Fræðaþing landbúnaðarins - 16.02.2007, Side 15

13 • Fræðaþing landbúnaðarins 4, 2007
area.
Total yield of swards
In the Swiss FACE system (Liischer et al., 2006) the strong increase in leaf photosynthesis
in response to elevated C02 was confirmed under field conditions (Ainsworth et al.,
2003). However, compared to controlled conditions, yield response was much weaker,
allocation of assimilates to non-harvested plant parts was increased (Hebeisen et al.,
1997; Suter et al., 2002), and the accumulation of non-structural carbohydrates was
increased in leaves and stubbles (Fischer et al., 1997; Isopp et al., 2000) indicating sink
limitation at elevated CO,. Results from the grassland ecosystems of the “GCTE (Global
Change and Terrestrial Ecosystems) pastures and rangelands core research project
network” showed that the stimulatory effect of elevated C02 on sward productivity
was only 15%. This response was independent of the aboveground productivity of the
ecosystem (Campbell et al., 2000; Luscher et al., 2004) and was similar to the effects
reported by Mooney et al. (1999). In the GCTE network most of the sites are improved
pastures under humid and cool/temperate climates; information on grasslands in other
climatic zones is rnuch more sparse.
Interspecific differences in yield response to elevated atmospheric C02
Much of the world’s grasslands are characterised by swards that are botanically diverse.
Thus, the response of individual functional groups and species within the sward are of
interest, as changes in these components can alter nutrient cycling, the quality of the diet
presented to animals and the maintenance of biodiversity. Significant differences in the
yield response to elevated C02 are consistently found between legumes and C3 grasses.
In the first six years of the Swiss FACE experiment, the yield response to elevated CO,
of Lolium perenne (perennial ryegrass) ranged from -11 to +25%, compared to +10 to
+ 49% for Trifolium repens (white clover) when grown in pure swards (Hebeisen et al.,
1997; Daepp et al., 2000). These differences were confirmed in separate studies for other
legume and grass species (Lúscher et al., 1998). Since mineral N in the soil was a major
limiting factor in this system (Daepp et al., 2001; Schneider et al, 2004), the crucial
advantage of legumes is their access to non-limiting atmospheric N through symbiotic
N2 fixation (Zanetti et al., 1996; 1997; Hartwig et al, 2000; Lúscher and Aeschlimann,
2006). Non N-fixing mutants of Medicago sativa (Luceme) consequently lost this
advantage in response to elevated CO, (Lúscher et al., 2000). Similarly, phosphorus
limitation of T. repens has been shown to significantly reduce its response to elevated
C02 in a growth room experiment (Almeida et al., 1999) and with micro swards from
calcareous grassland (Stöcklin et al., 1998). Both, the New Zealand and the Swiss FACE
experiments revealed that forbs showed a stronger response to elevated CO, than grasses
(Lúscher et al., 1998; Allard et al., 2003).
Changes in species composition
Interspecific differences in the yield response to elevated CO , were accentuated in bi-
species mixtures of the Swiss FACE experiment compared to pure swards (Hebeisen
et al., 1997), indicating changes in competition between species. In the first 6 years
the mean yield response to elevated CO, of T. repens averaged over all management