So what do oak trees do?
Trees are the dominant plant form on Earth on land. As such they are the hub of many of the World's richest
ecosystems - the forests and woodlands. In certain parts of Europe, such as England, the oak tree supports more
forms of wildlife than any other tree species. It is a source of food and shelter to many organisms, including all the
klingdoms of life (animals, plants, fungi, bacteria and protoctistans). Animals, such as the grey squirrel, are
dependent on the acorns for food, and they help spread the oak tree when they bury more acorns to last the winter
than they can eat, or if they forget where they buried them! This gives the acorn a suitable home to grow. The bark
of many oaks (depending how damp it is) is covered with a fine powder of single-celled photosynthetic
micro-organisms or with moss or lichens or with slime moulds and fungi. Large bracket fungi often adorn the sides of
oaks. Many of these fungi benefit the tree by rotting away the deadwood which the roots of the tree can then
reabsorb. The sulphur polypore (Laetiporus sulphureus) however, feeds on the living wood and will eventually kill
the tree and then feed on its dead remains. Oak apple galls contain the larvae of developing wasps (not the
common wasps that sting, but less familiar species).
Trees are the main store of nutrients in a woodland or forest. The soils of many forests are poor, because the trees
have taken out all the nutrients, after all they use what they can get. This is fine so long as nature is allowed to take
her course, because dead trees will decay and free up their nutrients to saplings that grow to plug the gap in the
canopy. However, if people simply remove the trees then the soil left behind is quickly exhausted if used for
agriculture. This has devastated vast areas of the Earth's surface as forest becomes desert.
Trees are valuable to nature when both alive and when dead. Dead trees are often left standing or fallen in nature
parks these days (so long as public safety is not obviously compromised) since dead trees provide shelter for
animals and food for fungi (and the fungi feed many insects and slime moulds) and insects. In my archives I have a
beautiful photo of a dead oak tree still standing, and in its hollow trunk was a bees nest.
Trees are busy creatures! - Why are leaves green?
The green leaves act like solar panels, trapping energy from the Sun and using it to make all the chemicals that the
tree needs, using raw materials from the soil and carbon dioxide and oxygen from the air. Photosynthesis is the
process whereby a plant makes fuel from light, water and carbon dioxide gas. Respiration is the process whereby
this fuel is burned with oxygen to release the energy to other parts of the plant where it is used for growth and
maintenance. Trees require a little water for growth and to fuel their chemical reactions, but the problem is that they
need to absorb oxygen and carbon dioxide from the air. They do this through pores in their leaves called stomata
(singular stoma). However, they unavoidably lose water through these pores (a process called transpiration) and so
they require vast amounts of water to keep these pores open and to maintain photosynthesis. The tree draws this
water up from the soil, in what is called the transpiration stream. This is where the wood comes in. A large oak tree
can transpire over 400 litres of water per day and this water must be pulled up the tree from the soil! That's 400 kg
(880 pounds) of water that an oak tree raises to a height of 30 metres every day! Who says trees are lazy! Tree
leaves are green because the green pigment, called chlorophyll, absorbs all the other colours of sunlight falling on
the leaf more than green. Trees also have other pigments that help, so some trees, like the copper beech, have red
leaves, but still contain green chlorophyll which is masked by the red pigments.
What is wood?
Wood is the material beneath the bark that makes up the best part of the stem of a woody plant (tree or shrub) and
consists of the heartwood in the centre and the conducting wood toward the outside. Everyone has heard about tree
rings - every year a deciduous tree deposits a growth ring of new wood on the outside of the old wood as the tree
grows. Indeed, the tree must do this in order to survive, so trees grow through their lives. The wood is actually made
up mostly of vessels, rather like branching straws through which the tree sucks up water from the soil, and which
give wood its fibrous texture. These 'straws' vary in diameter, but are rarely larger than half a millimetre across. Now
imagine sucking 400 litres of water through a 30 metre long straw that's about a quarter the thickness of a typical
straw and you can appreciate the amount of work an oak tree does every day! The tree is clever, however, rather
than waste energy it lets the heat of the Sun do this work for it. As the Sun evaporates water from the leaves
(transpiration) this creates a suction which draws the water up, all the tree has to do is grow the vessels and direct
the water to wherever it wants it to go and the Sun does the rest. Wood is also a very strong and yet very light
material and has a major role in supporting the plant, allowing it to grow taller than its competitors and hold up
millions of leaves to catch the sunlight. All those leaves can weigh a lot and drag a lot in high winds, so the wood
needs to be very strong. By being light, the wood has less weight of its own to support. Wood is what engineers call
a high performance material. Eventually, the vessels cavitate - that is they get blocked by air bubbles, especially in
cold weather (as do water pipes in houses!). Cavitated vessels may or may not recover, but eventually old vessels
stop working. Since the central cylinders of the trunk contain the oldest wood, this wood is no longer conducting and
often has a different colour and is called heartwood. Only the outermost younger wood outside the heartwood is
water-conducting. A tree must keep adding new wood and grow thicker in order to survive.
Trees are sensitive creatures!
All plants can sense their environment. If a plant is losing too much water, or if night-time comes, then it will close its
stomata to save water. Plants have both a built-in clock that tells them the time of day and the time of year, and are
able to detect light and temperature, so they know when its night-time and they know when its winter. The leaves of
some plants will turn to face the Sun as it crosses the sky (rather like a sunflower's flower does), maximising the
amount of light energy that they get. Indeed, the leaves of all plants can do this at least while they are growing, but
some plants have permanently movable leaves. Many (if not all) plants are also sensitive to touch. A vine will sense
when it touches something and curl its tendrils around the object. Many plants respond when their leaves are
touched and will close their stomata for a while if a leaf is damaged, again to conserve water. Some plants even
communicate with one another by releasing chemicals into the air. The roots of neighbouring trees often fuse
together beneath the soil and exchange water and perhaps signals too. Trees may even help one another, they
may send water through their roots to one of their own who is struggling for nutrients, and plants will release gases
into the air to warn other plants should they be attacked by herbivores, insects or fungi. The other plants will then
invest more energy in making defences, such as poisonous or bad-tasting chemicals that are otherwise expensive to
Trees can also sense gravity so that they know which end is up! Indeed, if a tree is dislodged and slanting, then if it
is small enough and light enough its wood will pull (in hardwoods) or push (in conifers) to lift the trunk back to an
upright position. Even in big heavy and mature trees, the branches are able to right themselves to the correct
position. The pushing or pulling is done by special wood that contracts or stretches, called reaction wood.
Why do trees branch?
Trees need to catch sunlight and extract carbon dioxide and oxygen from the atmosphere. Now, it will not profit the
tree if one part over-shadows another, because the shaded branch may then not be able to photosynthesise and
will require more food than it actually makes. Similarly, if one branch absorbs carbon dioxide in an area of air around
it, then there is not enough carbon dioxide for a branch that is too close. To avoid this branches are programmed to
avoid each other as they grow, or rather to seek out light and open space. Mathematical models, with the help of
computers, show us that the optimum solution is a branching pattern. Further more, they predict that some patterns
of branching are better than others and that all tree species adopt one of these optimum designs (unless
constrained by other requirements). Thus trees branch, but they also branch in a variety of different, but equally
optimum patterns. Thus, a birch tree and a plane tree branch very differently, giving each its characteristic
appearance, but both designs are optimised. However, the tree must grow taller, so inevitably some higher branches
will over-shadow lower ones. If the lower branches cannot get sufficient light then the tree deliberately sheds the
branch. Often tree surgeons will remove the lower branches, but trees shed their lower branches naturally, which is
why the branches that a tree had when it was say one metre tall are not to be found on the tree when it is 10 metres
tall - the branches are always at the top, not because they get pushed up as the tree grows (!) but because new
ones are added further up and the old ones are dropped.
How deep are tree roots?
Most roots of most trees are not that deep at all. Most of the roots of a typical English Oak are within the first metre
of the soil surface. These roots spread out far from the tree however, usually 2-3 times as far as the crown extends,
so the tree resembles more a lamp-stand with a very wide base than a pole hammered into the ground. The roots
also fuse together to form a lattice and a tree has what we call a root plate. This is essential - by covering a large
area of ground, the roots can catch more rain-water. Also, these spreading roots make the tree more stable. Over
time you may have noticed that a fence post gets loose in its hole and wobble about, but a tree's spreading roots
are covered with a vast weight of overlying soil and push on the soil beneath, making it very hard for high winds to
tip a tree. The tree also puts up many feeder roots that come through the soil and pock into the leaf litter, where
they can absorb nutrients that fungi and bacteria, insects and earthworms release as they decompose the leaves -
recycling the nutrients into the tree. Sinker roots go deeper, usually 1-2 metres and may grow until they reach
impenetrable rock or the water table. Some trees that grow in very dry soils do have one or more very deep roots,
however, to reach the water table for below, for example, a fig tree in South Africa was found to have a root
extending down to 120 metres!. Trees in wet temperate climates get most of their water from the rain rather than the
water table. Roots typically account for about 20-30% of the trees mass.
Trees are extremely complex machines
Trees are made up of complex tissues that are made up of plant cells. Like an animal cell, each plant cell is an
extremely complicated mechanical machine and chemical factory with thousands of working parts too small to be
seen with ordinary microscopes.
Above: a diagrammatic section through
a hardwood tree, such as oak. Click on
the diagram to learn more.