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Urban Vegetation
Urban vegetation includes all types of plant life found in
city environments, from preexisting native species to
those introduced to improve landscapes. It is found in
urban forests and parks, along roadsides, around ponds
and streams, and even in vacant lots. Properly managed
urban vegetation can help reduce air pollution, noise,
and dust, and add oxygen and visual appeal to otherwise bland cityscapes.
major component of seminatural vegetation in cities is the
anthropochory community (comprising companion
plants), which relies closely on anthropogenic (human)
interference under urban habitats and plays a special role
in composing urban vegetation, mainly grasses (Jiang
1989). Introduced plants can be categorized into roadside
trees, urban forests, parks, gardens, street greenbelts, and
so on.
U
Function
rban vegetation refers to all types of plants that
grow in urban environments, such as forests, parks,
roadsides, and wasteland areas (Jiang 1993). As a significant part of urban ecosystems, urban vegetation can not
only help clean and freshen air quality by reducing dust
and environment pollution, but it can also help maintain
the ecological balance of urban environments. Urban
vegetation also plays an important role in indicating and
monitoring environmental pollution.
Categories
Researchers have different ways of categorizing urban
vegetation. Some classify it as urban forest, parks and
green spaces, gardens and lawns, wall or roof plants, and
wetlands (Guntenspergen 1998); others identify roadside
trees, greenbelts in streets, green areas in parks, grasslands, and aquatic green spaces (Huang et al. 1990). More
simply, some have divided urban vegetation into three
types: relict (or remnant) natural communities retained as
they were before urbanization, weed communities occupying new urban habitats, and artificial green spaces
(Ohsawa and Da 1988). Another way of looking at urban
vegetation is according to its three main types: natural
plants, seminatural plants, and introduced plants. Natural
plants are those that existed before city construction. The
Urban vegetation has multiple functions. The main role,
however, is to help maintain the urban environment,
which is easily affected by all kinds of pollutants, thereby
improving human living conditions. For instance, urban
vegetation can adjust microclimatic conditions, clean up
air pollutants, reduce dust, dampen noise, and maintain
ecological balance. Urban vegetation can also serve aesthetic and educational purposes. Generally, the function of
urban vegetation is closely related to the type, for example,
forests, grasses, and/or wetlands. Urban forests improve
urban environments more dramatically than other types.
For instance, the temperature in urban forests can be
roughly 6°C–16°C lower than urban open space on sunny
summer days. In a case study in the city of Beijing, the air
passing through a fruit-bearing forest 80–100 meters wide
reduced the concentration of hydrogen fluoride in the atmosphere 22 percent compared to open space of the same
width (Wang 1998). A forest belt 40 meters wide also
reduced noise about 10–15 decibels (Wang 1998).
Development
Along with global urbanization, the distribution of urban
vegetation is being further specialized. According to the
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URBAN VEGETATION
United Nations Educational, Scientific and Cultural
Organization (UNESCO), 60 percent of the world’s
population will live in cities by 2030 (Wibly and Perry
2006). On one hand, people tend to change urban habitats into areas of development, which exposes them to
human disturbance. Adverse effects such as urban pollution become more and more serious, thus putting the
urban vegetation into a state of strong instability. A case
study conducted in Chiba City, Japan, showed that from
1952 to 1981 forest coverage declined from 51 percent to
8 percent. Meanwhile, land-use patterns had changed
dramatically: farmlands and forests had been turned into
residential areas and once-natural hills had been covered
with buildings (Ohsawa and Da 1998).
Exotic Species
On the other hand, various types of urban plant communities appear in cities, such as anthropochory
communities and introduced plant communities. Besides, human beings bring
numerous exotic species to cities, while
they destroy and discard a huge variety of native species. Whether
those influences or interferences
are conscious or unconscious,
direct or indirect, they ultimately
alter the natural features of urban
vegetation, its composition, structure, and function. As a result, much
urban vegetation has completely lost
its natural traits (Huang et al.
1990). For instance, urban vegetation depends largely on fertilization, pesticides, and irrigation to
live, similar characteristics to crops
in agricultural systems.
People like to bring exotic
plants to cities for various purposes,
but they often pay no attention to
local dominant species. Such activities
can destroy native urban vegetation. Although
relict communities can reflect the distribution of zonal
vegetation, the dominant species will gradually disappear
and be replaced by those that adapt to urban habitats. As a
result, the dominant species in urban plant communities
are often not obvious (Jiang 1993). People often neglect
comprehensive assessments before widely adopting exotic
plants, leading to the uncontrolled spread of invasive species. Such a phenomenon is more likely to occur during the
process of introducing herbaceous plants. Eichhornia
crassipes, or common water hyacinth (native to the Amazon
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• 399
basin), and Eupatorium adenophorum, a flowering shrub
native to Mexico (also called snakeroot), have brought
great harm to both urban and rural environments in China
(Bao 2008). In the beginning, people may bring in the
invasive species for economic purposes (e.g., Eichhornia
crassipes was supplied as feed for pigs in the 1950s) or simply to beautify the urban environment (e.g., the ornamental tree Rhus typhina), but they later realize the harmful
effects when those species become dominant in new spaces
such as urban landscapes.
Proposals
Since numerous plant species are immigrants to urban
habitats, urbanites have become increasingly disconnected from indigenous species and natural ecosystems
(McKinney 2006). To counter this, ecological principles
should be abided in the selection of urban plants.
Increasing awareness of the niche
that each species occupies would
provide indigenous species a
better chance for survival.
Choosing and making good use
of native species for the greening of urban habitats should be
fully considered in the future.
There are still many deficiencies in urban greening,
such as a decrease of native
species, scarcity of plant diversity, and a lack of ecological
background features (Bao
2008). A comprehensive investigation of native species and a
study of the genetic diversity
of dominant species should be
conducted. In addition, selection of indigenous species, especially trees, should be emphasized
because of their large biomass and
ability to provide habitats for birds
and other urban creatures. Introduction
of exotic species should be appropriately considered.
Furthermore, city designers need to conduct environment impact assessments to avoid the malignant spread
of invasive species before introducing exotic species into
urban settings.
JIANG Gaoming and BO Wenjing
Chinese Academy of Sciences
See also Best Management Practices (BMP); Brownfield
Redevelopment; Disturbance; Ecological Restoration;
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400 • THE BERKSHIRE ENCYCLOPEDIA OF SUSTAINABILITY: ECOSYSTEM MANAGEMENT AND SUSTAINABILITY
Ecosystem Services; Invasive Species; Landscape
Architecture; Light Pollution and Biological Systems;
Nitrogen Saturation; Permaculture; Pollution, Nonpoint
Source; Pollution, Point Source; Rain Gardens; Road
Ecology; Tree Planting; Urban Agriculture; Urban
Forestry
FURTHER READING
Bao, Mingzhen. (2008). On urban bio-diversity and landscape plants
planning in China. Chinese Landscape Architecture, 7, 1–3 [in
Chinese].
China Environmental Protection Network. (2009). China’s 283 invasive species cause 200 billion CYN economic losses [in Chinese].
Retrieved August 6, 2011, from, http://w w w.sei.gov.cn/
ShowArticle2008.asp?ArticleID=179267
Guntenspergen, Glenn R. (1998). Introduction: Long-term ecological
sustainability of wetlands in urban landscape. In Thomas R.
Detwyler & Melvin G. Marcus (Eds.), Urbanization and environment: The physical geography of the city (pp. 229–241). Belmont, CA:
Duxbury Press.
www.berkshirepublishing.com
Huang Xiaoyang; Lin, S. H.; Han, R. Z.; & Yao, Yuqi. (1990). Urban
vegetation in Beijing and its function. In Langzhou Chen & H. Y.
Zheng (Eds.), Ecological, social and economical designing for BeijingTianjin region (pp. 42–60). Beijing: Ocean Press [in Chinese].
Jiang Gaoming. (1989). Anthropochory in cities. Chinese Bulletin of
Botany, 6, 116–120 [in Chinese with English summary].
Jiang Gaoming. (1993). Urban vegetation: Its characteristic, type and
function. Chinese Bulletin of Botany, 10, 21–27 [in Chinese with
English summary].
McKinney, Michael L. (2006). Urbanization as a major cause of biotic
homogenization. Biological Conservation, 127, 247–260.
Ohsawa, Masahiko, & Da, Liang-Jun. (Eds.). (1988). Integrated studies
in urban ecosystems as the basis of urban planning (III). Chiba, Japan:
Chiba University.
Stanvliet, R.; Jackson, J.; Davis, G.; De Swardt, C.; Mokhoele,
J.; Thom, Q.; & Lane, B. D. (2004). The UNESCO biosphere
reserve concept as a tool for urban sustainability: The CUBES Cape
Town case study. Annals of the NY Academy of Science, 1023, 80–104.
Wibly, Robert L., & Perry, George L. W. (2006). Climate change,
biodiversity and the urban environment: A critical review based on
London, U.K. Progress in Physical Geography, 30, 73–98.
Wang, Bosco Shang. (1998). Urban vegetation and its construction
technology. Acta Scientiarum Naturalium Universitatis Sunyatseni,
37, 9–12 [in Chinese with English summary].
© 2012 Berkshire Publishing Group, all rights reserved.