![Hier klicken, um den Treffer aus der Auswahl zu entfernen](images/unchecked.gif) |
Titel |
Biological control of the terrestrial carbon sink |
VerfasserIn |
E.-D. Schulze |
Medientyp |
Artikel
|
Sprache |
Englisch
|
ISSN |
1726-4170
|
Digitales Dokument |
URL |
Erschienen |
In: Biogeosciences ; 3, no. 2 ; Nr. 3, no. 2 (2006-03-29), S.147-166 |
Datensatznummer |
250000885
|
Publikation (Nr.) |
copernicus.org/bg-3-147-2006.pdf |
|
|
|
Zusammenfassung |
This lecture reviews the past (since 1964 when the International Biological
Program began) and the future of our understanding of terrestrial carbon
fluxes with focus on photosynthesis, respiration, primary-, ecosystem-, and
biome-productivity.
Photosynthetic capacity is related to the nitrogen concentration of leaves,
but the capacity is only rarely reached under field conditions. Average
rates of photosynthesis and stomatal conductance are closely correlated and
operate near 50% of their maximal rate, with light being the limiting
factor in humid regions and air humidity and soil water the limiting factor
in arid climates. Leaf area is the main factor to extrapolate from leaves to
canopies, with maximum surface conductance being dependent on leaf level
stomatal conductance. Additionally, gas exchange depends also on rooting
depth which determines the water and nutrient availability and on
mycorrhizae which regulate the nutrient status. An important anthropogenic
disturbance is the nitrogen uptake from air pollutants, which is not
balanced by cation uptake from roots and this may lead to damage and
breakdown of the plant cover.
Photosynthesis is the main carbon input into ecosystems, but it alone does
not represent the ecosystem carbon balance, which is determined by
respiration of various kinds. Plant respiration and photosynthesis determine
growth (net primary production) and microbial respiration balances the net
ecosystem flux. In a spruce forest, 30% of the assimilatory carbon gain
is used for respiration of needles, 20% is used for respiration in stems.
Soil respiration is about 50% the carbon gain, half of which is root
respiration, half is microbial respiration. In addition, disturbances lead
to carbon losses, where fire, harvest and grazing bypass the chain of
respiration. In total, the carbon balance at the biome level is only about
1% of the photosynthetic carbon input, or may indeed become negative. The
recent observed increase in plant growth has different reasons depending on
the region of the world: anthropogenic nitrogen deposition is the
controlling factor in Europe, increasing global temperatures is the main
factor in Siberia, and maybe rising CO2 the factor controlling the
carbon fluxes in Amazonia. However, this has not lead to increases in net
biome productivity, due to associated losses. Also important is the
interaction between biodiversity and biogeochemical processes. It is shown
that net primary productivity increases with plant species diversity (50%
species loss equals 20% loss in productivity). However, in this
extrapolation the action of soil biota is poorly understood although soils
contribute the largest number of species and of taxonomic groups to an
ecosystem.
The global terrestrial carbon budget strongly depends on areas with pristine
old growth forests which are carbon sinks. The management options are very
limited, mostly short term, and usually associated with high uncertainty.
Unmanaged grasslands appear to be a carbon sink of similar magnitude as
forest, but generally these ecosystems lost their C with grazing and
agricultural use.
Extrapolation to the future of Earth climate shows that the biota will not
be able to balance fossil fuel emissions, and that it will be essential to
develop a carbon free energy system in order to maintain the living
conditions on earth. |
|
|
Teil von |
|
|
|
|
|
|