Forested slopes are always in our background view; we take them for granted. Despite their commonness, we fail to appreciate the many functions trees have in our environment and our watersheds. They have remarkable internal mechanisms and complex relationships with forest soils to prevent erosion, and they efficiently utilize water and store it for long periods.

Trees and Soils
Forest trees have evolved complex relationships with forest soils to efficiently hold and release water during times of drought or deluge. Alone, these relationships appear minor, but together, their effects are profound.

First, trees hold soil. Their roots typically spread out fan-shaped through the top layer of soil, binding soil particles within a loose, breathable mat, especially in the organically rich top inches of humus found in undisturbed forests. By binding soils, trees effectively stabilize steep slopes. Some tree species send down taproots to penetrate deep into clay soils. Taproots allow for water and nutrient infiltration in deep soil substrates.

Soils have a large capacity to hold water, with some soil types capable of holding up to 30 percent of their weight in water. [1]

In spring, summer, and fall, leaves on trees intercept the energy of raindrops falling from the sky. By breaking the fall of the raindrops, the trees shield the soils below so they don't splash away.

In late fall and winter, leaves and needles on the ground cover forest soils and keep soils in place during rains and snow melts. Leaves also help insulate forest soils from freezing and thawing too quickly, too often, or too deep.

When tree leaves hit the ground they begin to decompose and are worked on chemically by water; fungi and leaf molds extract nutrients and reduce leaves to smaller components while beetles, insects, and microbes continue the breakdown. Underfoot, tiny organisms are very active decomposing leaves and woody debris. Recycling nutrients to make soil is a slow process; it can take up to 100 years to produce an inch of humus.

Transpiration
Large amounts of water absorbed into forest soils are pulled up into trees in a process called transpiration.

When water is drawn up into the extremities of leaves, it contributes to the process of photosynthesis. Once utilized, water is returned to the air as water vapor. This evaporation of internal fluids from a plant's leaves or needles lowers the pressure in a plant's outer reaches so that a constant flow of moisture from the roots is drawn upward to replace what escapes to the air. In this way, trees are fantastic pumps, lifting water 100 feet or more into their trunks and to their outermost limbs.

A growing plant transpires 5-10 times as much water as it can hold.[2] In heavily forested areas such as Pennsylvania, New York, and the Allegheny Mountains, the conversion of rain (liquid) to water vapor (gas) greatly reduces the amount of runoff, stream flow, and potential erosion in natural systems. Between 80 to 90 percent of streams in western Pennsylvania occur in small, forested headwaters where transpiration regulates humidity and microsite temperatures, reduces overall stream flow, and wards off erosion by absorbing the potentially destructive energy of water. Transpiration also means that nutrients are retained that would otherwise be carried out of the ecosystem. [3]

Infiltration
Water that is not transpired penetrates further into forest soils and continues into groundwaters and aquifers. (20 percent of freshwater on the planet is stored in underground water).[4] Groundwater has an immense capacity to store and move water and slowly re-release it. Even in dry periods, groundwater supplies yield springs and seeps. Slow groundwater movement constantly feeds forest ecosystems and streams and contributes to the water cycle. Forests secure an ample supply of groundwater and have a huge capacity to hold water from long or intense rains. In this way, forests act like "sponges" and significantly reduce flooding.[5]

[1] The Climate Near the Ground, Rudolf Geiger, 4th edition, 1961. page 127.
[2] Environment Canada
[3] From Cape Cod to the Bay of Fundy: An Environmental Atlas of the Gulf of Maine, Philip Conkling, editor, Chapter 8, "Wetlands, Rivers, and Aquifers", Annette Naegel, page 146.
[4] Environment Canada
[5] Flood Commission Report, 1912, HJ Heinz president. Appendix