrestrial biodiversity resides in the canopy of tropical rainforests. Many rainforest animals have evolved to live solely in the canopy, and never touch the ground.
The canopy of a rainforest is typically about 10m thick, and intercepts around 95% of sunlight. The canopy is below the emergent layer, a sparse layer of very tall trees, typically one or two per hectare. With an abundance of water and a near ideal temperature in rainforests, light and nutrients are two factors that limit tree growth from the understory to the canopy.
Lowman, M.D., and H.B. Rinker (editors). 2004. Forest Canopies (Second edition). Academic Press. ISBN 0124575536, ISBN 9780124575530
Moffett, M.W. 1994. The High Frontier: Exploring the Tropical Rainforest Canopy. Harvard University Press, Cambridge, MA.
Russell, G., B. Marshall, and P.G. Jarvis (editors). 1990. Plant Canopies: Their Growth, Form and Function. Cambridge University Press. ISBN 0521395631, ISBN 9780521395632