The extent and causes of decline in productivity from first to second rotation blue gum plantations
Blue gum plantations in south western Australia are showing second rotation productivity decline that is as much as 50 per cent lower than in the first rotation. Using existing experimental data and productivity data from industry, this research has quantified the extent and severity of second rotation productivity and examined the primary causes. The report is in two parts; the first developed empirical growth curves and the second determined the main causes of any observed decline.
The results show that second rotation productivity is related to rainfall variability. During the first rotation trees accessed water stored in the soil, but by the second this water had decreased and trees were more dependent on rainfall. As a consequence, the second rotation trees were slightly more water-stressed, which made them more vulnerable to insect attack, particularly at the drier sites. While declines in soil nutrients are not having an affect on second rotation sites there are indications they are influencing third rotation productivity.
In light of these results, plantation managers should consider strategies that reduce second rotation water stress such as a fallow, selection of drought avoiding material and variation in stocking density. Management of harvest residue will also be important in the longer term.