Accelerated decay testing methods for Australian hardwoods: Evaluating shallow preservative treatments in Eucalyptus nitens

Juan Vargas Garcia

doi:10.25907/00993

Eucalyptus nitens (shining gum) is one of the two major plantation species in Australia, particularly in Tasmania, where it comprises nearly 70% of the plantation estate. Although initially established for pulp and paper production, changes in global pulp markets, the closure of processing facilities, and environmental policy shifts have redirected attention to its potential for high-value solid wood products. However, the heartwood of E. nitens is classified as non-durable, limiting its use in structural or outdoor environments unless treated with preservatives. A key challenge for commercial use in treated products is the refractory nature of E. nitens heartwood, which inhibits uniform and deep preservative penetration. Current standards for durability testing, including AWPA E10, EN 113, and protocols developed by the Australasian Wood Preservation Committee (AWPC), assume full and consistent preservative uptake—conditions not representative of treatment outcomes in E. nitens. In addition, standard methods fail to simulate long-term field degradation, particularly under conditions where surface checks form and create entry points for fungal decay. An urgent need exists for alternative laboratory methods that can simulate real-world treatment patterns and degradation processes, while enabling rapid and reproducible assessment of decay resistance. This thesis addressed this gap by designing a laboratory method to assess the ability of shallow preservative treatments in E. nitens to limit fungal attack. The project also evaluated redistribution of preservatives under artificial weathering and determined copper tolerance thresholds for a highly aggressive brown-rot fungus, Fomitopsis ostreiformis. These efforts provided new knowledge to support development of effective treatments for refractory hardwoods intended for above-ground use. A modified agar-based laboratory decay test was developed using F. ostreiformis to simulate fungal colonisation in shallow-treated wood with surface checks. This test accounted for uneven preservative distribution and enabled consistent replication under controlled conditions. The study also evaluated whether the Australian standard benchmark of 5 mm preservative penetration could be reduced without compromising fungal resistance. Preservative mobility was investigated by subjecting CCA-treated E. nitens samples to accelerated wet-dry weathering cycles. Elemental mapping confirmed migration of copper and arsenic toward surface checks, while chromium remained immobile. However, copper concentrations in checks remained below known antifungal thresholds reported in softwood studies, indicating limited protective value unless higher initial retentions are achieved. The chemical threshold of F. ostreiformis to copper was determined by treating Pinus radiata blocks with varying copper concentrations and exposing them to fungal decay under both leached and non-leached conditions. Two complementary approaches were used to estimate threshold concentrations: (1) a power regression model following the AWPC protocols, and (2) a graphical interpolation method based on the American Wood Protection Association (AWPA) Standard E10. The AWPC model estimated copper thresholds of 3160 ppm for 5% mass loss and 9360 ppm for 3% mass loss, while the AWPA interpolation method indicated a more conservative threshold of 1757 ppm to achieve biologically meaningful protection. All values substantially exceeded copper levels typical in H3- classified treated wood (~700 ppm), highlighting the strong copper tolerance of this fungal isolate. Notably, leached samples consistently showed greater decay resistance than nonleached samples, likely due to the removal of easily metabolised sugars and extractives, and possible redistribution of bioavailable copper to the wood surface. This suggests that pre-test leaching, and preservative redistribution can meaningfully influence decay outcomes.

Accelerated decay testing methods for Australian hardwoods: Evaluating shallow preservative treatments in Eucalyptus nitens

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