There are different reasons trees are removed. Sometimes they pose a safety risk, or are compromised by disease, insects, weather damage or construction. But in many settings these unstable trees may still serve a purpose, so it is always important to evaluate the site and consider the far-reaching effects before removing trees.
River Bank Stability
Trees hold the soil together along creeks, streams and rivers. The web of underground tree roots can be extensive and affect large areas. The roots reach at least the width of the trees' canopies. Once the trees are removed, erosion can become a serious problem. River banks can fail and mud can be introduced into once pristine waters. This can affect human water supplies, as well as that of the animal population. The roots and stumps may continue to provide some support to the banks, but will eventually rot away.
When trees are removed along riparian boundaries, shade becomes sunlight. This causes the normally cool water temperatures to rise, which can be perilous to fish and other underwater creatures. Even fallen trees have their place in waterways. Salmon and other fish routinely seek out the cool shelter of a fallen tree to spawn and raise their babies.
Plant Populations Affected
Many native plants establish themselves under shady trees. Certain species of wildflowers, such as trillium, lady slipper and shooting star, will die out completely in full sun. Some of these plants are rare and endangered. Moss and lichen also need moist shade to thrive. Removal of trees in a garden setting will also change the growing environment. Shade-loving plants will need to be removed and exchanged for sun-loving plants. When trees are removed and the area is left bare, opportunistic plants like Himalayan blackberry and scotch broom will move in.
Mushrooms and Fungi
Mushrooms grow in the perfect balance of shade and moisture under trees. Fungi have a symbiotic relationship with plants, which also rely on fungi for winter nutrients. In addition, it has taken years to learn to commercially propagate some mushrooms, and many only survive in perfect native situations.
Whether in the woodland or home landscape, trees are the perfect habitat for birds and other wildlife. Animals such as woodpeckers, owls and flying squirrels are displaced if large trees are cut down. Trees also provide food for birds and small animals that rely on cones, nuts and seeds as their primary food source.
Although it takes a very long time, conifer forests can recover from clear cutting. However, when an area of trees are cut down in a tropical rainforest, this may not be the case. The high turnover of foliage creates the thin layer of nutrients found on the rainforest floor. This nutrient-rich material is critical for new seeds to germinate. The heavy rains wash away this topsoil when there are no trees to create a barrier. Many rain forest plants will only thrive in the dense shade of the forest canopy. So when the trees are removed, the area may never recover.
Trees as a Windbreak
A row of trees can be one of the best barriers against wind. Mature trees at the edge of the woods or around a pasture will greatly reduce wind damage to other plants and structures. By taking out these protective trees, you could lose other trees you were trying to save. Trees will also reduce the impact of wind on agricultural soils that are otherwise exposed to the elements.
Clean Air and Water
Large trees take in large amounts of water and filter it. They take the water through their foliage and roots and release it back into the atmosphere cleaner than before. They also filter the air we breathe by taking in carbon dioxide and releasing clean oxygen. All plants go through this process, but trees do it on a much larger scale.