Chapter 4: Plant Succession and Disturbances in the Urban Forest ...

Chapter 4: Plant Succession and Disturbances in the Urban Forest Ecosystem 20
example, contains prairies, savannas, woodlands and
forests. The absence of fire has impacted these
ecosystems and their stages of succession in the
landscape. Oak savannas have been almost totally
excluded in the Chicago area and prairies have been
invaded by woody species. Historically, the
frequency and intensity of fire determined the
successional stage of these ecosystems, that is,
whether a given piece of land would be an open
grove or a dense forest (Figure 26). Restoration
efforts in this case are based on re-introducing fires.
To date, fire has been reintroduced in several areas
and native species typical of the region's ecosystems
are being planted. In some areas, native trees have
been cut to allow more light to reach the ground
(Figure 27). Such practices allow the landscape to
support several stages of succession, ranging from
open prairies to forests.
Figure 27.1 Photo courtesy of Chicago Wilderness
Figure 27.2 Photo courtesy of Chicago Wilderness
Figure 26. Historically, the frequency and intensity of fire
determined the successional stage of ecosystems
(whether a given piece of land would be an open grove or
a dense forest) in the Chicago area. Photo courtesy of
Chicago Wilderness
Some continuous or intermittent form of
management may be needed to create disturbances in
situations where human activity has severely
modified natural disturbances cycles. Efforts to
restore historical flooding cycles in the South Platte
River watershed illustrate the need for an integrated
restoration plan for a whole region. The floodplains
along the South Platte river in Nebraska consist of a
mosaic of different vegetation types. The presence of
wooded or open vegetation was historically
determined by natural periodic floods. Forests were
confined to drier sites, since native woody species,
such as willows (Salix spp.) and cottonwoods
(Populus spp.), would not survive flooding. Grasses,
Figure 27. Due to suppression of fires, the once open
savannas in the Chicago area (27.1) developed into
thickets of vegetation deprived of sunlight (27.2). Oak
savannas began losing their vast diversity of plants and
animals and were almost excluded from the landscape.
on the other hand, could tolerate flooding, allowing
for open areas along the river.
Channelization and upstream development
reduced the water flow and, consequently altered
flooding periods. As a result, previously open areas
of the floodplain are nowdrier and invaded with
adjacent native forest species. Before channelization
and development, migratory birds, such as the
endangered whooping crane (Grus americana) and
the sandhill crane (Grus canadensis) (Figure 28),
used the open grassy floodplains for feeding and
avoided roosting in areas with abundant woody
species. Because of these changes in natural