SUSTAINING PLANTATION PRODUCTIVITY

At the two extreme edges of Southern Australia - the Southwest of Western Australia and Northern Tasmania, substantial areas of E. globulus are now ready to harvest and research is centring on concerns that subsequent rotations retain high productivity levels.

Plantings of "bluegums" began in the mid-1980's, on former farmland with annual rainfall exceeding 700mm. Planting rates accelerated during the 1990's, mainly by forestry investors on land leased from farmers, and now totals over 100,000ha in WA and growing at 20,000 ha per year.

Favourable temperatures and access to water stored in deep soil profiles encourages fast growth. Also boosting productivity is the contribution made to soil fertility by fertilisers and pasture legumes in the years before the land was given over to plantations. A concern now is that with these inputs removed following tree planting, fertility and plantation productivity may decline over time.

CSIRO Forestry and Forest Products have set up a series of experiments near Manjimup and Busselton, WA, beginning in 1995. They are examining the impact of different management practices following first harvest and second and second rotation establishment. Also they are studying the effects of, plantations on soil Nutrient levels, by monitoring sites to compare the phosphorous, nitrogen and carbon status of soil under first rotation plantations, compared to land still managed for pasture.

Foresters from DPI Queensland and North Forest Products, Tasmania, inspecting the Savannah 1242 V-Rake mounted to a TD20 Dozer and the 1030 V-Shear mounted to D7G, at a comparison field day on 'one-pass' site preparation at Beerburrum Qld. last summer.

On sites clearfelled in 1994 and 1995 and replanted, researchers are comparing effects on soil fertility and tree growth from alternate methods of site preparation. These include retaining harvest residues or burning them and planting legumes between rows to boost nitrogen supplies. A key finding is that burning results in major nitrogen loss - more than 500kg per hectare form more fertile sites. Residue retention has an immediate positive impact on tree growth, which is being assessed on a long term basis.

A new set of experiments begun last year is examining practical ways to make merchandised forest operations compatible with fertility protection. This involves a range of options for replanting without removing harvest residues, including spot cultivation, chopper rolling, ripping only on rows, and V-blading residues onto inter-row areas and then ripping the planted rows. Researchers plan to monitor tree growth and soil fertility throughout the second rotation to see how effective each treatment proves to be and how they can be applied operationally.

In Tasmania, North Forest Products are carrying out their own series of experiments on residue retention and site prep operations, in the expectation that burning will ultimately be outlawed in Tasmania. A recent demo day with North Forest and DPI foresters was held on a DPI Forestry site in Queensland, to evaluate the alternative uses of a dozer-mounted V-shear and V-rake with following plow, to provide "onepass" ripping and plowing on a pine site.