Our world is filled with trees of different shapes and sizes, but what caused them to evolve the way they have?
To say that trees come in all shapes and sizes is an obvious statement. But have we stopped to consider why some species of tree have evolved into giant specimens while others are smaller with stunted, twisted growth.
The best examples of giant trees are of course the giant Redwoods, Sequoiadendron giganteum; the tallest in the world are growing in the forests of the National Parks in California. An explanation as to why they have grown to such terrific heights lies in the bark. This soft fibrous material contains tannic acid, a natural chemical which makes it fire resistant and offers protection to the trees so that when a forest fire roars past, other plant species are consumed by the fire and the Sequoiadendron is left to grow without competition for space, water and nutrients.
If we think of where we have seen trees that are low growing with stunted growth there are often similar environments factors that are common to each site. The locations could include a windswept hillside or cliff top, open moorland or chalk downland. In these conditions the trees may have a limited water supply that in turn restricts the amount of nutrients available to them which then limits their annual growth rate. In exposed conditions new plant growth can be scorched by hot or cold winds and on low plants new growth could be eaten by grazing animals, both of these causes having the effect of continuous pruning hence their shorter, stunted growth.
Public Parks and Countryside Hedges
In comparison the ornament trees that are planted in public parks and private gardens are planted in soil with adequate nutrients and water and usually have guards protecting against attack from animals. These trees get established quickly and have sufficient space to grow into trees with well shaped canopies. They can often be identified as being grown initially in a production nursery because of their straight, parallel trunks, which is due to the fact that they were grown against canes with supporting wires in their early years and had no need to grow thicker, stronger trunks at the base.
In the countryside field hedges containing native species are mechanically trimmed once a year to maintain a strong hedge in order to keep livestock in, sometimes it is possible to see a tree growing out of the hedge, often this was a strong shoot that had been missed by the hedge cutter and allowed to grow up into a tree.
The Browse Line
In the parklands of old estates across England the mature specimen trees very often follow their beautiful natural growth habits because they have had both the space and the time to mature. However what gives them their characteristic appearance is the neat horizontal base to their canopy known as the browse line. This is a method of pruning carried out by animals, usually deer or cattle as they reach up to eat the young growth on the lower branches.
In complete contrast to specimen deciduous trees growing in ancient parkland settings are the millions of coniferous trees planted as timber crops in vast plantations sweeping across the countryside. These may not be good for biodiversity of native wildlife but they can provide shelter from wind and help reduce soil erosion on steep hillsides. Current thinking by many land owners is to have more mixed planting throughout their plantations to encourage greater wildlife diversity, improve the visual impact and importantly reduce the risk, and rapid spread of forest fires destroying the trees.
Compare all this with the sheer beauty of English woodlands with their wide variety of deciduous species at many different ages and stages of growth including young saplings, coppiced stems and mature specimens hundreds of years old supporting a huge range of wildlife throughout the seasons.
The visual impact that trees have on our landscape is enormous. Without them we would see vast baron spaces and endless rows of houses and industrial buildings. When you are out and about in your own neighbourhood, or touring new areas take note of the trees around you, then visualise the location without them.