Water is one of the most important products your backyard woods produces. Even if a stream doesn’t flow through your woods, some of the rain and snow that falls on your land will reach a stream sooner or later. The path water takes through your woods determines how fast it moves, how much of it is available to the trees and other vegetation, and how clean it is when it reaches a stream.
Rain and melting snow can flow over the soil surface or through the soil. Overland flow travels fast and can carry soil with it. Water moving through the soil moves slower and does not transport soil. The structure of the soil determines the path water takes through your woods.
Woodland Soils
The soil beneath your feet is more than a place for the growth of plants and a provider of physical support, moisture, and nutrients. The soil is a dynamic system that serves as home for countless organisms, a disposal area for nature’s “wastes,” a filter of toxic substances, and a store-house for nutrients. The soil is a product of its environment, but its quality is a function of trees that grow in it.
Woodlands customarily occupy a site for many years, sending a portion of their roots deep into the subsoil. During this period considerable amounts of organic material are returned to the soil in the form of leaves, branches, and decaying roots. This organic material has a profound influence on the physical, chemical, and biological properties of the soil.
The layer of organic material on the soil surface (litter layer) is incorporated into the top layer of the soil by soil animals, such as worms and millipedes. The mixing of organic material with the soil particles creates structure in the soil. Soil structure is the arrangement of individual soil particles into aggregates or clumps.
The soil clumps increase the size and the amount of spaces in the soil. The soil spaces contain both air and water. The amount of water or air depends on the length of time since the last rain or snow melt. Soil spaces improve water and air movement into and through the soil and increase the amount of water and air the soil can hold. The amount of soil spaces, in most woodland soils, varies from 30 to 65 percent of the soil volume. Soils supporting a variety of tree species have a higher percentage of soil spaces than soils supporting a single tree species.
Infiltration
The movement of water into the soil is called infiltration. When the rainfall intensity exceeds the infiltration capacity, water will run over the soil surface. By virtue of the sponge-like action of the litter layer and the high infiltration rate of the soil below, overland flow is extremely rare in your backyard woods.
The litter layer in your woods is especially important in maintaining rapid infiltration rates. This layer not only absorbs several times its own weight of water, but it breaks the impact of raindrops, which would otherwise loosen soil particles and clog soil spaces and reduce infiltration rates.
Woodland soils also have a high percentage of larger channels through which water can move rapidly. Most of these channels develop from decayed roots or from burrows and tunnels made by insects, worms, or other animals.
The presence of stones increases the infiltration rate. The differences in expansion and contraction between stone and the soil result in channels and large spaces. However, stones reduce the water storage capacity of the soil.
If snow covers the soil before prolonged freezing temperatures, it protects the soil from freezing, thus favoring continued infiltration during the spring. But if the soil freezes before snow cover, a snowfall covering the frozen soil will delay thawing in the spring, reduce soil infiltration, and increase overland flow.
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