Principles of terrestrial ecosystem ecology.pdf

270 12. Community Effects on Ecosystem Processes
enrich soils on hilltops where they bed down
at night. Migrating salmon perform a similar
nutrient-transport role in streams. They feed
primarily in the open ocean and then return to
small streams where they spawn, die, and
decompose.The nutrients carried by the salmon
from the ocean can sustain a substantial
proportion of the algal and insect productivity
of small streams. These nutrient subsidies
can be transferred to adjoining terrestrial
habitats by bears and otters that feed on salmon
or by predators of insects that emerge from
streams.
Nutrient Turnover
Species differences in litter quality magnify site
differences in soil fertility. Differences among
plant species in tissue quality strongly influence
litter decomposition rates (see Chapter 7).
Litter from low-nutrient-adapted species
decomposes slowly because of the negative
effects on soil microbes of low concentrations
of nitrogen and phosphorus and high concentrations
of lignin, tannins, waxes, and other
recalcitrant or toxic compounds. This slow
decomposition of litter from species characteristic
of nutrient-poor sites reinforces the low
nutrient availability of these sites (Hobbie
1992, Wilson and Agnew 1992). Species from
high-resource sites, in contrast, produce rapidly
decomposing litter due to its higher nitrogen
and phosphorus content and fewer recalcitrant
compounds, enhancing rates of nutrient
turnover in nutrient-rich sites.
Experimental planting of species on a
common soil shows that species differences in
litter quality can alter soil fertility quite quickly.
Early successional prairie grasses, whose litter
has a low C:N ratio, for example, causes an
increase in the nitrogen mineralization rate of
soil within 3 years, compared to the same soil
planted with late-successional species whose
litter has a high C:N ratio (Wedin and Tilman
1990) (Fig. 12.5).
Seasonality of Resource Capture
Phenological specialization could increase
resource capture. Phenological specialization in
the timing of plant activity can increase the
Net N mineralization rate
(g N m -2 yr -1 )
14
7
0
(44)
(56)
(64)
As Ar Pp Ss Ag
Species
(109)
C:N ratios
(122)
Figure 12.5. Effects of prairie grass species on nitrogen
mineralization when grown on soils with containing
100gNm -2 (Wedin and Tilman 1990). Grasses
range from early to late successional in the following
order: Agrostis scabra (As), Agropyron repens
(Ar), Poa pratensis (Pp), Schizochyrium scoparium
(Ss), and Andropogon gerardi (Ag). Data are means
±95% confidence interval (CI). Numbers are the C:
N ratios of aboveground biomass.
total time available for plants to acquire
resources from their environment. This is most
evident when coexisting species differ in the
timing of their maximal activity. In mixed grasslands,
for example, C4 species are generally
more active in the warmer, drier part of the
growing season than are C 3 species. Consequently
C 3 species account for most early
season production, and C 4 species account for
most late-season production. Similarly, in the
Sonoran desert, there is a different suite of
annuals that becomes active after winter than
after summer rains. In both cases, phenological
specialization probably enhances NPP and
nitrogen cycling. In mixed-cropping agricultural
ecosystems, phenological specialization is
more effective in enhancing production than
are species differences in rooting depth
(Steiner 1982).
The ecosystem consequences of phenological
specialization to exploit the extremes of the
growing season are less clear. Evergreen
forests, for example, have a longer photosynthetic
season than deciduous forests, but most
carbon gain occurs in midseason in both forest
types, when conditions are most favorable