Principles of terrestrial ecosystem ecology.pdf

288 13. Temporal Dynamics
successional sites. Disturbance pattern on the
landscape influences the effective size of a disturbance
event. Disturbances often leave
islands of undisturbed vegetation that act as
propagule sources, causing the effective size of
the disturbance to be much smaller than its area
would suggest (Turner et al. 1997). A series of
dams on a river creates a chain of interconnected
lakes that can be colonized much more
readily than isolated kettle lakes formed after
glacial retreat or farm ponds formed by restricting
groundwater flow.
The timing of disturbance often influences its
impact. A strong freeze or fire that occurs
during bud break has greater impact than one
that occurs 2 weeks before bud break. Similarly,
anaerobic conditions associated with flooding
of the Mississippi River during the 1993 growing
season caused more root and tree mortality
than if the flood had occurred when roots were
inactive. Hydroelectric dams may eliminate
seasonal flooding associated with rain or
snowmelt and alter flow based on electricity
demand. Human activities often change the
timing of disturbances such as grazing, fire, and
flooding.
Disturbance is one of the key interactive
controls that governs ecosystem processes (see
Chapter 1) through its effects on other interactive
controls (microclimate, soil resource
supply, functional types of organisms, and probability
of future disturbance). Postfire stands,
for example, often have warm soils that have a
high water content; this occurs due to the low
albedo of the charred surface and the decrease
in leaf area that transpires water and shades the
soil. Fire both volatilizes nitrogen, which is lost
from the site, and returns inorganic nitrogen
and other nutrients to the soil in ash, thus altering
soil resource supply. The net effect of fire
is usually to enhance nutrient availability,
although the magnitude of this effect depends
on fire severity and intensity (Wan et al. 2001).
Fire affects the functional types of plants in an
ecosystem through its effects on differential
survival and competitive balance in the postfire
environment. Because of its sensitivity to, and
effect on, other interactive controls, changes in
disturbance regime alter the structure and functioning
of the ecosystem.
Succession
Successional changes occurring over decades to
centuries explain much of the local variation
among ecosystems. Although climate, soils, and
topography explain most of the broad global
and regional patterns in ecosystem processes,
disturbance regime and postdisturbance succession
account for many of the local patterns
of spatial variability (see Chapter 14). In
this section, we describe common patterns
of successional change in major ecosystem
processes. These successional changes are most
clearly delineated in primary succession, so we
begin with a description of primary successional
processes and then describe how the patterns
differ between primary and secondary
succession.
Ecosystem Structure and Composition
Primary Succession
Succession involves a change from a community
governed by the dynamics of colonization
to one governed by competition for resources.
Vegetation development after disturbance is
strongly influenced by the initial colonization
events, which in turn depend on environment
and the availability of propagules (Egler 1954,
Connell and Slatyer 1977, Bazzaz 1996). Severe
disturbances such as glaciers, volcanic eruptions,
and mining eliminate most traces of previous
vegetation and must be colonized from
outside the disturbed site. Most initial colonizers
of these primary successional sites have
small seeds that can disperse long distances by
wind. Fresh lava or land exposed by retreat of
glaciers, for example, is first colonized by winddispersed
spores of algae, cyanobacteria, and
lichens, which form crusts that stabilize soils
(Worley 1973). These are followed by smallseeded
wind-dispersed vascular plants (primarily
woody species), whose arrival rates depend
largely on distance to seed source (Shiro and
del Moral 1995). Late successional species with
heavier seeds generally arrive more slowly (Fig.
13.6).
Many of the species that become abundant
early in primary succession are free living or