This CSIRO led research project measured and assessed changes in productivity of blue gum plantations between the first and second rotation, particularly in Western Australia. The research showed that well established forestry management principles, such as soil and nutrition management, can sustain production through multiple rotations.
Since their establishment in the early 1980s, reported growth yields in eucalypt hardwood plantations across southern Australia have varied widely, with mean annual increments ranging from 5 to 30 cubic metres per hectare. In addition, in some areas of the bluegum plantation estate there has been a 25–40% productivity decline between the first and second rotations.
The study developed predictive modelling tools that can be readily incorporated into existing company inventory systems. The model calculates basal area and volume for the second rotation on a site based on the standardised precipitation evaporation index (which takes into account rainfall and evaporation at a site) for the site together with productivity information from the first rotation.
Using observations from a sample of bluegum plantation sites in south-western Western Australia the research also mapped the actual extent and revealed some of the causes of second rotation decline.
Analysis shows much of the reported decline was due to variation in rainfall and water available in the soil between rotations. For each millimetre of soil water lost before the start of the second rotation, there is a corresponding loss in wood production of approximately 0.015 cubic metres per hectare. In the worst case this will result in a 8–10% decrease in production.
Productivity decline was also associated with a higher level of insect damage particularly on drier sites. While soil nitrogen was reduced in all sampled plots it only affected second rotation productivity on one of the sample plots. However on sites where first rotation site fertility was low, soil nutrition is highly likely to limit productivity in the second rotation.
Importantly the research finds that burning or removing forest residues after harvesting will further increase second rotation decline on sites currently nutrient limited, and will decrease available soil resources in subsequent rotations on all sites.
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