18.104.22.168. Soil Properties and Plant Growth
The impact of climate change on soil is difficult to predict (Jenkinson et
al., 1991; Gifford et al., 1996a; IPCC 1996, WG II, Chapters A and 4; McMurtrie
and Comins, 1996; Tate et al., 1996; Thornley and Cannell, 1996). Soil nutrient
availability is particularly important in Australia, where a high proportion
of the continent has soils of low nutrient content. Increased CO2 concentration
and temperature will change the carbon-to-nitrogen ratios of biomass and hence
of decomposing organic material. Higher temperatures are likely to increase
the rate of decomposition of organic material; this loss may be partially offset,
however, by the small observed increases in primary production that can occur
with increased atmospheric carbon dioxide levels. Increases in soil carbon as
a result of improved stock and pasture management are likely to be significant
in Australian rangelands (Ash et al., 1996).
Soil water availability is likely to be the most important limiting factor
for productivity in much of the region, especially in inland Australia and eastern
districts of New Zealand. Soil water content will be affected by changes in
rainfall (increases or decreases) and by increased evapotranspiration occurring
as a result of increased temperature. Tropical forests that experience a long
dry season-such as those in northern Australia-may be more sensitive to changes
in rainfall than to changes in temperature.
Increased salinization and alkalization would occur in areas where evaporation
increased or rainfall decreased (Varallyay, 1994); this development could have
significant impacts on large areas of Australia's semi-arid zones. In areas
where salinity is a result of recharge processes, salinization would increase
if the upstream recharging rainfall increased (Peck and Allison, 1988). Increasing
atmospheric CO2 concentration can reduce the impact of salinity on plant growth
(Nicolas et al., 1993). An increased frequency of higher rainfall events would
increase soil erosion (IPCC 1996, WG II, Section 4.2.1), which would be a concern
in much of New Zealand's deforested hill country.
Rising levels of atmospheric CO2 will have a considerable impact on the growth
and morphology of plants, with likely increases in potential productivity through
increases in carbon assimilation, water-use efficiency, and possibly nutrient-use
efficiency. Increased water-use efficiency under higher CO2 conditions will
lead to higher productivity, especially in water-limited systems; but the magnitude
of the response will depend on other limiting factors such as soil nitrogen
(Eamus, 1991; Gifford, 1992). Some experiments have shown an "acclimatization"
or "acclimation" effect, in which the growth response to higher CO2 in the longer
term is less than in short-term experiments (Gunderson and Wullschleger, 1994);
whether this effect applies at the ecosystem level over many years remains untested,
In temperate zones, increased temperatures generally enhance the rate of plant
and soil biochemical processes and lead to greater plant productivity. Thus,
higher air temperatures are likely to increase plant growth in the mid-latitudes
of New Zealand and southern Australia, where productivity is currently limited
by lower temperatures. In Australia's tropical areas, however, higher temperature
stresses (above 35-40°C over extended periods) may result in more frequent damage
to the vegetation from desiccation and sunscald (IPCC 1996, WG II, Section 22.214.171.124).
In systems where C3 and C4 species co-exist, their relative proportions may
or may not change much, depending on the balance between increased photosynthesis
in C4 species at higher temperatures and increased photosynthesis in C3 species
arising from elevated CO2 concentration (Campbell and Hay, 1993).
Cloudiness has a major influence on the amount of solar radiation reaching
the Earth's surface. Any changes in cloudiness associated with changing weather
patterns would directly affect the characteristics of photosynthetically active
radiation and the amounts of solar UVB radiation, which is often detrimental
to biota and to the productivity of crop plants and trees (Hunt et al., 1996).
Many parts of Australasia experience relatively high levels of solar radiation
and solar UVB radiation. At present, climate models cannot provide reliable
predictions of how cloudiness might change in the future.