Forest soils are an important natural resource in the United States. Almost one-third of U.S. soil is in forests. These include highly managed forest plantations, farm woodlots, national forests and parks, urban forests, and vast wilderness areas. All these different types of forests require healthy soils to thrive and to provide goods, like timber, and fuel. Forests even provide “services”, like recreation and clean water. (For more information about soil ecosystem services, visit here.)
Clean and abundant water
It’s been estimated that about two-thirds of the water we drink comes from forested lands, and forests provide some of the highest quality water available. Forests have a layer, of accumulated leaves, seeds, wood, twigs and other organic matter. Many people call this the forest floor, or the litter layer, but soil scientists call this important layer the organic, or “O horizon”. This O horizon ranges from very thin to a few feet deep. All this organic matter increases the water infiltration capacity of soil, and slows or prevents surface runoff from rainfall. A good O horizon prevents erosion and loss of sediments into streams.
When forest soils are disturbed their ability to capture and filter water can be impaired. Examples of disturbance include harvesting trees, road building, wild fire, or overuse. That’s one reason why forest managers help create forest management plans – to protect the O horizon.
Soils sustain life
The biota that inhabit soils are also essential to the healthy forests. Soil dwellers are essential to helping to break down the leaf litter, fallen trees and other natural “trash” of the forest.
If the balance of life is thrown off by disturbance, then the equilibrium that naturally exists is thrown off. Microorganisms regulate cycling of nitrogen in particular. Nitrogen is the nutrient most often limiting forest growth and productivity. Soil microbes in disturbed soils – say a fire that increases soil acidity – can’t do their job. This throws off the balance not only of nitrogen but other nutrients and minerals in the soil. Plant life needs calcium and magnesium, but proper soil pH is needed to maintain the right levels. Changes in soil nutrient levels can result in physiological changes in the trees, making them less cold hardy, or perhaps more susceptible to drought.
Native earthworms work to aerate soils, reducing compaction and increasing water infiltration rates. However, in some parts of the country, introduced exotic earthworms can consume the organic matter in the O horizon. This changes the way nutrients and water move through forest ecosystems.
Healthy soils help ensure healthy forests. Trees that receive sufficient nutrients and water from the soil will grow better, and be more resistant to insects and disease. In the southeastern U.S, years ago, it was discovered that to produce a healthy plantation of pines the addition of phosphorus was required at the time of planting. Without sufficient phosphorus, the trees would not grow. Trees that are stressed by lack of nutrients or lack of water are also more susceptible to insects and disease.
Trees are long-lived organisms! It’s easy to think that trees and forest soils can take care of themselves. But, humans have changed the way forests are used. Unlike us, trees can’t leave when conditions are not to their liking! So, management of forest soils is important. Managing forest soils is mostly about protecting the organic matter in the O horizon. Forest managers work to minimize compaction and disturbance, ensuring that nutrient supply is balanced with the needs of the trees, and protecting the soil biota.
Answered by Mary Beth Adams, US Forest Service