Reduced forest cover means more flooding and erosion, because tree roots physically keep soil from washing away, but also because tree roots absorb and store water. Also a tree canopy can reduce the force of rain hitting the ground from a deluge to a gentle sprinkle, further preventing erosion, and reducing the chance of land-slides.
Trees reduce ozone levels. This effect is not just global, but local. In large cities, a reduction in ozone can mean milder temperatures and more breathable air.
Sustainable forests are a great source of timber and help maintain a vibrant economy.
Trees absorb sound and reduce noise pollution. This is especially important for people who live near motorways and industrial sites. In some cases, a well planted group of trees can reduce noise pollution by up to 10 decibels.
Trees trap dust and debris. Dust, smog, and other particles in the air collect on the leaves and tend to stick there. This creates cleaner air for people and animals to breath, which can be important for quality of life in both cities and dusty agricultural areas.
Forests provide some of the most important ecosystems in the world. Reduced tree cover means less habitat for animals, insects, fungi and plants. Overall there is a reduced biodiversity in our countryside.
Forests are an important part of the Carbon cycle. CO₂ is one of the major contributing elements to the greenhouse effect. Trees trap CO₂ from the atmosphere and produce carbohydrates that are used for plant growth. They give us oxygen in return. Mature trees can absorb roughly 20kg of CO2 a year. The tree in turn releases enough oxygen to sustain two human beings.
On the plus side there are some major tree planting projects on the go around UK run by the Woodland Trust, the Forestry Commission and the National Forest amongst others.