Effect of carbon dioxide on plants
The increase of atmospheric CO2
concentration will have many effects on crops and other plants, including the
following (1) -
- Gaseous ‘fertilising’ of
crops by doubling CO2 to
600 ppm is expected to enhance productivity by at least 10 - 15 %, and
even more in drier regions. In natural ecosystems a larger biomass and a
more rapid vegetation cycling will result from an increase in atmospheric
CO2 concentration, ceteris
- Water-use efficiency will
improve since partially closed stomata on the crop’s leaves will still
take in the usual amount of CO2
for plant growth, but the throttling of the stomata lessens the loss of
water vapour from within the leaves (2). Thus, either a given amount of
growth will require less rain, or a given rainfall may be expected to
yield more crop; which option is chosen by a plant will vary with rainfall
patterns and plant species. On the other hand, less evaporation from the
stomata implies less evaporative cooling, which means higher leaf
temperatures during the daytime. This may have a slight enhancing effect
on global warming, in warm, wet land environments, during the daytime.
- The efficiency of
photosynthesis depends on the CO2
vapour pressure gradient across the stomata, so an improvement will occur
in the use of the available solar energy. In other words, slightly greater
crop yields can be expected even when water is not the limiting factor.
- A higher CO2 vapour pressure would also
increase the mineral use efficiency, in particular that of potassium and
calcium. In many soils, in particular in the tropics, mineral deficiency
is the main growth-limiting factor.
- The global warming, and reduced daily temperature range,
resulting from increased greenhouse gas concentrations is likely to
enhance the direct effect of CO2
on plant growth, especially in mid- and high-latitude regions.
- In a CO2-enriched atmosphere plants appear
to be more susceptible to frost damage (3). At least this is the case for
snow gums, a kind of eucalypt that is particularly tolerant of frost.
Whereas 34% of leaves are damaged by frost in normal circumstances, this
rises to 68% if the CO2
is doubled. The incidence of frost may be reduced by global warming, but
- The ratio of carbon to
nitrogen (and phosphorous) in plant tissue should increase, and this will
reduce the concentration of nitrogen-rich compounds such as proteins. In
particular, this would reduce the protein content of wheat grain, however
adequate soil fertilisation can compensate for this effect.
- In most regions of a double
CO2 world, there would be
more leaf litter in forests. Litter production would be larger because of
the enhanced plant growth, and then its decomposition would be inhibited
by the increased C/N ratio of the leaf tissue, though accelerated by extra
warmth if there is adequate moisture. A thicker layer of humus would
result if the litter production temporally exceeds its decay. This would
reduce erosion but increase the threat of serious forest fires.
- Global warming will lead to
increased evaporation in some areas, causing drier soils and hence less
growth, and, in other regions, higher rainfalls which may outweigh the
evaporation increase. The extra warmth so far is apparently chiefly in the
form of higher nocturnal minimum temperatures. This would protect some
fruit crops, especially seed fruits, from spring frost damage, yet it
would inhibit the ‘setting’ of some stone fruits. In the case of rice,
higher temperatures reduce yields by inducing sterility of the spikelets,
a matter of increasing concern worldwide (4).
In view of the variety and complexity of the consequences of doubling the
atmospheric CO2 concentration,
the overall effect is impossible to predict. If the changes are slow enough,
most negative effects can be offset by the introduction of new crop varieties or
shifting the growing regions.
The change in vegetation patterns in response to a changing climate has, in
turn, an effect on the atmospheric CO2
concentration (5). The atmospheric CO2
concentration is increasing primarily due to the burning of fossil fuels, yet a global increase in biomass during this build-up
reduces the rate of CO2 increase. For instance, a young forest-plantation
area of about 1 Mha in Australia absorbed about 25 megatonnes CO2 annually in the 1990’s (1). In
Australia it would be necessary to establish about 0.4 Mha of new forest each
year in areas suitable to grow tall timber, to offset the 560 megatonnes CO2 emitted annually in Australia. This
includes 440 Mt/a of burnt fossil fuels and 120 Mt/a due to land clearing and
agriculture (6). Managed forests in Australia subtract only about 22 Mt/a at
this time. The Australian biosphere as a whole is provisionally reckoned to
take up approximately 350 Mt/a of CO2
from the atmosphere, i.e. almost enough to compensate the emissions by burning
Experiments on sour orange show 2.8 times more growth as a result of
doubling the CO2 over five years. The greatest enhancement of growth
of tree seedlings in New England (US) occurred when doubled CO2 was
accompanied by low light levels and added nutrients (7). The effects of
temperature, CO2 and UV-B on plan growth were found to be highly
specific to the species. Insects may also be affected by CO2 levels,
e.g. an agricultural pest moth. Their CO2-sensitive organs for
detecting food may be overwhelmed by current CO2 concentrations (8).
Computer simulations of spring wheat in Holland showed that a higher global
temperature by itself is likely to decrease yields, but there may be an
increase when the warming is combined with more CO2. The
increase becomes substantial in dry years, because of the enhanced
water-use efficiency. The global effect of warming is expected to cause a small
decrease of cereal yields overall, chiefly affecting developing nations.
- Noble, I. , R. Gifford and G.
Farquhar, 1997: What do we know about impacts of action or no action? A
perspective from the biological sciences. Paper to a National Academies
Forum, ‘The Challenge for Australia on Global Climate Change’, 29-30
April, Canberra, 8pp.
- Koch, G.W. and H.A. Mooney,
1996: Carbon Dioxide and Terrestrial Ecosystems (Academic Press)
- Matthews, R.B., M.J. Kopff,
D. Bachelet and H.H. v. Laar (eds), 1995: Modelling the Impact of
Climate Change on Rice Production in Asia. (CAB International,
Wallingford, UK) 289pp.
- Anon 1998. Frost finding
pours cold water on plant greenhouse theory. ANU Reporter
(July) 29, p3. Available from the Australian National University.
- Barson, M.M. and R.M.
Gifford, 1990: Carbon dioxide sinks; the potential of tree planting in
Australia. In Swaine 1990, p 433-43 (5).
- Swaine, D.J. (ed.), 1990: Greenhouse
and Energy (Commonwealth Scientific and Industrial Research
- Hengeveld, H. and P. Kertland
1995: An assessment of new developments relevant to the science of climate
change. Climate Change Newsletter (Australian Bureau of Resource
Sciences, Canberra), 7, August, 24p.
- Stange, G. and C. Wong 1993:
Moth response to climate. Nature, 365, 699.