Are C 4 Pathway Plants Threatened by Global Climatic Change?
539
Studies of C 3 and C4 plants in competition under elevated CO2 show
variable outcomes. Patterson et al. (1984) found that elevated CO2 improved
the competitive ability of soybean [Glycine max (L.) Merrill, C3] against its
major weed Johnson grass [Sorghum halepense (L.) Pers., C4].' Wray and
Strain (1987) found that the "old field" competitor Aster pi/otus Willd. (C3)
became more competitive with Andropogon virginicus L., (C4) at elevated
CO 2 . Other studies of annual species grown together have demonstrated
that the advantages conferred under elevated CO2 on C3 plants are not
always so marked, and that it is difficult to predict competitive outcomes.
For example, Bazzaz and Carlson (1984) found that the competitive advantage of Amaranthus retrofiexus L. (C4) over Polygonum pensylvanicum (C3)
in the current atmosphere and low soil moisture was only partially offset
by elevated CO2. In experiments with "mini-communities" Zangerl and
Bazzaz (1984) found that elevated CO2 concentrations increased relative
dominance of C3 species, with differing, species-dependent effects on
vegetative biomass and resource allocation to seed; however, C4 species
also increased their biomass and seed production at elevated CO2• Growth
analysis of competition experiments demonstrated that responses to elevated
CO 2 were greatest during early growth (Bazzaz et al. 1989). Surprisingly,
these effects were greater in Amaranthus retrofiexus (C4) than Abutilon
theophrasti medikus (C3), and this determined the competitive outcome.
In many cases, the above experiments are difficult to interpret because of
pot size effects (Arp 1991). Competition belowground is likely to be crucial,
and few experiments to date have addressed this problem. Such studies are
greatly facilitated by the measurements of ()13C value of total root biomass,
a technique already well proven in the field (Tieszen and Archer 1990).
Wong and Osmond (1991) followed interactions between the improbable
competitors Echinochloa frumentacea (C4) and Triticum aestivum L. (C 3 )
which had shootlroot ratios of 2 to 12, so shoot responses dominated. Shoot
growth of the C3 plants was routinely stimulated by elevated CO2 , but under
low nitrogen the C4 plants also responded. Surprisingly, C4 plants gained the
advantage belowground only under high nitrogen treatments, suggesting
that root access to nitrogen supply, rather than shoot nitrogen use efficiency,
determined competitive outcome. The conclusion of Bazzaz et al. (1989)
sums up our present position in that: "The precise competitive relationships
among and between the two groups will not be predictable from individual
responses as these will be modified by the other environmental factors that
impinge upon the plant community."
25.5 Present Distributions and Diversity of C4 Plants
The grass family is the most comprehensively studied plant family with
respect to the biology of C4 photosynthesis, even though it is one of only
two monocotyledonous plant families to contain C4 species (compared with
539
Studies of C 3 and C4 plants in competition under elevated CO2 show
variable outcomes. Patterson et al. (1984) found that elevated CO2 improved
the competitive ability of soybean [Glycine max (L.) Merrill, C3] against its
major weed Johnson grass [Sorghum halepense (L.) Pers., C4].' Wray and
Strain (1987) found that the "old field" competitor Aster pi/otus Willd. (C3)
became more competitive with Andropogon virginicus L., (C4) at elevated
CO 2 . Other studies of annual species grown together have demonstrated
that the advantages conferred under elevated CO2 on C3 plants are not
always so marked, and that it is difficult to predict competitive outcomes.
For example, Bazzaz and Carlson (1984) found that the competitive advantage of Amaranthus retrofiexus L. (C4) over Polygonum pensylvanicum (C3)
in the current atmosphere and low soil moisture was only partially offset
by elevated CO2. In experiments with "mini-communities" Zangerl and
Bazzaz (1984) found that elevated CO2 concentrations increased relative
dominance of C3 species, with differing, species-dependent effects on
vegetative biomass and resource allocation to seed; however, C4 species
also increased their biomass and seed production at elevated CO2• Growth
analysis of competition experiments demonstrated that responses to elevated
CO 2 were greatest during early growth (Bazzaz et al. 1989). Surprisingly,
these effects were greater in Amaranthus retrofiexus (C4) than Abutilon
theophrasti medikus (C3), and this determined the competitive outcome.
In many cases, the above experiments are difficult to interpret because of
pot size effects (Arp 1991). Competition belowground is likely to be crucial,
and few experiments to date have addressed this problem. Such studies are
greatly facilitated by the measurements of ()13C value of total root biomass,
a technique already well proven in the field (Tieszen and Archer 1990).
Wong and Osmond (1991) followed interactions between the improbable
competitors Echinochloa frumentacea (C4) and Triticum aestivum L. (C 3 )
which had shootlroot ratios of 2 to 12, so shoot responses dominated. Shoot
growth of the C3 plants was routinely stimulated by elevated CO2 , but under
low nitrogen the C4 plants also responded. Surprisingly, C4 plants gained the
advantage belowground only under high nitrogen treatments, suggesting
that root access to nitrogen supply, rather than shoot nitrogen use efficiency,
determined competitive outcome. The conclusion of Bazzaz et al. (1989)
sums up our present position in that: "The precise competitive relationships
among and between the two groups will not be predictable from individual
responses as these will be modified by the other environmental factors that
impinge upon the plant community."
25.5 Present Distributions and Diversity of C4 Plants
The grass family is the most comprehensively studied plant family with
respect to the biology of C4 photosynthesis, even though it is one of only
two monocotyledonous plant families to contain C4 species (compared with