Principles of terrestrial ecosystem ecology.pdf

340 15. Global Biogeochemical Cycles
CO 2 concentration (ppmv)
380
340
300
260
220
A
1000 1200 1400 1600 1800 2000
N 2 O concentration (ppbv)
320
310
300
290
280
270
Figure 15.3. Changes since 1750 in the atmospheric
concentrations of three radiatively active gases that
are influenced by human activities (Prather et al.
2001, Prentice et al. 2001). Data shown are a com-
to this sink per unit area of land, despite substantial
differences in climate and human
impact (Schimel et al. 2001). Boreal forests
may now be a source of carbon, in part due to
increased wildfire. Tropical forests appear to be
in approximate balance with the atmosphere,
due to similar magnitudes of net carbon uptake
from unmanaged forests and carbon loss from
deforestation.
Terrestrial Sinks for CO2
Four potentially important mechanisms contribute
to the Northern Hemisphere terrestrial
sink for CO2: land use change, CO2 fertilization,
inadvertent nitrogen fertilization, and climate
effects (Schimel 1995). The conversion of
forests to agricultural lands dominated land
use change in the middle and high latitudes
until the mid-twentieth century. Today, forest
C
CH 4 concentration (ppbv)
2000
1750
1500
1250
1000
750
260
250
1000 1200 1400 1600 1800 2000
Time
B
500 1000 1200 1400 1600 1800 2000
Time
posite of time series from air trapped in Antarctic ice
cores and from direct atmospheric measurements.
(Adapted from IPCC Assessement Report 2001;
Prentice et al. 2001.)
regrowth in previously harvested areas or in
abandoned agricultural lands has enhanced
carbon storage. The widespread suppression
of wildfire also enhances the mid-latitude
carbon sink, because it reduces fire emissions
and allows woody plants to encroach into
grasslands (Houghton et al. 2000). These are
probably the most important causes of
the north-temperate terrestrial carbon sink
(Schimel et al. 2001).
CO2 fertilization contributes to carbon
storage. Photosynthesis typically increases 20 to
40% under doubled CO 2 in short-term studies.
Carbon storage by ecosystems is, however,
much less responsive to CO 2 than is the shortterm
response of individual plants because
plant growth becomes nutrient limited as nutrients
become sequestered in live and dead
organic matter (Shaver et al. 1992, Schimel
1995) (see Chapter 5). Nutrient availability