The East Gippsland Silvicultural Systems Project. III: Site occupancy, species composition and growth to 12 years

Abstract

Summary A major silvicultural experiment was established in lowland forest in East Gippsland in 1989 and 1990 to evaluate alternative silvicultural systems. A range of harvesting and site preparation treatments was applied in a replicated design over two seasons. Harvesting treatments were based on a series of gap sizes (0, 0.03, 0.25 and 1-ha gaps, 4 and 10-ha clearfells) and retained overwood (clearfell and 7,22,35 and 100% retained basal area). Results of site preparation by slash-burn and by mechanical disturbance are compared. A primary criterion for evaluation of these treatments was the success of eucalypt regeneration, including site occupancy, species composition and early growth. Regrowth density, species, diameter and height were measured at ages 4,5,6 and 10 y after site preparation using small, dispersed plots. All harvesting treatments had a satisfactory level of site occupancy, but the level of site occupancy was lowest in the 10- ha clearfell at age 10 y. All harvesting treatments showed a significant shift in species mix of regrowth in favour of Eucalyptus sieberi, but this shift was less pronounced in the 7% retained overwood and clearfell treatments. Thus, the more intensive harvesting treatments stand out as being superior from the point of view of maintaining species composition. Rates of diameter and height growth in the regenerating stands were reduced by increasing levels of retained overwood and decreasing gap size. Regrowth basal area and volume were measured in large plots 12 y after site preparation. It was found that the initially-lower levels of site occupancy in the 10-ha clearfell may increase the rate of merchantable volume production in the long term. Regrowth basal area and volume growth increased with increasing gap size and decreasing retained overwood. Inclusion of the growth of retained overwood increased the ranking of the 35% overwood over the 22% retained overwood treatment. The differences between site preparation by slash-burn and mechanical disturbance were marginal. Although regrowth densities varied significantly at age 10 y in favour of mechanically-disturbed treatments, this did not translate into higher basal area or volume per hectare in these coupes. The initial competitive growth advantage of the slash-burnt treatments appears not to have been sustained.