In Spring the woods are light and airy and bluebells colour the floor and scent the air, followed by cherry
blossom. In Summer the canopy darkens the woods and in a particularly wet Summer they can seem dark
and rank, whilst in dry Summer weeks they are cool and lush. In Autumn they present the spectacle of
twilight colour and an abundance of fruit and leaf-fall. In Winter the woods have yet another character.
Their dark twisted de-leafed forms contrast against the white sky like elaborate inanimate carvings. After
a fresh snowfall they have a peaceful mystical quality. It is easy to imagine sylphs or dryads, fairies or
elves as almost tangible spirits manifesting as swirling snow and occasional light beams or hiding from
view, on the edge of our perception, in this otherworldly scene.
Depressions in tree boles, like the depression in the midst of this oak tree, are good for
retaining moisture and even water and when not dry or frozen will contain lots of wonderful
microscopic creatures (like Paramecium and other microorganisms). In dry and adverse
conditions many of these organisms are able to rapidly form dehydrated and dormant forms,
often protected by thick shells (cysts). These act like spores and can be carried about from
place to place on the wind.
Most of the trees in this temperate wood are deciduous - they shed their leaves in Autumn.
This protects the leaves from ice damage and reduces drag on the tree in high winds,
especially if the boughs are snow-laden. In any case, it has hard to photosynthesise if the
ground is frozen and water is hard to get and ice can have a dehydrating effect on leaves.
However, some trees are adapted for the cold and dryness of Winter and can grow even under
these conditions. These are the evergreens, tress like holly and conifers and box, but also
woody plants like English Ivy (Helix, Britain's only evergreen liana or climber). More sensitive
plants suffer damage at -10 degrees centigrade, but plants like the Scot's Pine (Pinus
sylvestris, a native of Scotland) is cold-hardy down to -60 degrees, and some trees are even
more cold tolerant than this!
The ways in which cold damage plants are subtle. The extracellular regions, outside of the living cellular
protoplasts, usually freezes first and as ice crystals form in-between the cells they draw ice out from the
cells dehydrating them. This can collapse leaves and damage them, but leaves of holly (Ilex aquifolia)
have very thick and waxy cuticles (especially on their upper surfaces) to reduce loss of water when ice
crystals form on the leaf surface, and tough thick-walled cells called sclerenchyma form a tough margin
around the leaves and continue into the leaf spines. Holly leaves maintain their shape even when
covered in ice. Thawing also damages tissues, especially if it is too rapid and holes may remain where
ice-crystals existed and upon thawing leaves may also collapse, but again the tough leaves of holly, and
the tough leaves of evergreen conifers, are resilient to this.
Low temperatures slow the chemical reactions occurring in cells, but decay and degeneration of tissues
are also slowed and many organisms can maintain growth at low temperatures so long as the water inside
them does not freeze. Cytoplasm is salty and full of sugar and other dissolved materials and does not
freeze at zero degrees, but rather supercools and freezes at much lower temperatures. Dehydration of
plant cells by extracellular ice does not kill them, but if a critical temperature is reached or if ice crystals
puncture cell membranes then the cell can freeze, and often flash-freezes in an instant when this critical
condition is reached. This usually kills the cells. If the protoplasts dry but do not freeze then they may
revive on thawing when they take in water. If tree tissues are frozen rapidly without ice forming, by dunking
in liquid nitrogen for example, then they revive upon warming - it is the ice and especially the thawing of it,
and not the low temperature that kills plant tissues. Very low temperatures may inhibit growth and life in the
long term, but it is ice that does the short-term damage.
In the middle photograph, above, is a small holly tree. Its leaves are covered with snow, but they have
maintained their integrity. Holly is dioecious - there are separate male and female trees, and the females
bear spectacular red berries in winter. Along with ivy and other evergreens, the holly gives splashes of
vibrant colour to the Winter wood. Tradition has it that the oak, King of trees, passes his crown to the
holly in Winter.
Above left: the bramble, or blackberry, is another evergreen shrub that creeps along the ground and
clings to nearby objects for support.
Very low temperatures can cause freeze-cracking in the trunks and branches of trees. A critical
point is reached, either as ice forms or thaws, and the wood suddenly splits open with a loud
retort like a gunshot! The gaping wound remains open in the cold, but when the wood thaws the
wound closes. Such wounds may open each Winter and close again each Spring. They seem to
be caused by wood shrinkage to differing degrees in different planes. The ice crystals draw water
out from the xylem vessels, which shrink and may close shut, causing wood to generally shrink
Earlier in the current Winter fog froze and by morning all the trees and other plants looked as if they
had blossomed, for they bore bunches of snow-icicles hanging down from their twigs like flowers of ice!
Despite their appearance, winter deciduous trees are mostly not dead of course. Close inspection of
the tree branches will reveal the dormant buds, the colour, size and arrangement of which can aid tree
identification. Sweeping away snow from a tree trunk may reveal a lush green mat of moss underneath.
In the absence of snow, it can be readily seen that a number of herbaceous perennial plants have
retained or begun to regrow rosettes of leaves. After the more upright shoots have died away, many
perennials retain a cluster of basal leaves, which all seem to emerge from the same height on the
much shortened stem (the internodes of the stem are very short, bringing the leaf-bearing nodes close
together). The rosette growth form helps protect the plant and its buds from cold and dehydration in
The growth centres of plants are well protected. The cambium inside the trunk will
Above: a winter rosette of Arum maculatum (or possibly a hybrid with Arum italicum). In early spring
Arum will bolt up and produce its distinctive flowering spike.
Some plants flower in winter, especially during a more mild winter. The wild Primula vulgaris below is
mostly in flower in early Spring (around March) but can be seen flowering in February here.
14 Jan 2017