|Shrub-Layer: woody shrubs and small trees
|In these woods the shrub-layer is predominantly small trees like hawthorn, shown above,
hazel, some rowan and holly. Saplings of larger trees, the oaks, beeches, maples,
sycamores, chestnut trees and ash trees also add to the shrub-layer.
|Above: hawthorn. The shrub layer of a woodland consists of shade-tolerant shrubs and
small trees that can photosynthesise using the dim light that filters through the tree
canopy. This small tree is a hawthorn. Hawthorns typically produce a considerable tangle
of branches and when grown in hedges and regularly pruned they respond by producing
more modified, stunted and pointed branches - the thorns that make this plant such an
effective barrier to contain livestock.
|Above left: the characteristic small lobed leaf of hawthorn. Above right: the underside of a holly leaf.
|The hawthorn is famous and much-loved for its May-blossom - an ancient fertility symbol. It flowers
more-or-less around May-time, producing a spectacular display of small white (sometimes pink) flowers - the
May blossom. In late summer and early winter it is well known for its bright red berries (haws), which provide
birds with valuable food well into the winter. The leaves are said to be very nutritious and to stave-off hunger
(could this be a defense against being eaten?) and the berries apparently make good wine. In summer the
hawthorn has neither its pretty white garment nor its red berries but the strange twisted and tangled form of
hawthorn trees is striking beneath the darkness of the late summer canopy above.
Hawthorns are very slow growing trees and typically live for 100 years, though quite a few make it to about
400. Its tangled and twisted growth habit always makes hawthorn trees look ancient. Being slow growing their
wood is extremely hard and dense (fine grained) but has few uses, perhaps because of the smallness of the
trees, sometimes being used for tool handles and walking sticks.
|Above: holly, Ilex aquifolia. The holly typically has spiny leaves with thick waxy cuticles, especially on the
upper surface. The tough leaves are well-adapted to survive the winter and the holly is evergreen. In winter
the holly is one of the main sources of colour in the woodland, with its lush green leaves and red berries,
famous as a Christmas decoration. Tradition has it that in late Autumn the oak hands-over his crown for the
holly to bear until the following Spring.
Folklore has it that both the hawthorn and holly make safe shelter during an electrical storm:
|Beware of an oak,
it draws the stroke,
Avoid an ash, It courts the flash,
Creep under a thorn,
It will save you from harm.
|Above: elder, Sambucus nigra. Elder forms shrubs or small trees up to 10 m tall. It prefers nitrogen and
phosphorus rich soils and is often associated with rabbit warrens (although rabbits do not eat it) and
eutrophic soils (soils polluted by excess fertiliser). It is pollution resistant and colonises open spaces rapidly.
It grows rapidly and has a light-weight construction, with white porous spongy pith in the younger branches.
The leaves and bark are toxic. The leaves contain cyanogenic glycosides (e.g. sambunigrin) which react
with enzymes, when eaten, to produce toxic hydrogen cyanide (HCN) gas! However, cases of poisoning are
isolated and deer will graze elder, but the toxicity undoubtedly makes it harder for animals to graze too
heavily on elder. The bark contains ribosome-inactivating proteins (RIPs) (constitute about 80% of bark
protein). The RIP forms a complex with lectin; the lectin molecule binds the target cell-surface membrane
and then the RIP enters the cytoplasm and kills the cells by inhibiting the ribosomes, which make proteins.
RIPs are also found in the fruit, and yet, the fruit and flowers make excellent wine! Elder is pollution resistant
and resistant to high levels of alkali, salt, ozone, sulphur dioxide emissions, and certain metals like copper
and lead. Being a fast-growing coloniser, elder is short-lived, with a lifespane of about 25-30 years,
occasionally over 40 years. Younger plants are shrubby with many stems branching from a single base, but
by 20-30 years of age a single trunk becomes dominant.
The leaves emerge in Feb?March, the flowers in May/June and the fruits in July, ripening by early
September. The flowers are nectarless and have five-fold radial symmetry, but are visited by beetles
(especially the longhorn beetle), flies and honeybees. The seeds are small (about 2-3 mg) and the fraction
that germinate is quite low, but increases when the seeds have been defecated by birds. The flowers
develop by the third or fourth year and each plant produces up to 100 000 seeds or more.
The wood of elder is diffuse-porous with small-diameter vessels (about 40 micrometres diameter in the stem,
but 2-3l times larger in the roots). Heartwood forms after 6-10 years. the bark is brownish-grey.
|Rowan (Mountain Ash)