Growth potential of Eucalyptus globulus subsp. bicostata provenances for the mid-altitude summer rainfall regions of South Africa

Abstract

Eucalyptus grandis is cultivated extensively in the humid, warmer temperate and subtropical regions in South Africa. With forestry expanding into mid-altitude drier and warmer, or drier and colder, sites the search to identify reasonably well-performing species/provenances for such ‘low productivity’ sites has extended to summer-rainfall provenances of E. globulus subsp. bicostata, which is closely related to E. globulus, a species known to have superior kraft pulping. Seed from New South Wales, Australia, provenances Narrow Neck Ridge, Nullo Mountain (West) and Nullo Mountain (North) was established in provenance/progeny trials on four sites in South Africa (Windy Gap, Enon, Petrusvlei and Speenkoppies). Eucalyptus globulus and E. globulus subsp. maidenii were the internal controls. Trials were planted at 1 667 stems ha–1, in single-row plots of six trees, with four replications in an unbalanced lattice design. Commercial seed of other eucalypts and clones was used as external controls. Mycosphaerella nobilosa susceptibility was measured at 12 and 30 months, and diameter at breast height (dbh) and height were measured at 80 months at Petrusvlei, 84 months at Enon, and 96 months at Windy Gap and Speenkoppies. Basal area and volumes were derived from these measurements. Mycosphaerella nobilosa infection was more severe at 12 months than 30 months. The Narrow Neck Ridge provenance, which was significantly less susceptible than other provenances and controls, also outperformed other E. globulus subsp. bicostata provenances for dbh, height, basal area and volume. The top Narrow Neck Ridge provenance families performed better than, or similar to, the best commercial controls for basal area and volume at three of the four sites tested. The inhibitory effect of M. nobilosa on growth emphasises the importance of site–species and site–provenance matching. Amongst the imported E. globulus subsp. bicostata material, the more M. nobilosa-tolerant Narrow Neck Ridge provenance was most suited to summer-rainfall, mid-altitude South African sites.

Keywords: Eucalyptus globulus subsp. bicostata, provenance/progeny trials, tree improvement