 
VOLUME 1, TOPIC 3
1 Corresponding author
2 Danish Forest and Landscape Research Institute, Hørsholm Kongevej 11, 2970 Hørsholm, Denmark
3. Significance of green areas to biodiversity and the environment
At the U.N. Conference on the Environment and Development in Rio de Janeiro, 1992,
all participating countries adopted Agenda 21, an action plan for how the world can work
towards sustainable development. It also obliges them to work towards sustainable development an
obligation that, in turn, devolves on the administrators of urban green areas.
Older gardens and parks, not to mention the churchyards, often contain noticeably rich
biodiversity. These are the main habitats of urban plants and animals. Older, wellestablished installations
attract, for instance, birds and mammals, the natural habitat of which is the forest. Since an
increasing part of the population lives in urban areas and receives its daily perception of nature
therein, nature in urban areas is important to environmental awareness and an understanding of nature.
Nature as created by man is often considered to be inferior to nature that evolves without
human intervention. In support of this, it is often asserted, e.g. that the number of species is often greater
in untouched nature. An English researcher, who is the owner of a garden of about 700 m in a
suburb of Leicester, in the Midlands, is of the opinion that this is not invariably the case. For 15 years,
she has collected and determined the species of all of the insects she has found in her garden. She
has, for instance, received visits from 34% of all indigenous species of butterfly, 30% of all
indigenous species of moth and 36% of all indigenous species of syrphus [Owen, 1992]. Although her garden
is quite ordinary, it has unusually variegated flower beds and a good mixture of cultivated and
uncultivated plants. Many urban green areas do not, however, boast particularly rich biodiversity. Most
of them were established with large paved areas, gravelled areas, wellmown greens and
isolated, individual trees.
Trees intercept particulate matter and absorb such as gaseous pollutants as ozone, sulphur
dioxide and nitrogen dioxide, thus removing them from the atmosphere. Trees also emit various
volatile organic compounds, such as isoprene and monoterpenes, that can contribute to ozone formation
in cities. By transpiring water and shading surfaces, trees lower local air temperatures [Nowak,
1995]. Because trees lower air temperatures, shade buildings in the summer, and block winter winds,
they can reduce the consumption of building energy and consequently reduce the emission of
pollutants from powergenerating facilities [McPehrson & Rowntree, 1993; Nowak, 1995].
While the role of urban forests in terms of carbon sequestration is limited in relation to forests
in rural areas, urban trees do play a role in control of global climate change through indirectly
decreasing the use of fossil fuels for building heating and cooling. By lowering air temperatures and shad
ing buildings, they can reduce the use of energy for air conditioning in the summer, and
through blocking winter winds, they can reduce the consumption of energy for heating [McPehrson
& Rowntree.
Even though plants absorb carbon dioxide and produce oxygen, it is still important not to
assign plants excessive significance to the urban environment. Harris [1992] reminds us that plants
really have only a minor effect on the carbondioxide and oxygen content of urban air. Photosynthesis
in the oceans accounts for between 70 and 90 per
cent of the world's total oxygen production,
for which reason it is absolutely vital that they be protected against pollution. However, even a
minor reduction in the oxygen content of the air will cause a large percentage increase in its
carbondioxide content, which would reinforce the greenhouse effect, thus leading to an rise in the global
temperature.
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