Influence of elevated atmospheric carbon dioxide on the productivity of Australian forestry plantations

Australia produced $2.7 billion worth of forest products in 1983-84 but a further $1.3 billion worth, principally softwood, were imported. Because of this ever increasing demand for softwood, there is a move away from utilization of native hardwoods and by 2020 AD, when the atmospheric CO2 concentration is likely to be greater than 450 ppmv, 75% of forest products are projected to come from coniferous plantations. This move towards Pinus radiata Zs a result of both demand for softwood and lack of indepth investigations of the potential of Australian native species, particularly eucalypts, for plantation forestry. Pinus radiata is the major plantation softwood in southern Australia and is presently grown at sites where phosphorus deficiency and repeated episodes of drought are common. Consequently, we are investigating the growth response of pines to elevated CO2 at a range of phosphorus and water levels. When phosphorus was adequate, doubling CO2 concentration more than doubled the rate of photosynthesis and increased the total plant dry weight by about 40%. However, there was no response when phosphorus was deficient. In contrast, there was a slightly higher response under simulated drought conditions. A further possible effect of rising CO2 levels is that the climatic range of P. radiata may be altered due to a reduction in water use or an increase in the drought tolerance of the trees. We found that CO2 enrichment did not affect either of these factors but the water-use efficiency was increased when phosphorus was adequate. All families of P. radiata do not respond to CO2 enrichment in the same manner. In a study investigating the response of four families to elevated CO2 at two phosphorus levels, we have identified a considerable variation between the families in their response to CO2 and phosphorus. To date our studies have indicated that the projected increase in atmospheric CO2 levels is likely to have a significant influence on the productivity of Australia's P. radiata plantations. But this will only occur if phosphorus fertilization is adequate. If the rise in CO2 results in climatic change the range of P. radiata may be even further restricted because there will be no concomitant decrease in water use or increase in drought tolerance. There is an urgent need for complimentary studies of the response of Australian native species to elevated CO2 at realistic levels of phosphorus and water to enable more accurate prediction of the productivity and water use of Australian native forests and eucalypt plantations.