Principles of terrestrial ecosystem ecology.pdf

292 13. Temporal Dynamics
that reduces the availability of resources to
other individuals. Both competitive and facilitative
interactions are widespread in plant
communities (Callaway 1995, Bazzaz 1996);
their relative importance in causing changes
in species composition during succession probably
depends on environmental severity
(Connell and Slatyer 1977, Callaway 1995).
Herbivores and pathogens account for much
of the mortality of early successional plants.
Selective browsing by mammals is particularly
important in eliminating early successional
species as succession proceeds (Bryant and
Chapin 1986, Paine 2000). In intertidal communities,
grazing by fish and invertebrates such as
limpets exerts a similar effect.
In general, life history traits generally determine
the pattern of species change through
succession, whereas facilitation, competition,
and herbivory determine the rate at which this
occurs (Chapin et al. 1994). These processes
interact with other less predictable events, such
as storms or droughts, to cause the diversity
of successional changes that occur in natural
ecosystems (Pickett et al. 1987, Walker 1999)
(Fig. 13.7).
Secondary succession can begin with soils
that have either high or low nutrient availability.
When initial nutrient availability is high,
early successional species typically have high
relative growth rates, supported by high rates
of photosynthesis and nutrient uptake. These
species reproduce at an early age and allocate
a large proportion of NPP to reproduction
(Table 13.1). Their strategy is to grow quickly
under conditions of high resource supply, then
disperse to new disturbed sites.These early successional
species include many weeds that colonize
sites disturbed by people. As succession
proceeds, there is a gradual shift in dominance
to species that have lower resource requirements
and grow more slowly. In ecosystems
with low initial availability of soil resources,
succession proceeds more slowly and follows
patterns similar to those described for primary
succession. Because there is a continuum in disturbance
characteristics between primary and
secondary succession, the patterns of establishment
and succession differ among ecosystem
types with different disturbance regimes and
even among different disturbance events in the
same ecosystem type.
Carbon Balance
Primary Succession
In primary succession productivity and decomposition
rates are often greatest in midsuccession.
Primary succession begins with
little live or dead organic matter, so NPP and
decomposition are initially close to zero. NPP
increases slowly at first because of low plant
density, small plant size, and strong nitrogen
limitation of growth. NPP and biomass generally
increase most dramatically after nitrogen
fixers colonize the site. The planting of
nitrogen-fixing lupines on English mine wastes
(Bradshaw 1983) and the natural establishment
of nitrogen-fixing alders after retreat of
Alaskan glaciers (Bormann and Sidle 1990), for
example, cause sharp increases in plant biomass
and NPP. In primary successional sequences
that lack a strong nitrogen fixer, successional
increases in biomass and NPP depend on other
forms of nitrogen input, including atmospheric
deposition, plant and animal detritus, and
floods.
Long-term successional trajectories of
biomass and NPP differ among ecosystems. A
common pattern in forests is that NPP increases
from early to mid-succession and then declines
after the forest reaches its maximum leaf area
index (LAI) (Fig. 13.8) (Ryan et al. 1997).
Several processes are thought to contribute to
these patterns. In some forests, hydraulic conductance
declines in late succession, causing
water to limit the leaf area that can be supported
and therefore the NPP that the ecosystem
can sustain (see Chapter 6). In other
forests, nutrient supply declines in late succession,
leading to a corresponding reduction in
NPP (Van Cleve et al. 1991). The mortality of
branches and trees often increases in late
succession, as trees age. The combination of
reduced NPP and increased mortality of plants
and plant parts in late succession slows the rate
of biomass accumulation, so biomass approaches
a relatively constant value (steady state)
(Fig. 13.9). There is little support for the earlier
generalization (Odum 1969) that the decline in