544
s. Henderson et al.
forecast in 2030 are shown in Fig. 25.6b. This projection suggests that the
90% C4 isoline would extend southwards by 200-400 km, and that the grass
flora of more than 80% of the continental area could be almost totally
dominated by C4 species. The magnitude of the potential shift is unexpected, until one realises that an increase in temperature of 4°C is 25% of
the present continental range in average January minimum temperature
(Fig. 25.5a). Whether this changed grass distribution will be realized, and
when, depends on many factors, especially species migration rate, barriers
to migration and present land use. We cannot predict the extent to which
elevated CO2 itself may modulate the temperature response of C 3 and C 4
species, but some territorial advance of C4 grasses still seems likely.
Factors other than the direct effect of CO2 and temperature and physical migration will also alter relative distributions of C 3 and C 4 plants.
These include intensity and seasonality of rainfall, the response of other
components of the vegetation to climatic change, and changes in land management practices as weather changes. Rainfall predictions, even for a
physiographically simple landscape such as the Australian continent, are less
certain than temperature projections in climate change. Current regional
climatic change projections for Australia (using several global climate
models) suggest that the summer rainfall region will become more extensive
over the continent (A.B. Pittock 1992, pers. comm.). This would again
favor C4 grasses. In fact, the southerly movement of summer rainfall
dominance is likely to complement to the temperature-driven southerly
migration of C4 plants. However, other potential changes, for example in
the Darwin-Gulf region of the Northern Territory in Australia could exclude
C4 grasses. If rainfall was more uniform over the year in a tropical or
subtropical area that had previously been a "wet-dry tropics" area, then
closed forest might replace open woodland or savanna woodlands, and C4
grasses (which are high light-requiring plants) might be excluded.
It is likely that representation of the three C4 types in grass floras will
also change in response to changed rainfall regimes. Based on regional
generalizations for New South Wales (Pittock 1991) and the whole Australian
continent (Pittock et al. 1991), we have estimated the percentage increase or
decrease in summer and annual rainfall for some selected regions. The
equations derived by Hattersley (1992) (Fig. 25.5b) were used to predict
changes in percent C4 type in grass floras. Preliminary analyses of the effects
of changed precipitation suggest little effect on percent C4 type in any
subdivision of the continent.
25.7 Conclusion
This essay is a crude, preliminary attempt to consider some of the dimensions of what, in effect, will be a global experiment in photosynthetic
s. Henderson et al.
forecast in 2030 are shown in Fig. 25.6b. This projection suggests that the
90% C4 isoline would extend southwards by 200-400 km, and that the grass
flora of more than 80% of the continental area could be almost totally
dominated by C4 species. The magnitude of the potential shift is unexpected, until one realises that an increase in temperature of 4°C is 25% of
the present continental range in average January minimum temperature
(Fig. 25.5a). Whether this changed grass distribution will be realized, and
when, depends on many factors, especially species migration rate, barriers
to migration and present land use. We cannot predict the extent to which
elevated CO2 itself may modulate the temperature response of C 3 and C 4
species, but some territorial advance of C4 grasses still seems likely.
Factors other than the direct effect of CO2 and temperature and physical migration will also alter relative distributions of C 3 and C 4 plants.
These include intensity and seasonality of rainfall, the response of other
components of the vegetation to climatic change, and changes in land management practices as weather changes. Rainfall predictions, even for a
physiographically simple landscape such as the Australian continent, are less
certain than temperature projections in climate change. Current regional
climatic change projections for Australia (using several global climate
models) suggest that the summer rainfall region will become more extensive
over the continent (A.B. Pittock 1992, pers. comm.). This would again
favor C4 grasses. In fact, the southerly movement of summer rainfall
dominance is likely to complement to the temperature-driven southerly
migration of C4 plants. However, other potential changes, for example in
the Darwin-Gulf region of the Northern Territory in Australia could exclude
C4 grasses. If rainfall was more uniform over the year in a tropical or
subtropical area that had previously been a "wet-dry tropics" area, then
closed forest might replace open woodland or savanna woodlands, and C4
grasses (which are high light-requiring plants) might be excluded.
It is likely that representation of the three C4 types in grass floras will
also change in response to changed rainfall regimes. Based on regional
generalizations for New South Wales (Pittock 1991) and the whole Australian
continent (Pittock et al. 1991), we have estimated the percentage increase or
decrease in summer and annual rainfall for some selected regions. The
equations derived by Hattersley (1992) (Fig. 25.5b) were used to predict
changes in percent C4 type in grass floras. Preliminary analyses of the effects
of changed precipitation suggest little effect on percent C4 type in any
subdivision of the continent.
25.7 Conclusion
This essay is a crude, preliminary attempt to consider some of the dimensions of what, in effect, will be a global experiment in photosynthetic