Options for seedbed preparation and seed supply for the regeneration of mixed eucalypt forests in north-eastern Victoria.  J. Di Stefano, J. Costenaro, R. Campbell and O. Bassett.  October 2010.  41pp. (unpubl.)

SUMMARY

Regeneration of forest trees is a primary objective of native forest silviculture and a variety of techniques are available to help foresters achieve success. The creation of a good seedbed by either burning or mechanical disturbance is central to the success of eucalypt seedling establishment.

The main objective of this study was to investigate the potential of pre-harvest soil disturbance and extended mechanical soil disturbance during harvest, as an aid to regeneration success. A secondary objective was to determine the contribution that unburnt slash made to seed supply and to eucalypt regeneration.

Silvicultural treatments, varying with respect to the degree of post-harvest tree retention, seed source, and seedbed preparation technique (either mechanical disturbance before or during harvesting, or slash burning) were applied to areas within three native forest coupes in north-eastern Victoria. The trial aimed to determine which of these and other site-level factors correlated most strongly with eucalypt regeneration success, which was measured as both the percentage of stocked 16 m² plots and seedling density. Regeneration was also evaluated in terms of the operational stocking standard, that is, plots with an "acceptable seedling" at 18 months or older.

Variability between sites made it difficult to make site performance comparisons. Silviculture system varied, the technique of treatment varied and seed supply was not consistent between sites. However, each site produced at least one treatment combination that was successful and these successes are discussed in the context of the relevant site.

Univariate correlations indicated that burnt area, bracken cover, seed supply, the proportion of plots containing receptive seedbed, and sparse slash cover had the strongest association with the two measures of regeneration success. Inclusion of these variables in a Principal Components Analysis indicated that sites with higher seed supply and more burnt area tended to have higher stocking and density values compared with sites with higher levels of bracken and sparse slash which tended to be less well regenerated. All four variables were moderately correlated with the first Principal Component suggesting no single factor dominated the regeneration outcome. Interpretation of the results suggested that seed supply and seedbed conditions are the variables most related to regeneration success. Nevertheless, regeneration success can be influenced by a number of different factors, and it is important to understand site-specific influences and to develop site-specific guidelines for regeneration success.

At the seed tree coupe where the slash seed experiment was conducted, 92% of seed present in the canopy prior to harvesting reached the ground as loose capsules or capsules attached to piles of slash. On average, plots under slash piles received 68 additional viable seeds per m² compared with plots not exposed to slash. Although this additional seed supply resulted in more germinants and small seedlings in the first few months after harvesting, most died over time. Nevertheless, developing techniques for maximising the contribution of slash seed to eucalypt regeneration on unburnt sites is seen as a worthwhile objective.