Are C4 Pathway Plants Threatened by Global Climatic Change?
543
the weed Echinochloa crus-galli (L.) P. Beauv. in North America was best
predicted by August maximum temperature, plants from the coolest treatments produced a small number of heavy seeds of superior germinabilty
(Potvin 1991). Precisely which trait, or which combination of traits, is most
relevant to the relative fitness of C3 and C4 species remains baffling.
25.6 Future Distributions of C4 Plants
In spite of our limited insights into processes which underlIe the relationships
between temperature parameters and the representation of C4 plants in
regional floras, these relationships are remarkably robust. Although there is
no compelling evidence that C4 plants will be physiologically debilitated by
elevated CO 2 itself, and although competitive interactions remain poorly
understood, it seems reasonable, nevertheless, to use present day algorithms
for C4 grass distribution vs. temperature to predict changes in composition
which might follow increases in temperature attributable to greenhouse
gases.
Global warming is the least disputed outcome from increases in greenhouse gases and there is a high degree of concurrence among different
global circulation models so far as future temperature in Australia is concerned. According to the Climate Change Scenario for the Australian Continent for 2030, issued by the CSIRO Climate Impact Group in November
1991, mean annual temperature is estimated to increase by 1 to 2°C in
northern coastal areas, by 1 to 3 °C in southern coastal areas, and by 2 to
4°C inland, with the larger increases being observed in dry regions. These
projections were used to adjust the average January minimum temperatures
of each region considered by Hattersley (1983), and then changes in percent
C4 grasses of regional grass floras were calculated using the linear regression
equation of Fig. 25.5a.
The present relative distribution of C3 and C4 native grasses in Australia
is shown in Fig. 25.6a, redrawn from Hattersley (1983). The isolines for
projected relative C3/C4 grass species' occurrence for the temperatures
Fig. 25.6a,b. Maps of Australia showing 90, 50, and 20% C4 isolines. a At present
temperatures. b At temperatures forecast for the year 2030. Percent C 4 refers to the
percentage of native species in a regional grass flora that is C4. a is based on Fig. 2 in
Hattersley (1983), with isolines interpolated between the centers of regional areas for
which percent C4 values are available. Isolines are not based on point data and their
positions are therefore only approximate (refer to Hattersley 1983 for source data). b is a
first approximation of the change in the position of the isolines using continental temperatures forecast by the CSIRO's Climate Impact Group (1991) for the year 2030,
adjusting the January minimum temperature by this amount, and using the linear regression equation from Fig. 25.5a to calculate percent C 4 values
543
the weed Echinochloa crus-galli (L.) P. Beauv. in North America was best
predicted by August maximum temperature, plants from the coolest treatments produced a small number of heavy seeds of superior germinabilty
(Potvin 1991). Precisely which trait, or which combination of traits, is most
relevant to the relative fitness of C3 and C4 species remains baffling.
25.6 Future Distributions of C4 Plants
In spite of our limited insights into processes which underlIe the relationships
between temperature parameters and the representation of C4 plants in
regional floras, these relationships are remarkably robust. Although there is
no compelling evidence that C4 plants will be physiologically debilitated by
elevated CO 2 itself, and although competitive interactions remain poorly
understood, it seems reasonable, nevertheless, to use present day algorithms
for C4 grass distribution vs. temperature to predict changes in composition
which might follow increases in temperature attributable to greenhouse
gases.
Global warming is the least disputed outcome from increases in greenhouse gases and there is a high degree of concurrence among different
global circulation models so far as future temperature in Australia is concerned. According to the Climate Change Scenario for the Australian Continent for 2030, issued by the CSIRO Climate Impact Group in November
1991, mean annual temperature is estimated to increase by 1 to 2°C in
northern coastal areas, by 1 to 3 °C in southern coastal areas, and by 2 to
4°C inland, with the larger increases being observed in dry regions. These
projections were used to adjust the average January minimum temperatures
of each region considered by Hattersley (1983), and then changes in percent
C4 grasses of regional grass floras were calculated using the linear regression
equation of Fig. 25.5a.
The present relative distribution of C3 and C4 native grasses in Australia
is shown in Fig. 25.6a, redrawn from Hattersley (1983). The isolines for
projected relative C3/C4 grass species' occurrence for the temperatures
Fig. 25.6a,b. Maps of Australia showing 90, 50, and 20% C4 isolines. a At present
temperatures. b At temperatures forecast for the year 2030. Percent C 4 refers to the
percentage of native species in a regional grass flora that is C4. a is based on Fig. 2 in
Hattersley (1983), with isolines interpolated between the centers of regional areas for
which percent C4 values are available. Isolines are not based on point data and their
positions are therefore only approximate (refer to Hattersley 1983 for source data). b is a
first approximation of the change in the position of the isolines using continental temperatures forecast by the CSIRO's Climate Impact Group (1991) for the year 2030,
adjusting the January minimum temperature by this amount, and using the linear regression equation from Fig. 25.5a to calculate percent C 4 values