4.3.1.2. 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, however.

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 1.4.3.2). 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.

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