SERVICES PROVIDED BY WETLANDS
Areas inundated by fresh, brackish, and salt water are all considered wetlands; among
many wetland types are fens and bogs, tidal marshes, riparian zones, and lakeshores.
Wetlands, which cover less than 9% of the Earth’s surface, can be extremely productive
and many are disproportio nately large providers of ecosystem services. Three of the key
services that wetlands provide are flood mitigation, water purification, and biodiversity
support. In the upper part of a watershed, many wetlands store water that flows overland
toward rivers and streams.
They can release this water into the main channel slowly, reducing and delaying flood
peaks. Downstream, wet lands can absorb and reduce peak flood levels, providing area
into which flood waters can spread, dissipating flood energy by slowing water movement,
and removing flood water through transpiration and infiltration. The same physical
characteristics of wetlands that slow and absorb overland flow related to flooding can also
provide a mechanism for storing and detoxifying urban and agricultural wastewater
before it discharges directly into a main channel.
Wetlands filter out various nutrients, other pollutants, and sediment: they support
anaerobic bacteria that denitrify waste; the plants take up and store nutrients; and by
slowing and redirecting water flow, wet lands enhance sedimentation – the accreting
sediments can effectively bury pollutants. While many wetlands can purify water very
economically, their effectiveness depends on many factors, including rate of inflow,
amount of sediment and organics in the wastewater, residence time of wastewater in the
wetland, and total surface area. A wide variety of animals rely on wetlands for survival.
Plant species that deliver flood abatement and water purification can also support
biodiversity, providing varied food and shelter.
A riparian wetland, for example, might provide food plants and underground burrows for
muskrats; seeds, food plants, and nest building materials for ducks; and food and shelter
for fish and invertebrates. Wetlands provide a variety of other services as well. Major
products associated with wetlands are peat, timber, and mulch. Regulating services in
addition to flood mitigation and water purification include waste detoxifi cation, carbon
storage, and control of pests and diseases. Wetlands provide many cultural services as
well, particularly recreation services such as bird watching, boating, and hunting.
Wetlands also provide key support ing services, such as soil formation and buffering
freshwater aquifers from saltwater intrusion.
Services Provided by Forests
Forests provide a wide array of services, such as timber production, climate stabilization,
provision of water quan tity and quality, and cultural benefits, such as recreation. Some
management options increase the supply of several services, but often one service is
enhanced to the detri ment of others. Forests are often managed for provisioning
services, particularly for timber. But even within the category of provisioning services,
management options differ. If a forest is considered exclusively a supplier of timber,
managers will encourage the growth of only certain kinds of trees, possibly nonnative fast
growing trees, and will cultivate them so that they grow in a uniform way, typically straight
and tall. When the trees are deemed mature, they will be cut down, often all at once. By
contrast, if a forest is regarded as a supplier of diverse benefits, it may be managed to
nurture a wide array of valued species that would not be available in the mono crop forest
described above.
Forests also have both short term and medium term impacts on climate. Temperature
regulation happens in forests when the canopy shades the ground and when dark colored
foliage absorbs heat. Forests can in certain circumstances also influence precipitation – in
cloud for ests, for example, trees and epiphytes intercept and condense water directly
from the air, and that water runs down trunks to plants and soil below. On a longer
timescale, forests play a role in carbon cycling and seques tration; when forest plants,
bacteria, and algae respire, they take CO 2 out of the atmosphere. Plants, soils, and the
animals that eat them in forests, grasslands, and other terrestrial ecosystems store 2000
billion tons of carbon worldwide, about half the amount of carbon stored in the ocean
and nearly three times that stored in the atmo sphere. However, if these ecosystems are
burned or destroyed, as happens when timber is harvested, the car bon they are
sequestering is released to the atmosphere.
Although most organic compounds do return to the atmo sphere as CO 2 when living
organisms die and decompose, in a functioning forest ecosystem some is buried and
sequestered. About 25% of the human caused increase in CO 2 concentration in the
atmosphere during the past 20 years resulted from land use change, primarily
deforestation. Forests in a watershed, on the hillslopes that drain into a river, influence
the water quality in that river. In part this is because higher intensity uses, such as
agriculture input pollutants like nutrients and pesticides into a system while forests do
not.
Forests themselves also reduce sediment and nutrient runoff. Clearing trees can have an
impact as soon as the next rainy season on sediment and nutrient loads in streams, as
demonstrated in the classic Hubbard Brook experiment. In some cases, water users have
invested in forests to keep their water supplies clean. New York City recently invested US$
250 million to acquire and protect land in the Catskills watershed that supplies water to
the city. By working with landowners to reduce pesticide and fertilizer application and to
plant buffer strips along water ways, New York City reduced potential contamination of
its drinking water.
In conjunction with related conservation investments amounting to US$ 1.5 billion, the
city thereby obviated the need to build a filtration plant pro jected to cost between US$
6 and US$ 8 billion. Forests can also play an important role regulating the timing and
quantity of runoff. The economic value of forests in the watershed of the Yangtze River
above Three Gorges Dam, in western Hubei Province, Central China, was quantified in a
study published in 2000. Here, the Gexhouba Hydroelectric Power Plant, the largest hydro
facility in China, producing 15.7 billion kW annually, requires a narrow range of flows on
the Yangtze in order to run at full power. If the water level is too high, then water must
be released through the sluice gates, causing the water level below the dam to rise,
reducing the amount of power that can be produced; at very high flows, turbines are
drowned and cannot work at all. If the water is too low, then generators cannot run at full
power. The goal of the hydroelectric facility’s managers is for the river to have flow depths
that vary as little as possible, as this has been shown to be much more important for
power generation than the total flow. Upstream forests damp fluctuations in stream flow
by reducing runoff in wet periods through canopy interception, leaf litter absorption, and
soil and groundwater storage; increased infiltration provides base flow in dry periods
through groundwater discharge.
Though water flow regulation is a function of vegetation, soil type, and slope, which occur
in a heterogeneous mix through the watershed, forests and even shrubs with all types of
soils and slopes consis tently provided better water regulation than grasses orchards, and
crop agricultural fields. This study esti mated the value of electricity produced by the
hydro facility due to water regulation by the forest at over US$ 600 000 per year (in the
early 2000s), or about 2.2 times the income derived from forest product services in this
area. Because trees lose water to the atmosphere through transpiration, however, the
total water available downstream was decreased by the forest. Different management
regimes will yield different suites of services.
Some services can never be coproduced; other services will almost always be produced in
tandem, though often to differing degrees. For the hypothetical forest illustrated in Figure
2, cattle and timber cannot be produced on the same parcel of land – conversion to
pasture optimizes livestock but reduces timber output dra matically. Under timber
maximization, once trees are harvested they are not available for climate or hydrologic
regulation, though before harvest those services will be produced, as well as some habitat
and hiking trails. Carbon sequestration, hydropower, recreation, and preser vation of
biodiversity tend to be coproduced, but there are tradeoffs in their optimal supply.
Maximizing biodiversity, for example, produces all four to their fullest extent but allows
for no timber supply. Bringing selective logging back into the management regime reduces
supply of the other services somewhat; maximizing timber yield reduces them much more
dramatically.
