The Woodland Canopy
lime leaf close
lime leaf
oak leaf
oak leaf close
copper beach leaf close
copper beach leaf close
The canopy is the topmost layer of the woodland and is comprised of the leaves of the dominant trees. In
these woods, these will be mostly oaks, but also beech, lime, birch, ash, maples and sycamores, yew trees
and pines and chestnut trees. Trees compete to reach the light and when grown in dense groups the trees
will grow thinner but taller in an effort to outreach their neighbours. Much of this light is absorbed by the
leaves and used to make sugars and other organic chemical building-blocks by
photosynthesis.
Photosynthesis needs a source of carbon and leaves absorb carbon dioxide gas from the air. The leaves of
plants gives them an enormous surface area for light and carbon dioxide capture, whilst keeping weight to a
minimum.
Left: the underside of an oak leaf.
Above: a close-up view showing
islands of cells (areoles) surrounded
by the smallest vessels (which are
blind-ending).  The central midrib and
veins help support the leaf but also
conduct xylem sap to the
photosynthesising cells in xylem
vessels. This xylem sap contains water
and mineral nutrients absorbed by the
roots. The vessels also contain phloem
vessels, the phloem sap carries the
sugars made by the photosynthesising
cells to other parts of the plant.
Above: the leaf of small-leaved lime, Tilia. The smallest areoles are about 1 mm in
diameter, or the diameter of about 20 cells.
Above: areoles of a copper beech leaf. Although the leaves of copper beech
contain plenty of green chlorophyll, it's colour is masked by the high concentrations
of red carotenoids in these leafs.
Why are plants green?
Above: a clear-varnish cast of the underside of a holly leaf seen
under the microscope. Clearly visible are the large number of pores
or
stomata (singular stoma) that allow the leaf to obtain carbon
dioxide from the air for photosynthesis. Most leaves have more
stomata on their under-surface and holly has no stomata at all on the
upper surfaces of its leaves.
Above: a close-up view of the varnish cast of three stomata. Each
stoma is flanked by two sausage-shaped
guard cells. These cells can
change shape (by swelling or shrinking) in order to regulate the
diameter of the central pore (stoma) by opening or closing it.
canopy
beech canopy

Above: a beech tree (Fagus sylvatica) canopy. Beech trees tend to produce such a complete canopy that the herb and shrub layers are reduced to a minimum in beech woods. In some places the shade is so dense that no vegetation occurs at all: the forest floor being covered with rotting beech leaves. Nevertheless, beechwoods can host a rich variety of orchids, particularly as they preferentially grow on calcareous limestone soils which are preferred by many orchids. Shade-tolerant orchids, such as the White helleborine (Cephalanthera damasonium), Bird's Nest Orchid (Neottia nidus-avis), Narrow-Lipped Helleborine (Epipactis leptochila), Common Twayblade (Neottia ovata) and Violet Helleborine (Epipactis purpurata) occur here. The Violet Helleborine is p[articularly shade tolerant and may grow in the darker parts of beechwoods where little else can grow. In contrast, Broad-Leaved Helleborine (Epipactis helleborine) which also grows in beechwoods, prefers more light and so is more typically found in glades, along track-ways and along the fringes of the woods, sometimes among scrub.

Above: the canopy of a mixed beech, ash and hornbeam wood.
oak canopy

Above: the canopy of an oak tree. Oak trees allow enough light to pass through their canopies to allow a rich diversity of shrubs and herbs to flourish beneath.Oaks often form mixed woodland with ash trees  (Fraxinus excelsior) and other large deciduous trees, whilst smaller trees such as Hazel (Corylus avellana), Crab Apple (Malus sylvestris) form a lower layer, often reduced to shrubs if coppiced. Silver Birch (Betula pendula) and Downy Birch (Betula pubescens) which are relatively fast growing and short-lived trees may spring up whenever an opening appears in the canopy, such as due to tree fall, but are eventually replaced by oak or ash trees.

Page updated: 19 May 2019.