Читать книгу Collins Complete Guide to British Trees: A Photographic Guide to every common species - Paul Sterry - Страница 19

Like all living things, trees have a finite life. Some live just a few decades while others may survive for hundreds of years; a few venerable specimens can claim to have seen out several millennia. However, whatever the life expectancy of a tree, it will pass through a series of reasonably well-defined stages, in much the same way as do all other flowering plants.

A tree’s life begins when a seed germinates. The first pair of leaves to emerge is simple and they are quite unlike the true leaves of the tree or shrub; they are derived from the cotyledons that contain the seed’s food store. These first leaves contain chlorophyll and can photosynthesise, supplying the tiny seedling with its first food made from sunlight energy. Tiny seedlings are vulnerable to environmental pressures – grazing, drought and trampling for example – and few make it to the next stage in a tree’s life.

Many acorns are collected and buried by Jays. Those that escape being dug up will germinate the following spring.

Recently germinated seedling.

Vigorous sapling.

Soon the seedling will start to produce true leaves and once these appear then rapid growth begins and continues for many years. In the case of a Pedunculate Oak, for example, all being well it could put on up to half a metre of growth each year, both in terms of height and, more significantly, in terms of crown size. Of course, the amount of growth is influenced by environmental factors, including soil type, grazing and insect damage, the proximity of neighbouring trees and prevailing climatic conditions in any given year.

Many large native tree species will not produce flowers and fruits in any quantity, or at all, until they are at least 30 years old. After that, they continue to grow for several decades more, but a typical Pedunculate Oak is likely to reach pretty much its maximum size at around 100 years old. All being well, it will continue to produce acorns for at least the next 200 years, with the crown filling out and the trunk expanding – unless, of course, it is struck by environmental catastrophe, disease or the actions of man.

Mature oak.

Fallen giant.

During their heyday years, native deciduous tree species follow, and indeed mirror, the four seasons experienced by Britain and Ireland. They are leafless, and seemingly lifeless, during the winter months, but the sap begins flowing in spring and soon fresh leaves appear, followed by flowers. The leaves mature, and fruits form, in summer, the foliage losing some of the fresh green colour it had in spring. With the approach of autumn, nutrients are withdrawn from the leaves, resulting in colour change, and eventually in late autumn they fall, carpeting the ground below.

The four seasons.

In the natural course of events, all trees eventually come to the end of their lives, although in the case of the Pedunculate Oak and several other native species, this can be after several hundred years of existence. However, sooner or later, often as the result of disease and damage, the tree begins to die back. The appearance of fungal fruiting bodies on the trunk is often a sign that all is not well and that the process of decline and decay has begun. However, from an ecological perspective, there is something life-affirming about this process. In the absence of man’s intervention, nothing goes to waste and all the nutrients from the venerable old tree enrich the soil, thanks to insect attack and fungal decay, thus creating ideal conditions for the next generation of trees. It should be borne in mind that the life of certain tree species can be prolonged by the actions of man. Notable coppiced Ash stools, for example, date back 800 years or more, much longer than an Ash tree would be likely to survive in its natural state.