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

Chapter 4: Plant Succession and Disturbances in the Urban Forest Ecosystem 18
natural removal of Australian pines allowed
managers to reestablish the ecosystems that existed
before by planting native species typical of that area.
For more information on invasive species see
Chapter 9 - Invasive Plants.
burning or mechanical techniques will require careful
attention to public education.
Re-introducing natural disturbances
When re-introducing disturbances, ecosystem
characteristics and site conditions should be carefully
considered. In the Southern U.S., for example,
upland ecosystems are adapted to frequent (every 1
to 15 years) low intensity fires. In the case where fire
has been absent for long periods of time, thinning of
trees and/or manual removal of excessive fuel loads
may be necessary prior to application of prescribed
fire. Such management practice would prevent
damage (and other associated risks) by a high
intensity fire to which this ecosystem is not
adapted.
On the other hand, where high intensity
disturbances have been excluded for excessively
long periods, other strategies may need to be
pursued. For instance, the sand pine scrub
ecosystems, also in the Southern U.S., are adapted to
infrequent (every 15 to 100 years) high intensity
fires. Historically, after a lengthy fire-free period, an
intense fire occurs. If fires become too frequent, sand
pines disappear, and the association becomes oak
shrub or changes to other pines. If fires become too
infrequent, a xeric hardwood forest develops. Most
scrubs naturally depend on fires, but these fires need
to be applied in such a way that various stages of
development are maintained within isolated
fragments. Without these fragments, species with
special habitat requirements (such as the endemic
Florida mouse, Podomys floridanus, the Florida
scrub lizard, Scelopors woodi, the gopher tortoise
(Gopherus polyphemus) and the sand skunk, Neoseps
reynoldsi) might be eliminated (Figure 23).
Although preliminary steps have been taken to
develop techniques to burn the scrub, reintroduction
of fires in scrub ecosystems within urban areas may
not be feasible (due to liability, fire control
considerations and public reaction). In such areas,
patches of the scrub ecosystem could be maintained
by cutting, scraping and chopping to simulate fires
(Meyers and Ewel, 1990). Implementation of either
Figure 23.1 Photo by Anne Birch
Figure 23.2 Photo by Dave Rich
Figure 23. In the scrub ecosystem of the southern U.S.,
the correct frequency and intensity of fire is critical. If fires
become infrequent and too intense, a sand pine
ecosystem develops, excluding the endangered scrub
lizard (Sceloporus woodi) (23.1) and gopher tortoise
(Gopherus polyphemus) (23.2).
In other ecosystems, small or large gaps may
need to be cut to stimulate further succession. Such a
practice is becoming common for restoration of
longleaf pine ecosystems in the Southeastern U.S.,
where dense hardwood thickets now dominate many
sites. Gaps are cut and regenerated (Figure 24), and
prescribed fire is used to keep hardwoods from
re-invading.
Re-instating several different stages of
succession in one area can only be achieved on very
large land areas. Small sites may prove not to be
functional, although a small mosaic of semi-natural
successional stages may, nevertheless, be effective in