46
H.-J. Bolle
1996; Valentini et al. 1997; Valentini et al. 1998) to measure carbon, water, and
energy fluxes in important terrestrial ecosystems. The European components of this
international activity, EUROFLUX (Valentini et al. 1999) and MEDEFLU
(Miglietta and Peressotti 1999), are concentrating on the carbon exchange in
European and specifically Mediterranean ecosystems, primarily forests.
Measurements conducted by the MEDFLUX project (Miglietta and Peressotti
1999) funded by the EC provide for specific ecosystems long term carbon dioxide
flux data. The CO 2 emission above Quercus ilex according to flux measurements for
a first annual cycle made on top of a holm oak stand in Southern France seems to
have a double annual period being a sink in the average of 5 g m- 2 d- I during late
autumn and spring/early summer and a small source (during major rain events)
respectively close to zero in August/September and January.
The result of one year measurements in a Mediterranean macchia forest in
Sardinia was that it is a small CO 2 sink of 70 g m 2 year-I. Only from mid August
to mid November and especially during a rainy period from mid September to
October the macchia was a small CO 2 source of the order of 2 - 3 g m- 2 d- I . The net
carbon sequestration was less than 80 kgC ha- I year- I which probably is due to the
low vegetation cover and arid environment.
The carbon sequestration of an average EUROFLUX forests (Valentini,
Baldocchi, and Olson, 1999) is 2 - 4 t C ha- I year- I but increases from low to high
latitudes. According to Valentini et al. (2000) the NEE offour tested Italian forests
range between 4.5 and 6.7 t C ha- I yr- I and at the same time the respiration is
between 4.45 and 6.4 t C ha- I yr- I resulting in a NEE/RE rate of about one. The
respiration (RE), to which also the comparably small flux of BVOC's belongs, is
calculated from the nighttime fluxes, whole day fluxes during leafless periods, and
daytime respiration which is estimated by extrapolation of nighttime fluxes to the
rest of the day taking into account the functional relationships with soil and air
temperature. While the NEE depends on the latitude, the GEE is nearly constant at
12 t C ha- I yr-I fori the investigated European forests. Younger stands accumulate
carbon to build up their biomass. Older stands are close to equilibrium and can
switch from being a carbon source during one year and a sink in another. Thus the
budget seems to depend on climate variability.
Another approach was made by Chirici et al. (1999) who combined the
ecophysical forest model (FOREST BGC) of Running and Coughlan (1988, see
Running et al. 1989) with Landsat -TM data for deciduous forest stands of Quercus
cerris near Radicondoli, Tuscany, Italy. The model was validated with measured
evapotranspiration fluxes (sap flow). According to this model the photosynthesis was
of the order of 80-100 kg CO2 hal d l during the vegetation period between May and
October 1997. This results in a total photosynthesis (GEE) of 8 - lOt C ha- I year- I
in the forest. It is estimated that a NEE of about half this figure results.
CO 2 fluxes also have been measured during the long term MUREX experiment
(Calvet et al. 1999) near Toulouse of a dense herbaceus agricultural fallow site
covered mainly with Brachypodium sp, Potentilla reptans, Geranium rotundifolium,
Erigeron canadesis, and Rumex acetosa. On 1 September 1997 under a clear sky
during daylight time (10 hours) average CO 2 flux densities of 0.24 mg CO 2 m- 2 S-I
H.-J. Bolle
1996; Valentini et al. 1997; Valentini et al. 1998) to measure carbon, water, and
energy fluxes in important terrestrial ecosystems. The European components of this
international activity, EUROFLUX (Valentini et al. 1999) and MEDEFLU
(Miglietta and Peressotti 1999), are concentrating on the carbon exchange in
European and specifically Mediterranean ecosystems, primarily forests.
Measurements conducted by the MEDFLUX project (Miglietta and Peressotti
1999) funded by the EC provide for specific ecosystems long term carbon dioxide
flux data. The CO 2 emission above Quercus ilex according to flux measurements for
a first annual cycle made on top of a holm oak stand in Southern France seems to
have a double annual period being a sink in the average of 5 g m- 2 d- I during late
autumn and spring/early summer and a small source (during major rain events)
respectively close to zero in August/September and January.
The result of one year measurements in a Mediterranean macchia forest in
Sardinia was that it is a small CO 2 sink of 70 g m 2 year-I. Only from mid August
to mid November and especially during a rainy period from mid September to
October the macchia was a small CO 2 source of the order of 2 - 3 g m- 2 d- I . The net
carbon sequestration was less than 80 kgC ha- I year- I which probably is due to the
low vegetation cover and arid environment.
The carbon sequestration of an average EUROFLUX forests (Valentini,
Baldocchi, and Olson, 1999) is 2 - 4 t C ha- I year- I but increases from low to high
latitudes. According to Valentini et al. (2000) the NEE offour tested Italian forests
range between 4.5 and 6.7 t C ha- I yr- I and at the same time the respiration is
between 4.45 and 6.4 t C ha- I yr- I resulting in a NEE/RE rate of about one. The
respiration (RE), to which also the comparably small flux of BVOC's belongs, is
calculated from the nighttime fluxes, whole day fluxes during leafless periods, and
daytime respiration which is estimated by extrapolation of nighttime fluxes to the
rest of the day taking into account the functional relationships with soil and air
temperature. While the NEE depends on the latitude, the GEE is nearly constant at
12 t C ha- I yr-I fori the investigated European forests. Younger stands accumulate
carbon to build up their biomass. Older stands are close to equilibrium and can
switch from being a carbon source during one year and a sink in another. Thus the
budget seems to depend on climate variability.
Another approach was made by Chirici et al. (1999) who combined the
ecophysical forest model (FOREST BGC) of Running and Coughlan (1988, see
Running et al. 1989) with Landsat -TM data for deciduous forest stands of Quercus
cerris near Radicondoli, Tuscany, Italy. The model was validated with measured
evapotranspiration fluxes (sap flow). According to this model the photosynthesis was
of the order of 80-100 kg CO2 hal d l during the vegetation period between May and
October 1997. This results in a total photosynthesis (GEE) of 8 - lOt C ha- I year- I
in the forest. It is estimated that a NEE of about half this figure results.
CO 2 fluxes also have been measured during the long term MUREX experiment
(Calvet et al. 1999) near Toulouse of a dense herbaceus agricultural fallow site
covered mainly with Brachypodium sp, Potentilla reptans, Geranium rotundifolium,
Erigeron canadesis, and Rumex acetosa. On 1 September 1997 under a clear sky
during daylight time (10 hours) average CO 2 flux densities of 0.24 mg CO 2 m- 2 S-I