This section gives a brief outline of the main benefits of trees and woodland.
The capability of trees to affect climate through the cycling of CO2 is becoming ever more important because of the connection with global warming.
Satellite studies made in 1999 have shown how the spread of mega-cities around the world has created vast ‘heat islands’, that cause smog, trigger thunderstorms and reduce the productivity of the land. In America, Atlanta has lost on average 55 acres of trees every day for two decades. The hard surfaces of the city soak up radiant energy during the day, and hold on to it at night. Consequently more energy is needed for air conditioning, creating more ozone and smog. The heat at night creates a low pressure system, with hot air rising and cooler surrounding air rushing in to replace it, so increasing the number of storms. Tree planting is one of the ways of reducing this heat island effect, but the scale of planting needed is massive. In China’s Pearl River delta the urban sprawl grew by 319% between 1988 and 1996. The need to plant trees and restore woodland cover in many parts of the world is urgent.
Trees directly affect climate and air quality in many ways; by oxygenating, humidifying, modifying temperature and wind patterns, and metabolizing pollution. Even smaller groupings of some species can be useful as oxygenators.
The foliage of a single mature beech tree, for example, can extract more than 2.5kg (5.5lb) of CO2 from the atmosphere, and produce 1.7kg (3.7lb) of oxygen in one hour, which in theory is enough for the needs of ten people in a year. Depending partly on leaf size, trees can also transpire large amounts of water, a process which cools and humidifies the surrounding air. A mature beech can transpire up to 440 litres (100 gallons) of water a day. Planted in a green network around and within built-up areas, trees work in several ways. They cool the air through transpiration, control the heat which radiates from roads and buildings, and disperse the warm, polluted air that tends to hover over inner cities.
Trees can help reduce the damage done by air pollutants and vehicle emissions through creating air turbulence. ‘Woodland edge’ plantings of shrubby species are particularly suited to this purpose. Pollutants can also be filtered and to some extent absorbed by the foliage of trees. Dust is easily filtered out from the atmosphere, becoming trapped on the leaves of trees and then washed down by rain.
The complex nature of native semi-natural woodlands in Britain is the result of long evolution and human interference. A newly planted woodland will take many decades before it begins to develop the wide range of dependent flora and fauna of a long established wood. To best encourage both immediate benefits for wildlife, and the gradual ingress of other species, many factors will need to be taken into account. These include the soils, location, most suitable species, planting layout and maintenance procedures.
Woodlands with a varied structure, with trees of different size, shape and stage of maturity will greatly increase the diversity of a woodland, and encourage more species. The number of bird species in a woodland, for example, is related much less to the variety of tree species than to a varied structure, which offers more opportunities for breeding, nesting and feeding. Woods with a limited structural variety can be improved for wildlife by introducing diversity in the number and types of woodland layers.
Different tree species create microclimates which influence what grows beneath them. They do this by controlling the amount of moisture and wind reaching the lower layers, the light intensity at ground level, the organic composition of the leaf litter in the soil, and the uptake of soil moisture and nutrients. The trees you plant will therefore influence the species which colonise in the layers beneath. The local seed zones set out which species may be appropriate in different parts of the country, according to their natural distribution.
Excessive noise from traffic or other sources is a form of environmental pollution, and trees can be useful in lessening it to some degree. The leaves of a tree tend to scatter sound, and its trunk to reflect it. Trees with dense foliage and large leaves, such as plane or large-leaved lime, are the best choice. Woodland plantings are even more effective, as the soft surface created by leaf fall, deadwood and ground cover tends to absorb sound. Where there is only room for a belt of planting, this should be 8-12m (27-30’) in width, with a thick understorey on either side. The belt should reach a height of at least 5m (16’) in the middle.
Clumps or belts of trees and tall hedges have been planted as windbreaks on farmland for many centuries to shield crops, buildings and livestock from exposure. Open spaces such as playing fields, parks, urban squares and industrial sites can be improved by the planting of strategic shelterbelts.
Trees reduce the force of the wind and modify its movement around buildings. Factors influencing the effectiveness of windbreaks and shelterbelts include the dimensions of the group of trees and the planting density. Very dense barriers are counterproductive, and some permeability is necessary. Shelterbelts around buildings can reduce fuel consumption in winter, and provide shade in summer.
Landscape, amenity and recreation
Trees, in woodlands, hedgerows, roadsides, parks, gardens and all types of location are a vital part of our landscape, in rural and urban areas. Some are remnants of semi-natural woodland, but many are planted, and the beauty of many wooded landscapes is due to tree planters of previous centuries.
Trees provide the backdrop for many urban views, and soften and disguise the harsh outlines of buildings and roads.
Wooded landscapes, commons, parks and places with scattered trees are favourite places for walking, picnicking and other forms of recreation.