Establishment of eucalypts by sowing on dieback-affected forest sites in East Gippsland.  P. C. Fagg.  April 1980.  62 pp. (unpubl.)

SUMMARY

A series of sowing trials was commenced in 1972 and 1973 on seven sites on which eucalypt crown dieback had occurred, in the lowland forest area of east Gippsland. The objective was to investigate the establishment, survival and early growth of eucalypts sown on sites infected with Phytophthora cinnamomi, the major agent causing the dieback.

Trials were established in the following order: small, medium, and large, or field-scale trials. The small-scale trial was replicated at three sites which ranged from high to low in dieback severity (prior to clearing). Another similar set of three sites was selected for the large-scale trial. The single medium-scale trial was located in a recently burnt forest exhibiting severe dieback.

In the small-scale trial, seeds of 6 indigenous eucalypts were sown on to cultivated and burnt seedbeds on each site, in May 1972. Seedlings were counted and their heights measured periodically up to May 1979. The currents of P. cinnamomi and Pythium spp. in soil samples was noted and rainfall was also recorded. In the medium-scale trial, seeds of five eucalypts were each sown on separate 0.4 ha plots, in June 1972. The seedbed had been lightly burnt and compacted. Seedlings and coppice were counted at regular intervals up to June 1979. In the large-scale trial, in May 1973, a seed mixture of four indigenous eucalypts was sown at two of the three sites. Both sites had been completely clear-felled and burnt. At the third site, 27 seed trees per ha were retained as seed sources after a partial cut, and the site was then burnt. Periodical measurements were made up to January 1978 of stocking and height of eucalypts, P. cinnamomi population levels, seedbed type, understorey vegetation and rainfall.

After seven years, results for the small-scale trial showed that seedling percents for each of the six species were satisfactory for both seedbed types, although the burnt seedbeds produced a lower percentage of seedlings. Height growth of most species was far superior on the burnt seedbeds. In the large-scale trial, 4½ years after sowing, stocking was satisfactory for all three sites despite quite high levels of P. cinnamomi infestation. The contribution made to stocking by lignotuberous coppice was substantial at the most heavily infested site. In the medium-scale trial, stocking was unsatisfactory for all species. This failure may have been due to a separate or combined effect of several factors including drought, a low amount of receptive seed bed, presence of P. cinnamomi, and severe understorey competition.

In the soils of the large-scale sites, P. cinnamomi appeared to be less frequent in the presence of dense understorey regrowth, which had been promoted by burning. Such vegetation may have reduced soil moisture and temperature to levels which were less favourable for the survival and spread of P. cinnamomi. In addition, root exudates from acacias in the understorey may have depressed P. cinnamomi population levels.

The partial-cut system, as currently practiced in east Gippsland, appeared to be a successful means of regenerating low to moderate-dieback sites with indigenous species. Early growth and vigour was poor on high-hazard sites, but mounding may overcome this problem by improving drainage on such sites. Alternatively, dieback-tolerant species, such as E. botryoides and E. globulus, could be planted.

Two commercially important species, E. sieberi and E. globoidea, which are generally susceptible to P. cinnamomi, were successfully re-established on sites representative of the full range of existing dieback hazards in east Gippsland. Forest managers should be encouraged to attempt to re-establish these two indigenous species on dieback-affected sites, given conditions similar to those over the trial period.

Although the density and growth rate of the eucalypts was satisfactory, on six of the seven experimental sites 4 to 7 years after sowing a longer study is needed to test the ability of the regeneration to grow to commercial maturity on sites known to be infected with P. cinnamomi. Eucalypts need to be tested by a combination of high summer rainfall and autumn drought, such as occurred in 1970-71.