Forest Soil

“Some years ago, scientists blocked off a small section of forest soil in New York state and removed the top layer of earth to a depth of one inch. In all, there was an average of 1,356 living creatures present in each square foot… Had an estimate been made of the microscopic population, it might have ranged up to 2 billion bacteria and many millions of fungi, protozoa and algae – in a mere teaspoonful of soil.”

Peter Farb, The Forest

Although it is often overlooked, forests are only as healthy as the soil they grow on. Soils are full of a wide variety of living organisms, again each with roles to play and susceptible to poisons. If we remove too many nutrients from the site, or degrade the soil, we can create the same results that humans get from drinking polluted water or eating poor quality food. When nutrients are lacking, unhealthy trees can easily succumb to insects, diseases and atmospheric pollution that stronger specimens might have weathered.

Soils are probably the most complex and least understood part of natural systems. Soil degradation is not easy to measure. It involves a combination of many effects; loss of biological diversity within the soil itself, destruction of soil structure, loss of nutrients and the enhanced development of acidic soils known as podzols.

Traditional harvesting on farm woodlots, while slowly degrading the quality of trees on site by removing only the best stock, were much less harmful to the soil than today’s methods. Individual trees or small groups were removed and a lot of brush remained on site to protect the soil and provide nutrients for the next crop. Small equipment was used, often in the winter, creating less soil compaction. Contrast this with large-scale clearcuts where brush is burned, or whole tree harvests, where almost all above ground brush is removed. In both cases, large amounts of nutrients are lost and heavy machinery is used that can lead to severe soil compaction.

Gordon Robinson, forester and author of The Forest and the Trees:
A Guide to Excellent Forestry, worries about increased exposure to solar radiation and evaporation after clearcuts. “The normal soil life of fungi, bacteria, worms and all types of micro-scopic plants and animals is destroyed or at least greatly changed.” Since health and vitality are important factors in preventing tree disease, trees growing on degraded soil are more susceptible to insect and disease damage.

What you can do:

• Wherever possible, maintain a closed canopy, where the tree tops are close together and shade the forest floor. Exposure to sun and wind can quickly burn off organic matter in the soil and lead to degradation. A closed canopy permits a unique moisture-temperature-light balance without which the proper soil conditions for a healthy forest are unlikely to exist. Multiple levels of undergrowth combine with a closed canopy to ensure a gentle transfer of water into the soil, even after a heavy rainfall, and more gradual snow melt. This leads to fewer dry periods, more humidity and less runoff.
• If a field is reverting to its natural state of forest, consider letting alders grow up and underplanting or interplanting with a mix of species. Alder leaves are unusually high in nitrogen and make yearly deposits to the soil bank. Alders also fix nitrogen from the atmosphere in root nodules and make it available to other plants. Acadian forest species can be planted this way, using tall transplants or protective cages to foil snowshoe hares.
• Make sure there are a lot of deciduous trees in the forest. Leaves increase bacterial action and are great soil builders. Conifers can create a dense mat of acidic needles that decay slowly.
• Not all plants feed from the same soil level and a mix of trees with shallow and deep roots will be less likely to exhaust the soil of specific nutrients. Also, each species cycles different nutrients through the soil, (for example, dogwood is known for making calcium available). Much work needs to be done in this area but encouraging a wide mix of plants just makes good sense.
• Remove only the extra, or even less, that the forest produces each year. For example, if the rate of growth is 2.5 cord/ha (1 cord/acre) per year, you may want to remove an average of 1/2 or 3/4 of that amount every year. This will allow soil to be improved and trees to grow older and more valuable.
• Avoid compacting forest soils by using light equipment when the land is dry and planning your extraction roads well. If you must use heavy equipment, it should have large `floatation’ tires to spread out the weight of the machine.