citation: FORSTREUTER, M (1993) Langzeitwirkungen
der atmosphärischen CO2-Anreicherung auf den
Kohlenstoff- und Wasserhaushalt von Rotklee-Wiesenschwingelgemeinschaften.
Dissertation, Universität Osnabrück, Landschaftsentwicklung
und Umweltforschung (Berlin) 91: 208 S.
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Langzeitwirkungen der
atmosphärischen CO2-Anreicherung auf den
Kohlenstoff- und Wasserhaushalt von Rotklee- Wiesenschwingelgemeinschaften
Manfred Forstreuter
Manfred.Forstreuter@fu-berlin.de
Long-term effects of increased atmospheric
CO2-concentrations on the carbon and water budgets of red clover and
meadow fescue communities
Manfred Forstreuter
Summary
This work studies the effect of increased atmospheric
carbon dioxide concentration on carbon and water budgets of model
ecosystems. In three long-term investigations, mixed plant stands
of Trifolium pratense Huds. and Festuca pratensis L.
were exposed to CO2-concentrations of 350, 450, 600 and 800 ppm
over a period of up to 1007 days.
For these experiments a measuring system was specially constructed
and was located outside. This made it possible to expose four model
ecosystems - consisting of the aforesaid species in a soil block
of 80 * 80 * 60 cm3 - to natural environmental conditions for several
vegetation periods.
The continuous measurements of the atmospheric concentration from
1984 to 1991 showed a significant yearly increase of 2.8 ppm.
The dry-matter accumulation (aboveground plus roots) of the herbaceous
plant stands were highly enhanced by the elevated CO2-concentrations.
A fertilization factor was determined by using all phytomass data
and developing a saturation model, showing that the average dry
matter accumulation was increased by 18% (450 ppm), by 50% (600
ppm) and by 52% (800 ppm). Particulary at the beginning of the vegetation
period and after mowing, the dry-matter accumulation was highly
enhanced.
Aboveground phytomass accumulation showed an optimum at nearly 600
ppm, whereas the roots were able to act as a "storage organ"
and incorporated additional amounts of carbon at CO2-concentrations
over 600 ppm.
The reproductive organs were shown to get heavier with increasing
CO2-concentrations. This was significantly the case for the weight
of the seeds and caryopses of the investigated species.
Leaf area index (LAI) of the whole plant stand was also higher at
increased CO2-concentrations. This change could be shown by an exponential
model. At 450 ppm the leaf area index was practically unchanged,
whereas at CO2-concentrations of 600 and 800 ppm the leaf area index
increased in average by 14% and 35 %.
In the model ecosystems taken as a whole higher CO2-treatments caused
higher net CO2 exchange rates (NCER), showing an increase between
350 and 450 ppm of 21 - 31%, and between 350 and 600 ppm of 48 -
58%, and between 350 and 800 ppm of 59%.
The study also showed that respiration rates of the model ecosystems
were greater under high CO2-levels, especially at increased temperature.
Duirng the daytime the net CO2 exchange rates (NCER) showed a "hysteresis
effect" and led to changes in the light compensation points
of the model ecosystems.
During the first 100 days the NCER values showed an optimum curve
at photon flux densities of 1000 µE m-2 s-1. The optimum level
of NCER was increased by CO2 enrichment.
In a "switch experiment" the CO2 gas exchange measurements
showed that under long-term exposure the model ecosystems were able
to acclimate to the enriched CO2 conditions: At high CO2-concentration,
NCER values were higher in the model ecosystems, which were grown
under high CO2 treatment, in comparison to model ecosystems grown
under low CO2 treatment.
At all CO2 treatments a saturation level of NCER was determinde
at high LAI. With increased CO2 levels higher NCER occured at higher
LAI.
At all phenological stages of the investigated model ecosystems
a linear correlation between the daily sums of photon flux densities
were observed. On days with high sums of photon flux densities the
model ecosystems at high CO2-concentration showed higher net CO2
exchange rates, on cloudy days the CO2 amounts were reduced or showed
greater CO2 losses in comparison to the plant stands at 350 ppm
CO2-concentration.
The investigation covered the period from 1984 to 1989. In the months
from April to August the model ecosystems at 600 ppm showed a higher
carbon gain in comparison to the system at 350 ppm of up to 40%.
In the other 7 month there was at 600 ppm a decrease in the net
CO2 gain or an increase in CO2 loss. A comparison showed that the
model ecosystem at 600 ppm had a yearly carbon budget 16% higher
than that of the 350 ppm system.
Thus grassland ecosystems are "sinks" for anthropogenic
"sources" of CO2, but the "sink" is considerably
lower than expected.
At photon flux densities above 10 E m-2 d-1 higher NCER was observed
at 600 ppm compared to 350 ppm, whereas below this level the system
at 350 ppm took up more CO2.
As expected an exponential relationship was found between the average
daily CO2 uptake of a given month and the average monthly temperature.
At the start of the vegetation period (at temperatures above 5°C)
both model ecosystems showed positive CO2 gas exchange rates. The
highest CO2 gas exchange rates were observed at an average temperature
of 13°C in May. Higher temperatures in June caused a reduction
in CO2 exchange rates in both systems.
The water budget of the model ecosystems were influenced by elevated
CO2-concentrations. At 450 ppm the evapotranspiration was reduced;
at 600 ppm they were hardly affected. However, the evapotranspiration
observed at 800 ppm was higher than that of the 350 ppm system.
This led to reduce soil water storage in the system at 800 ppm,
whereas the soil water storage at 450 ppm was increased.
At higher CO2-concentrations water use efficiency was significantly
increased and resulted in a more efficient productivity.
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