Ancient British Woodland

This essay gives a brief introduction to the forest ecosystem, with special emphasis on deciduous woodland
in Britain. The distribution of species in British woodland is affected by a number of biotic and abiotic factors.
Every ecosystem is said to be made-up of
biotic and abiotic components. Abiotic components are physical
factors, such as light and water, whereas, biotic components are the organisms themselves: the primary
producers, herbivores, carnivores, parasites, etc. Each organism is affected by a range of abiotic and biotic
factors/components. Of particular relevance to ecology, the geographic distribution of each species is
affected by these factors.

Trees

The natural habitat of the British Isles is mostly deciduous temperate woodland and woodland once covered
some 80%-90% of these isles. This
wildwood appeared some 13 000 years ago after the last Ice Age. The
first trees to appear were those that are relatively short-lived, fast-growing and produce huge numbers of
tiny airborne seeds that could be carried across to Britain from areas less affected by the spread of ice,
namely
birch trees (Betula). This percentage cover was partly reduced by human activity in prehistoric times,
right through to the 19th century when it reached a low of 5%, but is now on the increase approaching 10%.
The Domesday Book written in 1086 records that 20% of Britain was covered by woodland and forest.
However, only 2% of what remains today can possibly be considered as more or less original wildwood.
However, much ancient woodland survives; this is woodland that has existed continuously since at least 1600
AD, but which has seen some form of management by people.

‘Trees mean life’

Most of the woodland that you see in lowland and inland areas such as the Midlands is primarily English oak
and hazel on clay soils with
beech (Fagus sylvatica), ash (Fraxinus excelsior) and rowan (mountain ash)
dominating on well-drained acidic, sandy or chalky soils. All trees have a preferred soil type, but all can
tolerate a range of conditions and this, coupled with local variations in soil moisture on slopes for example,
mean that all woods contain a mixture of species, though several will dominate in a particular area. Birch is a
good colonizer of clearings (say where a large tree has fallen) since it grows fast and its small winged seeds
spread far and wide in huge numbers. Birch is short-lived for a tree (about 80-100 years on average) and
when dead its wood rots very quickly, and so birch eventually gets replaced by other trees, such as oaks, in
England. In colder climates, however, birch may form a major part of the climax community, along with aspen
and Scotts pine (
Pinus sylvestris) in the ancient Caledonian forest of Scotland, for example. Conifers
compete best in colder climes, such as in Scotland. Shortly after the Ice Age, the Scots pine thrived in much
of Britain, but today occurs naturally only in the Highlands of Scotland in forests of pine, birch and aspen.

Oak trees support more known life forms (insect, birds, animals etc.) than any other tree species in England.
They are currently the dominant tree species in the UK. The Oak tree (
Quercus) typically lives about 900
years (it takes 300 years to mature, stays for 300 years and then senesces over 300 years) though often
makes it past 1000 years of age! Our longest lived native tree species, however, is the
yew tree (Taxus
baccata
), which can probably reach 5000 years or more! Scots pine (Pinus sylvestris), yew and juniper (an
evergreen shrub) are the native gymnosperms (non-flowering seed-bearing plants, including conifers) of
Britain. Other conifers are relatively recent imports grown for timbre. Even dead oak trees are useful to other
living creatures, apart from
decomposers like fungi and wood-boring beetles, for example, bees will often
nest in hollow trees.

The Effects of Abiotic Factors in British Deciduous Woodland

Sunlight

Sunlight has an obvious major effect on the primary producers of the woodland. Woodland is divided into
several plant layers: the tree
canopy, the shrub layer, the herbaceous and grass layer, and the woodland
floor (one or more of these layers are collectively called the
field layer) and other surfaces where algae and
lichens and photosynthetic bacteria thrive.

Many shrubs, herbs and grasses must be shade-tolerant to survive in the low light levels beneath the tree
canopy. Some, like
bluebells, will undergo their whole life-cycle in spring (bluebells flower in April/May) so
that they can make full use of the light before the trees completely unfurl their leaves. Others will only appear
where there are clearings.
Ferns, though shade tolerant, will make the most of a woodland clearing and often
form miniature ‘fern forests’ in large clearings.

Some plants will use the trees to gain height and so light, such as the parasitic mistletoe which also
photosynthesizes and sits high-up on the tree where there is more light. Climbers like
ivy (Hedera helix) will
entwine themselves around trees for support, enabling them to reach the sunny heights without investing in
the production of huge amounts of wood for support.

Many plants, algae, lichens and photosynthetic bacteria are
epiphytes (epiphyte: a plant that grows attached
to another plant but does not feed off it, it simply uses the plant for support or as a surface) growing on the
trunks and branches of trees to gain elevation of the ground and benefit from light in the tree canopy. E.g.
mosses, lichens, algae. (These plants also benefit from moisture from rain held in the outer layers of the tree
bark).

Other plants are quick to exploit the light available when a tree falls. Herbaceous plants, ferns and shrubs
may quickly expand into such a clearing, but before long the first fast-growing trees spring up, such as birch
trees. Birch trees are colonizers that easily exploit tree clearings. Birch trees are relatively short-lived,
however, and most do not exceed 100 years. They decompose quickly, giving a chance for seedlings of
other trees that had begun to grow, but were beaten to the light by the fast-growing birch. These seedlings,
often oak or beech, may now spring-up and take there place as the dominant tree species in mature
woodland.

Water/moisture/rain

Depression and holes in tree boles hold water and are whole micro-ecosystems in their own right: protozoa,
algae, bacteria, round worms and mosses thrive here. Algae, moss and lichens thrive on damp tree-trunks.
Moisture may help fallen logs, dead wood and leaf litter to decay more rapidly and these moist decaying
habitats form an ideal home to bacterial and fungal
decomposers and to slugs and creeping slime molds that
eat these fungi (these slime molds are single cells with thousands of nuclei and may grow up to one metre
across!). It is common practice these days to leave fallen logs in woodland to support these various species
of decomposers and the creatures that eat them. Many beetle larvae also feed on rotting wood.

Some trees prefer wet soils. Alders and willows grow in wet soil and alongside streams and ponds. The crack
willow has very fragile wood and often splits and fragments under its own weight – but that is useful, willows
can grow from rooted twigs, and so fragments falling into the stream may root in the bank downstream and
grow into new crack willow trees!

Temperature

Conifers can out-compete many angiosperms in cold climates. Their tough needles resist drying by cold
winds and their xylem vessels are better adapted to conduct water in the cold, since they are narrow
tracheids. Wide vessels, found in many angiosperms, are prone to becoming blocked by air bubbles in cold
conditions.

The Effects of Biotic Factors in British Deciduous Woodland

Decomposers and food chains

A saprophyte is an organism that lives on decaying organic matter. There are many other decomposers in
the forest, apart from fungi (though fungi are among the main decomposers of wood). Woodlice require
damp dark conditions in which to hide during the day and emerge at night to feed, since they lack waterproof
cuticles (they are crustaceans!). Woodlice eat mostly decaying plant matter and are able to digest cellulose
(but also eat dead animal flesh). Common shrews eat vast quantities of woodlice. Small owls eat shrews (and
mice and rats). Woodlice, shrews and owls are all nocturnal. Millipedes also eat dead and decaying plant
material and are common amongst the leaf litter.
Earthworms are detritovores and eat fallen rotting leaves
and organic detritus (organic remains already broken down into small fragments) in the soil. In England
earthworms form the staple diet of badgers (and a badger can grow to 10kg or more – that is a lot of
earthworms!). What other creatures eat lots of earthworms? Scavengers. Crows, beetles and fly larvae
(maggots) eat carrion (amongst other things).

Animals of the Leaf Litter

Beneath Scots pine on heathlands, the acidic soil reduces the rate of decay by inhibiting fungal growth, and
pine needles take up to nine years to decompose. Conifer needles also produce acidic humus. Beech, which
grows best on chalky soil, also produces acidic humus, though this may be neutralized if mixed in with chalk.
In oak, hazel and birch woods, however, there is typically a rich diversity of life amongst the
leaf-litter,
especially if the soil is well-drained and not waterlogged and is a medium loamy soil. The leaf litter layer may
be several centimeters thick and underneath this there may be a fermenting layer of similar thickness.

Herbivores

Much of the woodland in Britain today is grazed by sheep, but it is not natural to have so many sheep grazing
in woodland and sheep do cause problems by eating young saplings and preventing woodland from
regenerating. Britain’s largest natural herbivore is the red deer (stags may weigh up to 130 kg). Red deer will
eat a large range of plants, including heather moss, tree seedlings, tree bark, berries and grasses. The
numbers of red deer in Scotland are on the increase, with some 300 000 living there today. These deer have
existed in Scotland since the Ice Age and have also had to adapt to living on moorland as much of the
natural forest has been destroyed by humans. Not all herbivores are large! Many invertebrates, such as
snails and caterpillars graze vegetation. Squirrels, mice and many birds eat nuts, seeds and fruit.

Carnivores

What were the natural enemies of red deer? Wolves used to be a top predator in Britain. The last of these is
thought to have been killed in Scotland in 1743 people feared wolves and wolves could kill their livestock,
though rarely attacked people, and so humans drove the British Wolf to extinction. Ironically, when the first
post-glacial humans arrived in Britain as hunter-gatherers, their way of life was perhaps more like that of the
wolf than any other wild animal and it is thought that the wolf was highly regarded as a sacred animal. Many
people want to see the wolf reintroduced to Britain today.

Why do we need carnivores? The population of a predator and its prey are intimately linked with each
regulating the other. Today deer and sheep are kept in such high numbers that overgrazing is a serious
problem. Many woods are dominated by old trees, with seedlings struggling to grow up since they are easily
grazed and destroyed by herbivores. What will happen when the old trees die? Continual growth of saplings
is needed to regenerate the woodlands. It has proven necessary to control populations of deer. This is done
by culling, red deer are hunted for sport. Whatever one thinks of the moral implications of this sport, it does
serve to control the red deer population, and is in fact a principle reason why these populations are
encouraged to grow. Humans can not escape from the ecosystems of which they are an integral part.
Humans have replaced the wolf as a top predator in Britain. Like it or not, Britain needs top predators!
(Though it can be argued that we eat too much meat, on both health and ecological grounds – fewer sheep
would almost certainly be good for the ecosystem).

Other top predators in the forest: owls may eat mice, rats, voles, squirrels, shrews, frogs, fish and lizards as
well as insects and other grubs. Sparrowhawks eat small birds. Goshawks eat birds and small mammals.
Foxes will eat rodents, rabbits, frogs, birds, lizards and insects. Adders frequent the open edges of woods
and eat small animals, birds eggs, snails and insects.

Not all forest predators are large! In fact, the majority of predators are small. Dragonflies eat other insects,
which they catch on the wing, whilst their larvae may eat tadpoles and even small fish. Bats and many small
birds eat insects. Badgers are quite large but manage to sustain themselves by eating mostly earthworms!
Predators are often linked in a food-chain of predators. Beetles may eat the slugs and caterpillars, and birds
may eat the beetles, and these birds may be eaten by birds of prey. Don’t forget the spiders! Did you know
that there is on average of 150 spiders per square metre in the UK (averaged over a range of habitats)! Wolf
spiders do not trap their prey in webs, but jump on their prey, which includes insects and other spiders, and
devour it. It is estimated that across the UK spiders devour 700 000 tonnes of prey per year! So, like them or
not, it is a good job we have so many!

Symbionts: living together!

So far we have considered one type of relationship between organisms – eating each other. Sometimes,
however, relationships between organisms are especially close. Symbionts are organisms that live closely
associated with each other in symbiosis. One organism may live within or attached to another, or organisms
may provide a special service to one another. A parasite and its host is one example of a symbiosis, in which
the parasite derives most or all of the benefit by feeding on its host.

Plants parasitizing plants: The mistletoe is a parasite of trees, including oak. The mistletoe penetrates the
wood of the host tree with special projections, called haustoria, which push and enzymatically dissolve their
way into the host tissues and then connect to the host vessels and draw out water and nutrients. The
mistletoe, however, makes some of its own sugars as it is photosynthetic. Some mistletoes have less
chlorophyll than others, and these will draw more heavily on the tree’s own sugars.

Fungi parasitizing plants: many fungi parasitise plants in different ways. The honey fungus is a major
parasite of trees and shrubs in woodland. You may find its bootlace-like black strands that creep across the
forest floor among the leaf litter from an infected tree, searching for a new host. (Though some fungi that
only feed off dead and rotting wood also produce these ‘bootlaces’, and these tend to be thicker and more
numerous and so are the type most commonly encountered). These bootlace strands will infect the roots of
new trees that they encounter, penetrating the bark and investing the tree beneath the bark in a net-like
lattice of strands, which draw out sugars from the phloem sap. Their toadstools (which are edible) can be
seen at the base of infected tree trunks.

Of course not all fungi are parasites! Apart from the decomposers mentioned above, that you will find on
fallen logs, there are many fungi that live attached to trees, but only feed on dead wood. As a tree grows,
new xylem is added to the outer layers of the tree trunk and branches. The old xylem in the centre stops
conducting and becomes ‘dead’ heartwood. If a tree is damaged, then this heartwood may become exposed
to the outside elements and to attack by fungal decomposers. Over time the whole inside of a trunk may rot
away, living a hollow (and perfectly healthy) tree trunk. If you look inside these hollows then you may see that
the tree has even put down roots inside the hollow to absorb nutrients from the pile of rotting wood on the
floor. Thus it recycles its own nutrients with the help of fungi! Can you think what other creatures benefit from
hollows in trees?

The English elm (
Ulmus procera) is a well-known disaster story. Certain bark-boring beetles carried a fungal
disease that infected elm trees across Europe. In England the elm was actually on the edge of its climatic
range (though in the past conditions suited it much better) and so usually failed to reproduce from seed, but
successfully spread asexually to become a dominant countryside feature. In the 1970’s the disease ravaged
the English elm and permanently transformed the countryside. The story has not ended, however, and both
the elm tree and the disease continue to coexist and both may eventually reach a stable equilibrium.

Mycorrhizas

Mycorrhiza literally means “fungus-root” and is an association between fungus in soil and the roots of plants.
Most plants form these associations, and most plants grow poorly without them! In soil, a fungus consists of a
mass of interconnected hair-like threads, called
hyphae (singular hypha) which form the mycelium (fungus
body). This structure gives the fungus an enormous surface area with which to absorb nutrients from the soil.
In a mycorrhiza, some of these hyphae form a sheath around a root and penetrate the root tissues. These
hyphae draw sugars out of the phloem of the plant for their own nutrition. However, these fungi are not
parasitic and the plant cells assist the fungus and enhance nutrient transfer, because the plant benefits too!
The large absorptive capacity of the fungal hyphae makes them very good at drawing minerals out of the soil
and they give some of these nutrients, especially phosphorus, to the plant in exchange for the sugars. Many
soils are low in phosphorus, and plants can not grow well in such soils without mycorrhizal fungi to help them.
The toadstools that you often see growing near to trees are often the sporing bodies of mycorrhizal fungi.
Some plants associate with very specific mycorrhizal fungi and grow poorly in soil lacking the right fungus.

The bird’s nest orchid is a saprophyte, it is found in beech woodlands and has no chlorophyll, and in fact its
leaves are reduced to colourless scales. Its roots are associated with mycorhizal fungi that themselves feed
on decaying vegetation and pass some of the nutrients to the orchid. In return, the orchid synthesizes certain
complex organic chemicals for its fungal partner. Early purple orchid is photosynthetic and forms a
mycorrhizal association with honey fungus.

Trees even help each other. The roots of neighbouring trees, especially trees of the same species, often
naturally graft together beneath the soil, in such a way that xylem can flow from one tree to another and
carry water and mineral nutrients with it. It has been shown that nutrients tend to flow from the stronger trees
to the weaker ones. If a tree is felled, the stump may die or it may put out new shoots that can grow into a
new tree. Although a stump can not photosynthesis, it will draw on its root reserves and is helped by
nutrients from other trees.

Lichens

Lichens grow in all woodland, but are especially abundant in clean unpolluted air and in damp conditions.
They may grow on exposed rocks and bare soil, but also may drape themselves from the branches of trees
as epiphytes. Lichens are photosynthetic, but are not technically plants. They are actually a compound
organism: they consist of a specialized fungus body that contains photosynthetic algae or bacteria. In what
ways do you think that both the fungal and algal/bacterial partners benefit from this symbiosis?

Pollination

Bees are important pollinators of fruit trees and other flowering plants. The flower produces sugary nectar as
food for the bees (which also eat pollen), and signals their presence with attractive smells and colours that
the bees can recognize. (Flowers often contain patterns that are only visible in ultraviolet light which the bees
can see, but we can’t! These patterns guide the bee to the nectary). Many insects pollinate plants. The
honeysuckle, for example, is pollinated by bees in the daytime and by moths at night and produces different
smells at night to attract the moths. Evening primrose flowers opens at about 6:00 pm and are moth
pollinated.

Many flowers rely on the wind to pollinate them, including the flowers of alder, ash, birch, elm, hazel,
hornbeam, poplar and oak trees. These trees produce flowers before their leaves unfurl, and these flowers
are typically small and not brightly coloured and may occur in groups, like the catkins of birch trees. Pine
pollen grains each have a pair of air sacs to help them float on the breeze.

Carrion flowers

Wild arum (Arum) emits a foul odour of decay and also generates a lot of heat that helps vapourise and so
disperse the odour. This attracts carrion flies, which get trapped in the flower chamber overnight, until
pollination of the female flowers is complete, the flies are then released, laden with male pollen to pollinate
another arum flower.

Seed and Spore Dispersal

Many plants rely on the wind to disperse their seeds. Birch, ash, hornbeam and maple (including sycamore)
have winged fruit that may rotate in the breeze to gain lift (like helicopter blades). Animals and birds eat
attractively coloured fruit loaded with sugars, the seeds resist digestion and pass through and are so
dispersed.  There are many other ways in which seeds can be dispersed. (Can you think of any?). Fungal
spores are dispersed by rain and wind.

The importance of ants

The total mass of ants (biomass) in a wood or forest may equal or exceed the biomass of mammals. Ants
perform a very wide range of ecological roles. Ants may collect and store seeds for later consumption, they
may eat other invertebrates, and they may pollinate plants and disperse their seeds. Ants can change the
physical and chemical properties of soil, and some ants even grow fungus in ‘fungal gardens’ as a source of
food.


Answer the following questions that draw your attention to the main points of the text:

Q.1. Which is Britain’s only native pine?
Q.2. Woods and forests are layered, with different plants collecting light from different heights within the
forest. What are the four main plant layers in a typical woodland?
Q.3. What is a detritovore? Give one example.
Q.4. It has been said that fungi provide ‘life support for ecosystems’. In what ways are fungi beneficial to the
woodland ecosystem?
Q.5. What are the benefits of dead wood and dead trees to other life forms?
Q.6. Give an example of a forest food-chain with a primary producer ‘link’, a herbivore ‘link’ and two ‘links’ of
predator.
Q.7. Give an example of an animal that scavenges carrion.
Q.8. Apart from light, state two other variable abiotic factors that affect life in woodland.
Q.9. How is wind important is determining the distribution of different species?
Q.10. How would the populations of deer and wolves have kept each other in check in the ancient wildwood?
Q.11. Two other animals that once roamed the woods, but are now extinct in Britain, are the wild boar and
the bear. Use the Internet to find out what foods these creatures would have eaten in the woods. Were these
animals herbivores or carnivores?



Abiotic Components/Factors

Sunlight
Temperature
Precipitation
Water/moisture
Soil or water chemistry (pH, nitrogen content, etc.)
Drainage
Aspect
Wind/waves
Tides
Etc.

Biotic Components/Factors

Primary producers
Herbivores
Carnivores
Omnivores
Detritovores
Parasites and other symbionts
Etc.
Woodland
Sunny woodland