A decadal multi-site study of the effects of frequency and season of harvest on biomass production from mallee eucalypts

Mallee eucalypts are hardy, woody perennials that are being developed as a short-rotation coppice crop in Australia for the production of eucalyptus oil, biofuels and other biomass products. The economic viability of this prospective crop is dependent on its ability to survive and regenerate following repeated harvesting of the above ground component. Here we report on survival and biomass yield of mallee belt plantings of Eucalyptus polybractea, E. loxophleba ssp lissophloia and E. kochii ssp plenissima, at 19 sites, under two harvest-frequencies (3–8 year cycles) and harvest seasons (autumn or spring) over a decade from 2006 to 2015. 16 sites had post-harvest mortality ranging from 1.0% to 12.2% while the remaining three sites with either shallow saline water tables or a silcrete hardpan failed. Average site dry biomass yield across treatments ranged from 2.2 to 32.8 Mg ha⁻¹ yr⁻¹. Higher yielding sites were generally characterised by pH between 3.8 and 8, ECₑ below 15.0 dS m⁻¹ and high soil fertility. Lower yielding sites were generally near saline valley floors. After 7-years, biomass yield from unharvested treatments exceeded the average cumulative yield of harvest treatments at eight of the 16 sites, including all three E. kochii sites. For E. loxophleba, significant interactions were found between season and frequency of harvest with highest yields in long rotation spring treatments. There were also interactions between site and frequency of harvest, which were mainly driven by the variable performance of the uncut treatment. On average E. loxophleba yielded more biomass following spring harvests whereas E. kochii yielded more following autumn harvests. E. polybractea yield was unaffected by season or frequency of harvest; however, harvest treatments yielded more biomass than uncut treatments. After 10 years, at eight of the nine sites subjected to three 3-year cycles, no decline in biomass yield was observed. The site that declined in production was attributed to depletion of a sandplain aquifer by extensive mallee plantings. Overall, the results from this decadal study indicate that in warm-temperate semi-arid areas, such as the south-west of WA, mallees biomass can be harvested sustainably at most sites even in short (3-year) rotation cycles.