Forests and water - the connections

Often articles about water briefly mention forests, and articles about forests briefly mention water. What exactly is the connection? The Yodeler asked forester and hydrologist Fred Euphrat to summarize for us the connections between forests and water.

Let's start with a classic conundrum.

• When a forest is destroyed, first the land releases more run-off water, but over time the soil and springs dry up;
• yet when a forest is planted, it first struggles with very little water, but over time can even create springs and a moist soil environment.

Why do forests, which use so much water to grow, ultimately create an environment productive of cool water, healthy streams, and abundant fish?

The answer is "forest influences", a term coined by George P. Marsh in Man and Nature (1863). Forest influences are the effects of forests on the landscape. Forests deflect wind and sun, slow and moisten the air, protect the soil from raindrop impact and landslide erosion, and promote the infiltration of water to soil and groundwater. In a well-established older forest with a high canopy, a dense understory, and significant downed wood on the ground, the influence of a forest on water is profound.

A forest canopy reduces the impact of raindrops by intercepting them above the ground and rerouting much of the flow down the tree trunks. The raindrops moisten leaves, from which they may fall or evaporate. The rain in the canopy also supports an entire tree-top ecosystem of fungi, lichens, and epiphytic plants.

The atmosphere under the canopy is distinctly different from that outside. Cooling shade creates wetter air, and days and nights vary less in temperature. Birds, such as owls, woodpeckers, and doves, can fly, find refuge, and forage in and beneath the canopy. Plants and animals that are too fragile for direct sun prosper here.

The forest floor is the product of the influences from above, including moderation of rainfall, a cooler, wetter environment, and abundant stemflow down trees. Snow builds up less on the forest floor than in adjoining clearings. The soil of the forest is made more porous and rich in humus by old wood, fungi, and subterranean animals, which constantly burrow and turn over the soil in small but important ways. Charles Darwin studied earthworms, more effective at turning the soil than falling trees or the pull of a plow.

When forests get wet, in storm or snowmelt conditions, soils may be saturated near streams or where they are too stony to accept water. But most forest soils absorb water very quickly, and pass what they can't absorb down through rodent burrows, old root holes, or soil pores straight into groundwater. The absorbed water in the soil, which stays into the summer and is drawn out by roots, can account for up to 10 inches of rainfall.

In California's dry climate, the most productive forest sites are near springs or in floodplains, with water sources throughout the year and no limits on water. Magnificent redwood groves, such as at Muir Woods or on the South Fork of the Eel at Bull Flats, show how forests can grow with abundant water.

Compared to a cleared landscape, forests are better at absorbing water and releasing it into groundwater and streams at a slow and gentle rate. The water, having passed through the soil and water table, is filtered and clear. The tree roots support the soil, reducing landslides near streams and on steep slopes.

Other soils and surfaces, from meadows and golf courses to roofs and parking lots, create a more `flashy' runoff, quicker to shed water when rain falls or snow melts, and are more in need of water in the summer months. During storms, farmlands, construction sites, country roads, and city streets all generate runoff that combines in rivers and creeks. The water from forests will be the last to arrive, carrying the least sediment, oil, and trash, and arriving after urban peak stream flows have receded.

But forests receive external influences as well. Logging, road-building, air pollution, adjacent development, and the vast expansion of people living `in the woods' all take a toll on the influences of forests on water. Old problems can live on, eroding soil or preventing the growth of native forests. Poorly planned logging, road-building, stream crossings, and `no management' can all increase likelihood of erosion or fire, and of ultimately significant watershed damage - affecting stream flows, stream channels, fish and the aquatic ecosystem, and people.

In poorly managed forests, clearings may be so big that forest influences disappear. Soils and seedlings are then exposed to wind, rain, and heat. Clearings grow only sun-tolerant tree and shrub species, so that forest diversity is reduced. Rainfall impact can be profound, and the compacted ground of roads and logging trails, especially when built without standards, can create disasters.

Historically, logging has had severe impacts on streams and caused flooding. Sometimes, when rain and bad forest practices have combined, the damage is so profound it is remembered for decades. Here in Northern California we still talk about - and look at the remnants of - the Flood of 1964. In response, California now has licensing for foresters, forest-practice rules, the Clean Water Act, environmental voices, and greater public scrutiny.

Forest management is a critical component of watershed management. It requires controlling erosion from roads, spacing trees so that the stand can resist canopy-destroying fires, planting trees so that there is always a productive forest, and working with neighbors and the industrial community to assure that lands can stay in profitable, productive forests. Without a doubt, environmentally sound forest management starts from a watershed-based approach, functioning in a viable manner for landowners, large and small, public and private.

Here in California about half the land is forested, including the vast majority of the watershed lands for our reservoirs. Sediment deriving from forests - a low influx from healthy forests, a much higher one from badly managed lands - determines the longevity of dams. The preservation of forest influences is the key to preservation and distribution of water; it is a win-win scenario.

Maintenance of forest influences does not mean preservation of the status quo. It means active pursuit of forests with broad, well-defined canopies. On lands used for logging, it means that forests must be sustainably managed to preserve both habitat values and economic viability. It means a forest floor with downed wood, understory vegetation, and low fire hazard. It means recognizing fire as an intrinsic natural part of the forest ecosystem and as a management tool, not merely fearing it as a menace.

In our present state, however, fire is not an easily used tool for forest management. These ecosystems have developed with fire: it prepares the soil as a seedbed (some seeds require fire to germinate), it clears understory vegetation so that predators may more easily hunt and large trees may more easily grow, it creates ash that returns nutrients to the soil, and it significantly reduces subsequent fire danger. But the best solution to the lack of fire is, of course, fire. Controlled (prescribed) burning with good weather conditions is an extremely effective tool at opening up the forest floor, recycling nutrients, and reducing fire danger. Like unmanaged fires, prescribed burns have hot spots and miss some areas, increasing forest diversity.

But today's air regulations, small ownerships and a culture built on fire suppression have taken this tool away from most landowners, limiting it to large ownerships and public lands. Fire, these days, must therefore generally be mimicked, not lit. Fire mimics include removing the small, dense patches of trees (`dog-hair fir'); felling individual trees to create small openings; and using torches to manage invasive plants, and chainsaws and chippers to manage downed wood. While not identical to fire, these practices reduce fire danger and promote forest influences. The alternative, letting unmanaged forests burn, creates catastrophic firestorms.

Ultimately, no mere set of rules can protect forests and water quality; the ultimate protections are the intent of the landowners and the ability of the operators. Maximizing lumber profit while minimizing forest investment is a poor approach to protecting streams and their ecosystems, including water for people. Good operations that combine selective logging with skilled protection of resources, including a balance of practice and preservation, are good for water, good for ecosystems, and good for the forest.

You can influence forest influences. Support family forests, logical logging, road management, and protection of your local watershed and creek. Join your local watershed organization. If you live in the country, get to know your neighbors, and consider managing properties together. You don't need to be a forest to have a great deal of influence.

Fred Euphrat, Ph.D., is a licensed forester and consultant in forestry, hydrology, and fisheries. He lives, logs, and writes in Healdsburg, CA.