Principles of terrestrial ecosystem ecology.pdf

Net primary production is the net carbon
gained by vegetation. It includes new plant
biomass produced, root exudation, carbon
transfers to root symbionts, and the emission of
volatile organic compounds by plants. Biome
differences in NPP correlate with climate at the
global scale largely because temperature and
precipitation determine the availability of soil
resources required to support plant growth.
Plants actively sense the availability of these
resources and adjust photosynthesis and NPP
to match this resource supply. For this reason,
NPP is greatest in environments with high
availability of belowground resources. After
disturbance, NPP is often reduced below levels
that the environment can support. Plants
maximize production by allocating new growth
to tissues that acquire the most limiting
resources. Constantly shifting patterns of allocation
reduce the degree of limitation of NPP
by any single resource and make NPP in most
ecosystems responsive to more than one
resource.
Tissue loss is just as important as NPP in
explaining changes in plant biomass. Programmed
loss of tissues provides a supply of
plant resources that supports new production.
Biomass and NPP are greatest in warm,
moist environments and least in environments
that are cold or dry. The length of the photosynthetic
season and leaf area are the two
strongest determinants of the global patterns
in NPP. Most ecosystems have a similar (1 to
3g biomass m -2 of leaf d -1 ) daily NPP per unit
leaf area.
Net ecosystem production is a measure of
the rate of carbon accumulation in ecosystems.
It correlates more strongly with time since
disturbance than with environment. NEP is
generally greatest in midsuccession, when
ecosystems accumulate plant biomass and
SOM. NEP is greater under conditions that
promote NPP (e.g., elevated CO2, N deposition)
than under conditions that promote decomposition.
Net biome production integrates NEP at
the regional scale, taking account of regional
patterns of disturbance and stand age. Human
activities are altering most of the major controls
over NEP at a global scale in ways that are
likely to affect global climate.
Review Questions
Additional Reading 149
1. What controls the partitioning of carbon
between growth and respiration? Explain
why the efficiency of converting sugars into
new biomass is relatively constant.
2. What factors influence the variability in
maintenance respiration?
3. Describe the multiple ways in which climate
affects the NPP of grasslands or tundra.
4. There is generally a close correlation
between GPP and NPP. Describe the
mechanisms that account for short-term
variations in GPP and NPP (e.g., diurnal
and seasonal variations).
5. Describe the mechanisms that account
for the relationship between GPP and NPP
when ecosystems from different climatic
regimes are compared.
6. How does allocation to roots vs. shoots
respond to shade, nutrients, CO2, grazing, or
water?
7. How does variation in allocation influence
resource limitation, resource capture, and
NPP?
8. Why do plants senesce tissues in which
they have invested carbon and nutrients
rather than retaining tissues until they are
removed by disturbance or herbivory?
How does this physiologically programmed
senescence influence NPP?
9. Describe the carbon budget of a plant and
of an ecosystem in terms of GPP, respiration,
and production. How would you
expect each of these parameters to respond
to changes in temperature, water, light, and
nitrogen?
10. How do the controls over NEP differ from
the controls over GPP and decomposition.
Why are these controls different?
Additional Reading
Chapin, F.S. III. 1991. Integrated responses of plants
to stress. BioScience 41:29–36.
Chapin, F.S. III, E.-D. Schulze, and H.A. Mooney.
1990. The ecology and economics of storage in
plants. Annual Review of Ecology and Systematics
21:423–448.