Forests have been depleted throughout history and less than one third of the world’s original forests remain. Vast areas were felled to support human communities and allow them to expand. In Roman times, for example, many of the large oak forests in Western Europe were destroyed, the wood being used to fire lead smelters.
Today, forests are more highly valued and as environmental concerns grow – about climate change, in particular – there is a better appreciation of the benefits that forests provide, including soil and water conservation.
Soil Conservation
Forests conserve and enhance the soil’s ability to sustain diverse ecosystems. They conserve the productive capacity of the land by reducing soil and nutrient loss.
Erosion is the primary cause of soil and nutrient loss. Erosion is the removal of the top one or two layers of the soil profile – the most important in sustaining ecosystems. Degradation of removal of natural vegetation accelerates erosion which is caused by water and wind.
Leaching is the loss of soluble nutrients which water transmits down the soil profile causing soil degradation.
Forests also restrict soil and nutrient loss by reducing the volume of rain falling to the ground. The forest canopy absorbs much of the kinetic energy of rain, reducing its ability to loosen and displace soil particles and cause surface run-off. Volume and velocity would otherwise combine to create an erosive force. Run-off velocity is also reduced by the mechanical action of roots and vegetation litter which cause the water to disperse and take a longer, slower path downhill.
Vegetation and surface litter shelter the soil surface from wind and reduce surface water evaporation – water sticks soil particles together.
Water that does not run-off soaks into the soil. It moves down until it reaches the ground watertable from where it slowly seeps into streams, lakes, oceans or underground aquifers. Trees root deeply and are able to intercept a significant proportion of the nutrients carried downwards by the water and draw them up into the tree, eventually returning them to the soil surface.
A type of erosion known as ‘mass movement’ involves a large mass of soil and can happen very quickly. It generally occurs irrespective of vegetation cover, but its frequency and severity are greatly reduced in forests where the tree roots bind soil particles together, interlock and also penetrate down into deep soil layers.
Water Conservation
Forests produce clean water on a sustainable basis. They reduce sedimentation of fresh water and prevent nutrients from being removed from the soil. Water is cycled back to the atmosphere helping to maintain rainfall levels.
Water with a high nutrient content is a problem as it causes high growth rate in aquatic plants and micro-organisms. These can pollute and choke waterways, making them uninhabitable for other aquatic life and undrinkable. In New Zealand this is a problem with algal blooms in the major lakes of central North Island such as Lake Taupo and Lake Rotorua.
Sedimentation can also make water unsuitable for many uses and habitats. Its effects can extend to lakes and coastal areas such as marshes and coral reefs. High sedimentation can choke swamps and marshes, reducing the ability of the ecosystem to support a diverse number of species. Sediments can settle in river channels, slowly raising the beds until they are higher than the surrounding land. Rivers then find new paths to the sea – usually with catastrophic flooding as a consequence.
Forests can reduce the severity of floods. By increasing the time water takes to move through a drainage system, peak flows are reduced. Forests also inhibit sedimentation which can add significantly to the volume of floodwater and the amount of destruction caused.