Succession and Climax Communities

Ecosystems are dynamic, in that the populations constituting them do

not remain the same. This is reflected in the gradual changes of the vegetational

community over time, known as succession. It begins with the colonization

of a disturbed area, such as an abandoned crop field or a newly

exposed lava flow, by species able to reach and to tolerate the environmental

conditions present. Mostly these are opportunistic species that hold

on to the site for a variable length of time. Being short-lived and poor

competitors, they are eventually replaced by more competitive, longerlived

species such as shrubs, and then trees. In aquatic habitats, successional

changes of this kind result largely from changes in the physical environment,

such as the build-up of silt at the bottom of a pond. As the pond

becomes more shallow, it encourages the invasion of floating plants such

as pond lilies and emergent plants such as reed mace. The pace at which

succession proceeds depends on the competitive abilities of the species involved;

tolerance to the environmental conditions brought about by

changes in vegetation; the interaction with animals, particularly the grazing

herbivores; and fire. Eventually the ecosystem arrives at a point called

the climax, where further changes take place very slowly, and the site is

dominated by long-lived, highly competitive species. As succession proceeds,

however, the community becomes more stratified, enabling more

species of animals to occupy the area. In time, animals characteristic of

later stages of succession replace those found in earlier stages.

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CONIFERS

Conifers, or softwood trees, form a distinct group, which has become

very important in the world's economy, because they grow fast on poor

soils even under harsh climates, and yield timbers that are very suitable for

industry. They are now being planted on a growing scale in most countries

as a source of wealth.

Distinctive characters of the conifers include:

a) narrow, needle-like or scale-like leaves;

b) foliage usually evergreen (the only common exception being the

larches);

c) scaly buds;

d) regular almost geometrical, branching habit;

e) resinous fragrance of foliage, buds, bark and timber;

f) male and female flowers always borne separately, though usually on

the same tree;

g) flowers always wind-pollinated, and therefore catlike, lacking

showy petals or nectar,

h) fruit in the form of a woody cone (rarely, as in yew and juniper, a

flesh berry).

Most conifers flower in spring, their cones may ripen during the following

autumn, the following spring, or in some species eighteen months

after pollination.

In natural forests conifers grow readily from seed, unaided by man. In

cultivation they are raised in nurseries nearly always from a seed, since

most kinds are very hard to grow from cutting.

The timber of conifers is always called softwood, though in a few species

it is quite hard. On the whole, however, it is softer and easier to work

than the hardwood yielded by broadleaved trees. Today the great bulk of

timber used in house building, fencing, packing cases and boxes of all

kinds, and as railway sleepers, telegraph poles, or pitprops, is softwood.

For paper making, which uses about half of the output of wood in the

main timber-growing countries, softwood is more suitable than hardwood

because, amongst other features, its fibres are substantially longer. Softwood

is very suitable for the manufacture of most kinds of artificial board,

an expanding industry that gives us wood chipboard, hardboard, arid insulation

board.

At the present time about nine-tenths of all the timber used in Britain,

whether in the unaltered state or made up into paper or manufactured

board, comes from coniferous trees.

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BROADLEAVES

The broadleaves, or hardwood trees, are the leading feature in the

British landscape of woods and hedgerows. In the past they were the country’s

main source of building material, fencing and fuel. Today, when there

are other sources of heat and power, and steel and concrete play so large a

part in building, the hardwood timbers of these attractive trees are less important

to the British economy. But there is still a very substantial trade in

good sound oak, beech, ash, sycamore and elm, for the better classes of

furniture making and the joinery, while poplar is used in matches, and willow

for cricket bats.

To a growing extent, the country’s needs of timber in bulk are nowadays

met by the conifers, or softwood trees.

Most forest planting today is done, inevitably, with these conifers. But

the landscape, shade and shelter values of the broadleaves are so great that

they are always likely to play the larger part in hedgerows, as street trees.

Most of them are natives, and these are firmly established in old natural or

semi-natural woodlands throughout the British Isles.

Our broadleaved trees form part of the vast natural broadleaved forest

of Northern Europe. The key feature of all these woods and trees is, as the

name implies, the broad leaf, which is shed each autumn as the colder

weather approaches. Every leaf holds in its tissue a remarkable substance

called chlorophyll, which gives it its green colour. In sunlight and in the

presence of sufficient moisture and mineral salts, this chlorophyll ‘fixes’

some of the carbon dioxide gas that is always present in the air. Once

fixed, the carbon dioxide if transformed by intricate chemical processes

into sugars, and eventually into all the other complex materials that make

up the tree’s substance — wood, roots, flowers and seeds as well as the

leaves themselves.