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

Chapter 4: Plant Succession and Disturbances in the Urban Forest Ecosystem 10
Figure 11.2 Photo by Lyn Topinka
Figure 11.4 Photo courtesy National Park Service
Figure 11. The eruption of Mt. Saint Helens is an example
of primary succession. It eliminated most traces of life in a
substantial area of the northeastern part of the mountain
(11.1). Less than a decade later, pioneer and early
successional plants have colonized the area (11.2).
Eventually, Douglas-fir forests will become established
(11.3) and, without further disturbance, over several
hundred years a Western hemlock forest may eventually
occupy the area (11.4).
where, after the disturbance, the substrate (i.e., soil)
remains relatively intact.
Figure 11.3 Photo courtesy of R. Emetaz, U.S.
Department of Agriculture
primary succession can be hastened through the
addition of top soil.
Secondary succession
Secondary succession occurs in an environment
that has supported mature vegetation in the past, and
Secondary succession also occurs in urban areas.
Suppose you decide to give up the fight with weeds
in your backyard and no longer mow your lawn. The
changes that take place will be typical of "old-field"
secondary succession. First, your backyard would be
colonized by a variety of plants, mostly annuals.
Within a few years, these plants would be joined by
perennials and smaller shrubs and the grass would
start to disappear. Later, a mix of taller shrubs and
tree species would seed in. Then, maybe 50 years
from now, you would have a successional forest in
your backyard.
Additional examples of secondary succession
include the changes in vegetation and ecosystem