The following article is a brief description of how forests do this and why this function of our private woodlots should be valued more directly.
Fall along a forested stream.
Water arrives in the watersheds that supply cities, towns and individual homes in Ontario as precipitation in all of its forms, in annual amounts averaging 890 mm of rain. It leaves these watersheds, to enter drinking water supplies, after traveling a path ranging from direct to tortuous, through land cover ranging from pavement to dense forest, and after a time period ranging from minutes to years. In a forested watershed, the erosive energy of even the most driving rainfall is absorbed, and pollutants are purged by the relatively long path and time frame from precipitation to the outlet. The erosion of sediments and nutrients is minimized and high-quality raw water is delivered to receiving surface and groundwater sources. There simply is no other watershed cover or land use that exceeds the purifying, protective value of forests for drinking water supplies. A forest provides water treatment at all scales, from the individual tree to the relatively homogeneous forest stand to the more diversely structured forested watershed.
An individual tree captures and slows precipitation through passive interception and evaporation and active transpiration (collectively, evapotranspiration or ET). While many factors determine ET rates, a mature, open-grown deciduous tree may have in excess of 200,000 leaves, which on a summer day can transpire as much as 3,400 litres of water.1 In hydrological terms, water that is not consumed by ET moves through forests as subsurface flow, during which it is filtered both mechanically and chemically, or, much more rarely, as overland flow, when the soil is either frozen or saturated. By reducing soil saturation, ET limits overland flow and the associated transport of nutrients and sediments. While the deep organic soils of the forest represent the actual “filter” in this system, individual trees a) anchor the soil; b) produce soil macro pores as roots penetrate and die, increasing water infiltration; c) capture and utilize inorganic nutrients in the soil water as the basis for growth and metabolism; d) deliver organic materials (leaves, twigs) to the forest floor, reducing the erosive impact of rain; e) provide shade that regulates the pace of decomposition and the temperature of streams; and f) produce seeds that germinate in the soil and enable the forest to regenerate and recover from disturbances. In addition, trees process pollutants directly in a variety of beneficial ways. Some airborne pollutants are simply trapped on the surfaces of the tree, removing them from the air and stalling their entry into released waters. Biochemical reduction by plants (the basis of “bioremediation”) is a varied and complex combination of processes that further neutralizes pollutants.2 As long as pollutant levels are not toxic to the trees, these tree-level processes continuously cleanse the water that passes through the forest and its soils.
Early morning by a mid-forest vernal pool.
A forest stand protects water supplies through the multiplication of the effects of individual trees and understory plants, but also provides collective effects that go beyond those of individual plants. When an individual tree in a stand begins to decline, leaf area, transpiration, root penetration, growth and nutrient uptake, shade and eventually seed production are all reduced. This process may result from simple stem exclusion, through which an initial stand of perhaps a million seedlings per acre is reduced by competition to a mature forest of 100–200 trees. Or it may result from a large array of defoliators, fungi, or viruses, or from injuries following wind or ice storms. Regardless of the cause of tree decline, the influence of a forest stand is that the living, thriving trees surrounding a tree in decline will capture and utilize the surplus resources, including sunlight, water and nutrients, and the result for water quality is uninterrupted protection; the system persists even as its components are recycled. Furthermore, the diverse, interchangeable seed sources in a mixed stand maintain a regeneration response even in the event of a species-focused disturbance. When a disturbance eliminates groups of trees, the rapid regeneration of the openings by the surrounding stand again maintains the protective effects of the soil and forest vegetation on water quality.
The quantity of water leaving a forested area (yield) is affected by stand types. Stands dominated by evergreen conifers generally reduce annual water yield below that produced by stands of deciduous trees on similar sites, primarily because evergreens continue transpiring throughout the year and because they intercept a higher percentage of snowfall, a portion of which either melts and evaporates or directly sublimates. The quality of the water leaving a forest, in particular its nutrient content, may be affected by the stand age. Young, established stands of any species mix are accumulating biomass more rapidly than older, maturing stands, and therefore assimilating available nutrients more aggressively, which keeps them out of the water.3,4 As expected, this demand is highest during the growing season, which is reflected in the seasonal patterns of nutrient flux in streams. The capacity for nutrient capture may decline as trees and stands mature, but as openings are created through management or natural tree death, the forest rapidly refills these with young, nutrient-hungry seedlings that quickly restore the uptake of available nutrients.
After a storm on Quabbin Reservoir Massachusetts.
The forested watershed accumulates the effects of individual trees and forest stands to provide highly resilient protection for drinking water supplies. The natural range of seed sources, topographic positions, water regimes, aspects, soil types and bedrock composition conspire to maintain a diversity of stand types, while the range of disturbances maintains age diversity. The combination builds ecosystem inertia that maintains forest cover. While some disturbances may temporarily overwhelm the controlling influence of even a diverse watershed forest (e.g., hot crown fires following severe droughts or catastrophic wind events or ice storms), disturbed forests are quick to recover biotic control of stand level nutrient mobility.5 The inherent species diversity across a forested watershed provides a level of redundancy in the living, green filter that rivals the most responsibly engineered water treatment plant. The diverse age structure in the watershed forest, like diversity in an investment portfolio, yields more consistent performance through the vagaries of climate fluctuations, wind, snow and ice, intense rainfall, and damaging native and alien pests than a forest (or an artificial filter) built to a single design. The range in structural and species compositions together represent built-in multiple barriers, providing a forest biofilter that functions continuously, without pause, and does so on free solar energy.
So, how much forest-filtered water does a hectare of woodlot in Ontario provide? Given annual rainfall of 890 mm, each hectare receives 0.89 m x 10,000 m2 = ~8,900 cubic metres (8.9 million litres) of water annually. The forest typically uses, through evapotranspiration, approximately 50% of annual rainfall. Therefore, about 4,450 cubic metres of water flows out of a forested hectare annually, almost as much as 50 people use in a year in Ontario. (We each use ~250 litres daily). Even if water sources are taken for granted, Ontario residents place a significant dollar value on water. In Toronto, drinking water costs residents $3.20 per cubic metre, and Ottawa charges $1.70 per cubic metre. So the municipal retail value of the water delivered by one hectare of forested land is in the range of $7,500 to $15,000 per year. Although further treatment costs may be required to meet drinking water standards, the raw water leaving a forest is remarkably clean and therefore relatively cheap to finish.
Large wetland in late spring.
Ontario woodlots provide water supply protection without the burden of infrastructure or routine recalibration or power costs, but compensation for this benefit is absent, taken for granted downstream. Yet woodlot owners are faced with both annual taxation and ever-increasing pressures to convert forestland to other purposes, some of which are lucrative but all of which will compromise the water supply protection provided by a forest and sacrifice the values placed on owning woodland. If the hydrological connection between a private woodland and a municipal drinking water supply can be established, perhaps a “water protection” rate for property taxes could be gained in exchange for a long-term commitment to maintaining forests as forests.
Thom is retired after 26 years as a Forester on the 100,000 acres (40,500
hectares) of actively managed oak and pine forests that protect Boston’s drinking water, a forest-filtered system that serves 2.2 million customers.
- DeCoster, L.A. and J. Herrington, 1988. Is a tree a heavy drinker or does it just pump water? American Tree Farmer. May-June pp. 17.
- Pulford, I.D. and C. Watson. 2003. Phytoremediation of heavy metal-contaminated land by trees – a review. Environment International 29:529-540.
- Bormann, F.H. and G. E. Likens. 1979. Pattern and process in a forested ecosystem. Springer-Verlag. New York, NY. 253 p.
- Vitousek, P.M. and W.A. Reiners. 1975. Ecosystem succession and nutrient retention: a hypothesis. Bioscience 25(6):376-381.
- Cooper-Ellis, S., D.R. Foster, G. Carlton, and A. Lezberg. 1999. Forest response to catastrophic wind: results from an experimental hurricane. Ecology 80(8): 2683-2696.