Climate, Climate Variability and Impacts in the Mediterranean Area: An Overview
45
5.2 Carbon Fluxes
During daytime plants are taking up carbon dioxide from the air through their leaves
or needles due to the photosynthesis process which exceeds the losses by respiration
that takes place at the same time, Carbon dioxide emerges also from soils resulting
from respiration by soil organisms, roots, humus decay and myccorhizal activity. It
is replaced by organic matter input to the surface (surface litter) and root detritus.
Part of the CO 2 released from the soil during daytime will again be taken up by the
leaves and only the rest returns to the air outside the canopy. Therefor the net
amount of carbon dioxide integrated into the biomass through the photosynthesis
process, the net assimilation, is higher than the net CO 2 flux measured in the
atmosphere (Cal vet et al. 1999) provided there is enough photosynthetic radiation
(PAR), water, phosphor and nitrogen.
The total input to the ecosystem due to photosynthesis is called gross ecosystem
exchange (GEE), gross photosynthesis or gross primary production (GPP). This
leads to the production of biomass and drives the output of the system, the CO 2
respiration (RE). Respiration occurs directly from the living plants or at the end of
an internal cycle by which litter and humus is produced from the primary production
of biomass and decays after some time (Aber, 1998).
The atmospheric CO2 net flux measured at the top of the canopy represents the
difference between GEE and RE and are called Net Ecosystem Exchange (NEE =
GEE - RE). It represents the uptake respectively loss of carbon by the ecosystem
during a specified time period. The instantaneous CO2 net t1ux measured at the top
of the canopy provides no information about the carbon budget because of the time
lag between the inhalation of carbon dioxide which is restricted to the vegetation
period and daytime and its release by decomposition oflitter which occurs any time.
The annual net flux of carbon dioxide at the top of an ecosystem is calculated from
the summation of carbon dioxide fluxes at the top of the canopy and leads to
budgets. It includes the CO2 consumed for production of sustaining biomass, the
direct re-emission into the atmosphere (due to respiration from leaves, fires,
decomposition of dead timber above the surface), and the respiration from the soil
of which a fraction is taken up again by the vegetation.
Raich and Schlesinger (1992) estimated the soil respiration rates for different
ecosystems on the basis of CO2 fluxes measured at the surface and not at the top of
the canopy. From the CO 2 t1uxes they calculated the following carbon t1uxes relevant
for the Mediterranean ecosystems in units of g C m- 2 year-I: Temperate coniferous
forests 681 ±95, temperate deciduous forests 64 7±51, Mediterranean woodlands and
heath 713±88, croplands, fields etc. 544±80, and desert scrub 224±38. If one
assumes that in the long run the soil carbon flux cannot exceed the biomass
production on top of the soil, these values provide an upper limit for the net primary
productivity (NPP). A least square regression analysis resulted in an average
relationship between the NPP and the soil respiration (SR) of SR = 1.24 NPP + 24.5.
For the Mediterranean woodland and heath the relationship would even be SR =
1.75 NPP (all units in g C m- 2 year-I).
IGBP in 19961aunched the international initiative FLUXNET (Baldocchi et al.
45
5.2 Carbon Fluxes
During daytime plants are taking up carbon dioxide from the air through their leaves
or needles due to the photosynthesis process which exceeds the losses by respiration
that takes place at the same time, Carbon dioxide emerges also from soils resulting
from respiration by soil organisms, roots, humus decay and myccorhizal activity. It
is replaced by organic matter input to the surface (surface litter) and root detritus.
Part of the CO 2 released from the soil during daytime will again be taken up by the
leaves and only the rest returns to the air outside the canopy. Therefor the net
amount of carbon dioxide integrated into the biomass through the photosynthesis
process, the net assimilation, is higher than the net CO 2 flux measured in the
atmosphere (Cal vet et al. 1999) provided there is enough photosynthetic radiation
(PAR), water, phosphor and nitrogen.
The total input to the ecosystem due to photosynthesis is called gross ecosystem
exchange (GEE), gross photosynthesis or gross primary production (GPP). This
leads to the production of biomass and drives the output of the system, the CO 2
respiration (RE). Respiration occurs directly from the living plants or at the end of
an internal cycle by which litter and humus is produced from the primary production
of biomass and decays after some time (Aber, 1998).
The atmospheric CO2 net flux measured at the top of the canopy represents the
difference between GEE and RE and are called Net Ecosystem Exchange (NEE =
GEE - RE). It represents the uptake respectively loss of carbon by the ecosystem
during a specified time period. The instantaneous CO2 net t1ux measured at the top
of the canopy provides no information about the carbon budget because of the time
lag between the inhalation of carbon dioxide which is restricted to the vegetation
period and daytime and its release by decomposition oflitter which occurs any time.
The annual net flux of carbon dioxide at the top of an ecosystem is calculated from
the summation of carbon dioxide fluxes at the top of the canopy and leads to
budgets. It includes the CO2 consumed for production of sustaining biomass, the
direct re-emission into the atmosphere (due to respiration from leaves, fires,
decomposition of dead timber above the surface), and the respiration from the soil
of which a fraction is taken up again by the vegetation.
Raich and Schlesinger (1992) estimated the soil respiration rates for different
ecosystems on the basis of CO2 fluxes measured at the surface and not at the top of
the canopy. From the CO 2 t1uxes they calculated the following carbon t1uxes relevant
for the Mediterranean ecosystems in units of g C m- 2 year-I: Temperate coniferous
forests 681 ±95, temperate deciduous forests 64 7±51, Mediterranean woodlands and
heath 713±88, croplands, fields etc. 544±80, and desert scrub 224±38. If one
assumes that in the long run the soil carbon flux cannot exceed the biomass
production on top of the soil, these values provide an upper limit for the net primary
productivity (NPP). A least square regression analysis resulted in an average
relationship between the NPP and the soil respiration (SR) of SR = 1.24 NPP + 24.5.
For the Mediterranean woodland and heath the relationship would even be SR =
1.75 NPP (all units in g C m- 2 year-I).
IGBP in 19961aunched the international initiative FLUXNET (Baldocchi et al.