20 – Urban and Mixed Environs
Howard L. Ferguson
Geographic Distribution. Urban habitat occurs throughout Oregon
and Washington. Most urban development is located west of the Cascades of both
Oregon and Washington, with the exception of Spokane, Washington. However, urban
growth is being felt in almost every small town throughout the Pacific
Northwest.
Physical Setting. Urban development occurs in a
variety of sites in the Pacific Northwest. It creates a physical setting unique
to itself: temperatures are elevated and background lighting is increased; wind
velocities are altered by the urban landscape, often reduced except around the
tallest structures downtown, where high-velocity winds are funneled around the
skyscrapers. Urban development often occurs in areas with little or no slope and
frequently includes wetland habitats. Many of these wetlands have been filled in
and eliminated. Today, ironically, many artificial “wetland”
impoundments are being created for stormwater management, whose function is the
same as the original wetland that was destroyed. Landscape Setting.
Urban development occurs within or adjacent to nearly every habitat type in
Oregon and Washington, and often replaces habitats that are valuable for
wildlife. The highest urban densities normally occur in lower elevations along
natural or human-made transportation corridors, such as rivers, railroad lines,
coastlines, or interstate highways. These areas often contain good soils with
little or no slope and lush vegetation. Once level areas become crowded, growth
continues along rivers or shores of lakes or oceans, and eventually up elevated
sites with steep slopes or rocky outcrops. Because early settlers often modified
the original landscape for agricultural purposes, many of our urban areas are
surrounded by agricultural and grazing lands. Structure. The
original habitat is drastically altered in urban environments and is replaced by
buildings, impermeable surfaces, bridges, dams, and planting of non- native
species. Some human-made structures provide habitats similar to those of
cavities, caves, fissures, cliffs, and ledges. With the onset of urban
development, total crown cover and tree density are reduced to make way for the
construction of buildings and associated infrastructure. Many structural
features typical of the historical vegetation, such as snags, dead and downed
wood, and brush piles, are often completely removed from the landscape.
Understory vegetation may be completely absent, or if present, is diminutive and
single-layered. Typically, three zones are characteristic of urban habitat.
High-density Zone The high-density zone is the downtown
area of the inner city. It also encompasses the heavy industrial and large
commercial interests of the city in addition to high-density housing areas such
as apartment buildings or high-rise condominiums. This zone has >60% of its
total surface area covered by impervious surfaces. This zone has the smallest
lot size, the tallest buildings, the least amount of total tree canopy cover,
the lowest tree density, the highest percentage of exotics, the poorest
understory and subcanopy, and the poorest vegetative structure.4a, 116a,
185a
Human structures have replaced almost all
vegetation.23b, 148a Road density is the highest of
all zones. An example of road density can be seen from Washington’s Growth
Management Plan requiring Master Comprehensive Plans to set aside 20% of the
identified urban growth area for roads and road rights-of-way. For example,
Spokane’s urban growth area is approximately 57,000 acres (23,077 ha);
therefore >11,000 acres (4,453 ha) were set aside for road surfaces.
In the high-density zone, land-use practices have removed most of the native
vegetation. Patch sizes of remaining natural areas often are so small that
native interior species cannot be supported. Not only are remaining patches of
native vegetation typically disconnected, but also they are frequently missing
the full complement of vertical strata.149 Stream
corridors become heavily impacted and discontinuous. Most, if not all, wetlands
have been filled or removed. Large buildings dominate the landscape and
determine the placement of vegetation in this zone.30a
This zone has the most street tree strips or sidewalk trees, most of which are
exotics. There is virtually no natural tree replacement, and new trees are
planted only when old ones die or are removed. Replacement trees are chosen for
their small root systems and are generally short in stature with small
diameters. Ground cover in this zone, if not synthetic or impervious, is
typically exotic grasses or exotic annuals, most of which are rarely allowed to
go to seed. Snags, woody debris, rock piles, and any other natural structures
are essentially nonexistent. There are few tree cavities because of cosmetic
pruning, cavity filling, snag removal, and tree thinning.149
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Medium-density Zone This zone, continuing out
from the center of the continuum, is composed of light industry mixed with
high-density residential areas. Housing density of 3-6 single-family homes per
acre (7-15 per ha) is typical. Compared with the high-density zone, this zone
has more potential wildlife habitat. With 30-59% impervious soil cover, this
zone has 41-70% of the ground available for plants. Road density is less than
the high-density zone.
Vegetation in this mid-zone is typically composed of non-native plant
species. Native plants, when present, represent only a limited range of the
natural diversity for the area. The shrub layer is typically clipped or minimal,
even in heavily vegetated areas. Characteristic of this zone are manicured
lawns, trimmed hedges, and topped trees. Lawns can be highly
productive.82a, 97a Tree canopy is still discontinuous
and consists of 1-2 levels, if present at all. Consequently, vertical vegetative
diversity and total amount of understory are still low. Coarse and fine woody
debris is minimal or absent; most snags and diseased live trees are still
removed as hazards in this zone.119a, 119b Isolated
wetlands, stream corridors, open spaces, and greenbelts are more frequently
retained in this zone than in the high-density zone. However, remnant wetland
and upland areas are often widely separated by urban development.
Low-density Zone
The low-density zone is the outer zone of the urban-rural continuum.
This zone contains only 10-29% impervious ground cover and normally
contains only single-family homes. It has more natural ground cover
than artificial surfaces. Vegetation is denser and more abundant
than in the previous two zones. Typical housing densities are 0.4-1.6
single-family homes per acre (1-4 per ha). Road density is lowest
of all three zones and consists of many secondary and tertiary roads.
Although this zone may have large areas of native vegetation and
is generally the least impacted of all three zones; it still has
been significantly altered by human activities and associated disturbances.
Roads, fences, livestock
paddocks, and pets are more abundant than in neighboring rural areas. With many
animals and limited acreage, pasture conditions may be more overgrazed in this
zone than in the rural zone; overgrazing can significantly affect shrub layers
as well. Areas around home sites are often cleared for fire protection. Dogs are
more likely to be loose and allowed to run free, increasing disturbance levels
and wildlife harassment in this zone. Vegetable and flower gardens are
widespread; fencing is prevalent. Many wetlands remain and are less
impacted. Water levels are more stable and peak flows are more typical of
historical flows. Watertables are less impacted and vernal wetlands are more
frequent; stream corridors are less impacted and more continuous. This
zone has the most vertical and horizontal structure and diversity of any of the
three urban zones.30a, 80a, 140a, 187a In forested areas,
tree conditions are semi-natural, although stand characteristics vary from
parcel to parcel. The tree canopy is more continuous and may include multiple
levels. Patch sizes are large enough to support native interior species. Large
blocks of native vegetation may still be found, and some of these may be
connected to large areas of native undeveloped
land.220a In this zone, snags, diseased trees, coarse
and fine woody debris, brush piles, and rock piles are widespread. Structural
diversity approaches historical levels. Non-native hedges are nearly nonexistent
and the native shrub layer, except for small areas around houses, is relatively
intact. Lawns are fewer and native ground covers are more common than in the
previous two zones.
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Composition. Remnant isolated blocks of
native vegetation may be found scattered throughout a town or city mixed with a
multitude of introduced exotic vegetation. As urban development increases, these
remnant native stands become fragmented and isolated. The dominant species in an
urban setting may be exotic or native; for example, in Seattle, the dominant
species in one area may be Douglas-fir (Pseudotsuga menziesii), whereas a
few blocks away it may be the exotic silver maple (Acer saccharinum).
Dominant species will not only vary from city to city but also within each city
and within each of the three urban zones. Nowack167
found that in the high-density urban zone, species richness is low, and in one
case, four species made up almost 50% of the cover. In the same study, exotics
made up 69% of the total species. In urban and suburban areas, species
richness is often increased because of the introduction of exotics. The
juxtaposition of exotics interspersed with native vegetation produces a diverse
mosaic with areas of extensive edge. Also, because of irrigation and the
addition of fertilizers, the biomass in the urban communities is often
increased.149 Interest in the use of native plants for
landscaping is rapidly expanding,135, 172 particularly
in the more arid sites where drought-resistant natives are the only plants able
to survive without water. Across the U.S., urban tree cover ranges from
1 to 55%.167 As expected, tree cover tends to be
highest in cities developed in naturally forested areas with an average of 32%
cover in forested areas, 28% in grasslands, and 10% in arid areas. Yakima,
Washington, has an overall city tree cover of 18%, ranging from 10% to 12% in
the industrial/ commercial area to 23% in the low-density residential
zone.167 Remnant blocks of native vegetation or native
trees left standing in yards and parks will compositionally be related to
whatever native habitat was on site prior to development. In the Puget Sound and
Willamette Valley areas, Douglas-fir is a major constituent, whereas the Spokane
area has a lot of ponderosa pina (Pinus ponderosa). Other
Classifications and Key References. Many attempts have been made to
classify or describe the complex urban environment. The Washington GAP
Analysis37 classified urban environments as
“developed” land cover using the same three zones as described
above:
(1) high density (>60% impervious surface); (2) medium density (30-60%
impervious surface); and (3) low density (10-30% impervious surface). The Oregon
Gap II Project126 and Oregon Vegetation
Landscape-Level Cover Types127 represented this type
as an urban class. Several other relevant strudies characterizing the urban
environment have been reported. 182, 129, 34, 70, 151
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Natural Disturbance Regime. In many instances, natural
disturbances are modified or prevented from occurring by humans
over the landscape and this is particularly true of urban areas.
However, disturbances such as ice, wind, or firestorms still occur.
The severity of these intermittent disturbances varies greatly in
magnitude and their impact on the landscape varies accordingly.
One of the differences between urban and nonurban landscapes is
the lengthening of the disturbance cycles. Another is found in the
aftermath of these disturbances. In urban areas, damaged trees are
often entirely removed and if they are replaced, a shorter, smaller
tree, often non-native, is selected. The natural fire disturbance
interval is highly modified in the urban environment. Fire (mostly
accidental or arson) still occurs, and is quickly suppressed. Another
natural disturbance in many of our Pacific Northwest towns is flooding,
which historically altered and rerouted many of our rivers and streams,
and still scarifies fields and deposits soil on flood plains and
potentially recharges local aquifers. Floods now are more frequent
and more violent than in the past because of the many modifications
made to our watersheds. Attempts to lessen flooding in urban areas
often lead to channelization, paving, or diking of our waterways,
most of which fail in their attempt to stem the flooding and usually
result in increased flooding for the communities farther downstream.
Succession and Stand Dynamics.
Due to anthropogenic influences found in the urban environment, succession
differs in the urban area from that expected for a native stand.
Rowntree185 emphasized that urbanization is not in the
same category as natural disturbance in affecting succession. He points out that
urbanization is anthropogenic and acts to remove complete vegetation
associations and creates new ones made of mixes of native residual vegetation
and introduced vegetation. Much human effort in the city goes toward either
completely removing native vegetation or sustaining or maintaining a specific
vegetative type, e.g., lawns or hedges. Much of the vegetative community remains
static. Understory and ground covers are constantly pruned or removed, seedlings
are pulled and lawns are planted, fertilized, mowed, and meticulously
maintained. Trees may be protected to maturity or even senescence, yet
communities are so fragmented or modified that a genuine old-growth community
never exists. However, a type of “urban succession” occurs across the
urban landscape. The older neighborhoods with their mature stands are at a later
seral stage than new developments; species diversity is characteristically
higher in older neighborhoods as well. An oddity of the urban environment is the
absence of typical structure generally found within the various seral stages.
For example, the understory is often removed in a typical mid-seral stand to
give it a “park-like” look. Or if the understory is allowed to remain,
it is kept pruned to a consistent height. Lawns are the ever-present substitute
for native ground covers. Multilayered habitat is often reduced to one or two
heights. Vertical and horizontal structural diversity is drastically reduced.
Effects of Management and Anthropogenic Impacts. These
additional, often irreversible, impacts include more impervious surfaces, more
and larger human-made structures, large-scale storm and wastewater management,
large-scale sewage treatment, water and air pollution, toxic chemicals, toxic
chemical use on urban lawns and gardens, removal of species considered to be
pests, predation and disturbance by pets and feral cats and dogs, and the
extensive and continual removal of habitat due to expanding urbanization, and in
some cases, uncontrolled development. Another significant impact is the
introduction and cultivation of exotics in urban areas. Native vegetation is
often completely replaced by exotics, leaving little trace of the native
vegetative cover. Status and Trends. From 1970 to 1990,
>30,000 mile2 (77,700
km2) of rural lands in the U.S. became urban, as
classified by the U.S. Census Bureau. That amount of land equals about one third
of Oregon’s total land area.12 From 1940 to 1970,
the population of the Portland urban region doubled and the amount of land
occupied by that population quadrupled.201 More than
300 new residents arrive in Washington each day, and each day, Washington loses
100 acres (41 ha) of forest to development.215 Using
satellite photos and GIS software, American Forests9
discovered that nearly one third of Puget Sound’s most
heavily timbered land has disappeared since the early 1970s. The amount of land
with few or no trees more than doubled, from 25% to 57%, an increase of >1
million acres (404,858 ha). Development and associated urban growth was blamed
as the single biggest factor affecting the area’s environment. This urban
growth is predicted to continue to increase at an accelerated pace, at the
expense of native habitat.
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