The Life Span of Plants continue…

The leaves fall to form a deep carpet beneath the trees, adding to the dead twigs, flowers and unripe fruit remnants already there. Every year trees shed more than 3,000 kg of waste products in every hectare of woodland and all this breaks down, together with the herbs of the forest floor to form a deep layer of litter. As this litter breaks down so the minerals and other organic substances which were stored in the leaves are released once more, and the resulting layer of humus acts as a natural fertilizer. In this way the nutrients from the soil are part of a continuous cycle, being drawn up from the deeper layers of the soil by the tree’s roots, passed through its system and eventually returned to the surface soil by leaf fall and the final decay of the tree itself.

Where the ground is utilized by man, this natural cycle is broken. If agricultural crops are grown, much of the soil’s nutrients are removed at harvest time, while grazing animals similarly feed on the nutrient-rich grass converting it to meat for the table. Man replaces these essential foods as far as he can by application of artificial fertilizer, but it is often not enough to keep the soil really fertile. Furthermore, the structure of the soil is damaged by continuous removal of humus.

Remarkably enough, a plant growing in fertile, humus-rich soil will not achieve a greater age than one in an unfavourable habitat, in fact, quite the reverse. Many small, soft-stemmed plants have a relatively short life, Red Clover (Trifolium pratense) and Lucerne (Medicago sativa) normally living from 2-5 years, White Clover (T repens) from 5-8 years, Zig-zag Clover (T medium) for 10 years, Solomon’s Seal (Polygonatum multi forum) and Herb Paris (Paris quadrifolia) for about 17 years. Many perennial plants live far longer than this, but with clump-forming species it is often difficult to be sure if it is the same plant which is living on, or whether its living space has been taken over by surrounding offsets.

With trees, determining the age presents fewer difficulties and a sample boring will usually give an accurate dating. The sequoias of the western USA are still healthy trees at 1,000 years, by which age they have reached a height of 110 metres and a girth of 10 metres. The exceptional tree known as ‘Father of the Forest‘ was 142 metres when felled and 36 metres across its base. Far older than these however, are the specimens of Pinus longaeva found in the Sierra Nevada and between 4,000 and 5,000 years old. In marked contrast to the big coast redwoods of the coastal forests, these pines are small and stunted, each year’s growth rings being so small as to be discernible only under a microscope.

It is noticeable that the tallest conifers, (Sequoia sempervirens), the tallest broad-leaved trees, (Eucalyptus regnans) and the oldest trees, (Pinus longaeva), are all evergreen. As they are never without foliage it is often thought that their leaves live almost as long as the tree, but a glance at the deep litter of fallen needles under a pine soon shows that this is not so. In fact many evergreens replace their leaves at least every two or three years and some annually. The only basic difference between evergreen and deciduous trees or shrubs is that the former lose their leaves, a few at a time, almost throughout the year, while the latter shed all their almost at once.

What limits the life of a woody plant? There is no simple answer to this question because many factors affect the onset of senility of a tree. Storage of waste products and minerals absorbed by the roots probably affects the efficiency of the conducting system. This is associated with the fact that plants, unlike animals, never stop growing. Every extra bit of length added to the tips of branches, especially to the tree’s leader, means a greater number of leaves and wood to be supplied with water and minerals by the roots which may be over 100 metres away. It does seem that once each species reaches a certain size, efficient conduction becomes more difficult and small branchlets will die off as they fail to obtain sufficient supplies.

It is not unusual to see trees with dead branches at the top. These trees are often called stag-headed from the resemblance to a spread of bare antlers above the foliage. This condition is partly tied up with age, but stag-headed trees are far more common in hedges beside agricultural land and this is largely the result of a build-up of herbicides in the lower layers of the soil. The roots of the tree become damaged and can no longer supply the whole tree.

Once limbs die, then fungal infections are able to get into tissue and they often spread into healthier parts of the tree hastening its final end. Trees which are unhealthy, especially from disease which affects the roots, are particularly prone to damage in severe gales, sometimes becoming completely uprooted.

As the heartwood of a tree is made up of dead cells with the living wood surrounding it, it is possible for the outer part to remain alive even after the centre decays. Some very old oak (Quercus) and yew (Taxus) trees show this characteristic and the living branches have to be propped up artificially to prevent them from falling because they have no natural trunk weight to counterbalance them. Trees which have been cut to near ground level (coppiced) regularly through their life will also continue to grow from separated parts of the living wood long after the central core has gone. These stumps from which new shoots can arise are known as stools. Ash (Fraxinus) stools which originated more than a thousand years ago are known.

As well as supplying the current year’s growth with food, all deciduous plants must store sufficient nutrients for the development of new shoots and leaves in the spring. Evergreen plants do not have this problem as they are able to manufacture their food at any time of the year when sunlight and the availability of water make this possible.

Foodstuffs are stored in the parenchyma, usually in the form of starch. In some species, such as lime trees (Tilia), they are changed into fats and sugars. These supplies are used up in spring and then replaced through the summer by the surplus produced by photosynthesis. This is why the amount of growth put on by a plant in spring, and the number of flowers and fruits formed, is largely dependent upon the conditions of the previous season.

Throughout the winter season plants of cold regions must be able to survive severe conditions. Those which remain above ground may stand while temperatures fall as low as —50°C. Only those species which have particularly dense cell contents can survive, the thickened sap acting rather like an antifreeze solution. Plants from milder parts of the world do not have this protection and if grown in cold countries, the winter frost will freeze their sap. The expansion of the water as it turns into ice ruptures the cell walls and causes the breakdown and death of the tissues. Plant breeders, trying to create hardier forms of commercial crops, purposefully breed from strains adapted to the coldest conditions, but growing slightly tender plants on a large scale is always a hazardous business.

During an unusually cold spell in temperate climates, many plants show signs of damage very similar to that experienced in drought. This is because when the soil is frozen, the roots can obtain no water and plants which do not become totally dormant will suffer.

Cold and drought are the two greatest enemies to plant life and their adaptations to combat these both seasonally and on a world wide scale are of endless interest.

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