Pine Plantations Keep Soil Fertile

"The soils under pines have been found to store as much carbon as pasture soils. This suggests that replacing pasture with pine plantations will not lead to a long-term reduction in soil carbon or a large net release of carbon to the atmosphere with adverse greenhouse effects, as some feared," says CSIRO's Dr Clive Carlyle, who leads the research.

The findings come from research by CSIRO and the Queensland Forest Research Institute in the 'Green Triangle' - a concentrated area of mostly pine plantations centered on Mt Gambier, South Australia, and near Gympie, Queensland.

For the pine/native forest comparisons, researchers selected about 20 sites where plantations - radiata pine (Pinus radiata) in the Mt Gambier region and slash pine (P. elliottii) in Queensland - meet native eucalypt forest. The plantations, mostly first rotation, were in the 23-37 years age range.

The scientists analysed soil samples for a wide range of chemical properties affecting soil fertility - notably organic matter content, levels of the major nutrients nitrogen and phosphorus, and soil acidity.

"We found no major changes in soil properties - in fact some have improved," says Dr Carlyle.

"The key finding, though, was the lack of a major difference in organic matter levels - in fact, these were marginally higher in the plantation soil. This has important implications for carbon accounting and carbon credits as it indicates that the long-term effect of converting land from pasture to pine is neutral," he says.

Researchers found the average levels of organic matter under pine plantations were around 16% higher than in native forest soil.

Dr Carlyle suggests this is a consequence of the higher productivity of the plantations.

"Under the management systems developed for the Green Triangle region, pine productivity is currently increasing from one rotation to the next, making the plantations there a good example of sustainable forestry", he says.

Research also found that pine and native forest sites showed no difference in nitrogen levels; an important result in the light of earlier concerns about declining nitrogen levels under plantations. Phosphorus levels were substantially higher under pine because of the use of phosphorus fertilisers.

"The plantation soils were slightly more acid, but their acid levels remained safely in the range where any acidity impact on a subsequent non-tree crop would be negligible," says Dr Carlyle

"Comparisons between pine and pasture showed much higher levels of phosphorus and nitrogen under pasture than under the plantations."

According to Dr Carlyle, this is the result of nitrogen fixation by clover and frequent applications of superphosphate in the improved pasture.

The Forest and Wood Products R&D Corporation funded this project, which began in 1998.