Fræðaþing landbúnaðarins - 16.02.2007, Síða 16

14 • Fræðaþing landbúnaðarins 4, 2007
treatments increased from 25% in pure stands to 87% in the mixture. This resulted in an
increased content of T. repens in the swards under all examined management treatments.
Significant increases in both annual and perennial legume content at elevated C02 have
also been measured in the NZ FACE experiment (Edwards et al., 2001; Ross et al., 2004).
The results of the grassland ecosystems of the GCTE research project network showed a
mean increase in legume content in the swards of 8.5% (Liischer et al., 2005).
Do plants show genetic variability in their response to elevated C02?
Variability in the response of individual genotypes to elevated C02 provides the raw
material for natural selection or selection by breeders for the adaptation of plant species
to the increasing concentration of atmospheric C02. If appropriate genetic variability
exists in the response to elevated C02, selection could act to increase the frequency of
the fittest genotypes under elevated C02. Thus, interspecific differences in the response
to elevated C02, as they are found today may change with time because of evolutionary
adaptation of the species (Patterson and Flint, 1990; Bradshaw and McNeilly, 1991;
Woodward et al., 1991). Consequently, to make long-term predictions of how plant
communities and individual plant species will respond to the increasing concentration
of atmospheric C02, it is necessary to determine whether a significant intraspecific
variability in the response to elevated C02 exists.
Very few studies of intraspecific variability in the response to elevated C02 have been
published and the results are inconsistent. Intraspecific variability in the response to
elevated C02 was reported for lifetime fecundity in Raphanus raphanistrum (Curtis et
al., 1994), for the production of biomass in Arabidopsis thaliana (Norton et al., 1995)
and for seed maturation and seedling development in Plantago lanceolata (Wulff and
Alexander, 1985) in controlled environments without competition. On the other hand,
Fajer et al. (1992) found no variability in the response to elevated C02 of growth and leaf
biochemistry among clones of Plantago lanceolata. In 12 plant species from permanent
grassland Liischer et al. (1996; 1998) found no genetic variability in response of biomass
production to elevated CO,. In artificial communities, intraspecific variability in the
response to elevated C02 was detected for Bromus erectus but not for Festuca ovina
(Leadley and Stöcklin, 1996). When grown under field conditions with competition
Schmid et al. (1996) did not find heritable variability in the response to elevated C02 for
Prunella vulgaris and Prunella grandiflora.
Does elevated C02 act as a selection force?
Differential growth enhancements for species or genotypes under elevated C02 can
lead to changes in the composition of plant communities. Under a “rich-get-richer”
hypothesis, species that constitute a large proportion of a community (the dominants)
will increase their dominance at elevated C02 (Bazzaz and Garbutt, 1988). Under
the altemative “catch-up” hypothesis the smaller components of communities will
benefit proportionally more than dominants from elevated C02 conditions (Wayne and
Bazzaz, 1997; Ramseier et al., 2005; Stinson and Bazzaz, 2006; Stinson et al., 2006). A
recent review (Poorter and Navas, 2003) provided no evidence for differential growth