Instant Notes: Plant Biology

consequences of global warming. World sea levels have risen by more than

10 cm in the last century and their continued rise will result in losses of coastal

land and island communities.

At present, photosynthesis in C3 plants (Topic J2) is limited by CO 2 levels and

growth rates in at least some species increase if CO 2 concentrations are increased.

Growth rates may increase by 30–60% if CO 2 concentrations are doubled, but this

may only be temporary as growth and development are regulated in other ways

(Topic F1). In natural communities, growth is likely to be limited by other factors

such as nutrient availability. Yields of some glasshouse crops, such as tomato,

have been increased by enriching the air around them with CO 2. C4 species

(Topic J3) are not likely to show enhanced yields with rising CO 2 , as they do not

show photorespiration and have a much lower CO 2 compensation point (Topic

J3).Figure 3illustrates net photosynthetic rates of a C3 plant (e.g. wheat) and C4

plant (e.g. maize) at different CO 2 concentrations but otherwise optimal environ-

ments. While the C4 plant is more efficient at current global CO 2 concentrations of

around 360 ppm, this efficiency advantage will diminish and eventually C3

species will exceed C4 in efficiency if CO 2 concentrations continue to rise.

While in the short term, plants reduce atmospheric CO 2 by fixing carbon, in the

long term this only decreases global atmospheric CO 2 concentrations if the plant

material is not burnt or rotted, processes which release CO 2 to the atmosphere

(Fig. 1).

Overall, the impact of changing climate is likely to be greater than direct effects

of CO 2 concentration. While crop yields may increase in optimal environments,

losses due to drought, flooding, sea level increases and disease will also

increase. Plant communities will be changed as the competitive abilities of some

species increase while others decline. Biomes likely to be sensitive to climate

change include tundra (loss of permafrost); savannah and deciduous tropical

forests (drought and fire) and deserts (rising temperatures). Other biomes, like

tropical rain forests, are at risk from both human activity and drought. If a

species is to survive, it must either adapt to its new surroundings or migrate

through seed dispersal at a rate equal to that of climate change. For many long-

lived species this may not be possible and they may become extinct.

Global climate
and biodiversity

Plants and rising
carbon dioxide

182 Section K – Plant communities and populations

60

50

40

30

20

10

200 400 600 800 1000

CO 2 concentration (ppm)

CO

assimilation ( 2

μmol m

–2

–1s

)

C4

C3

Fig. 3. Photosynthetic responses of a C3 plant and a C4 plant to rising CO 2 concentrations

under experimental conditions. Atmospheric CO 2 concentrations are >360 ppm and increasing

by about 10 ppm every 10 years.