Investigating the environmental adaptability of promising subtropical and cold-tolerant eucalypt species in the warm temperate climate zone of KwaZulu-Natal, South Africa

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During 2001, a series of four site-species interaction trials was established by the ICFR (Institute for Commercial Forestry Research) in the warm temperate climate zone of KwaZulu-Natal, South Africa (mean annual temperature (MAT) range 16–19°C), to investigate the relative growth performances of a range of promising new eucalypt species. Trees were assessed for frost sensitivity at 15 months, and height and basal area at 48 months. Eucalyptus longirostrata, E. pilularis, Corymbia citriodora subsp. citriodora, C. henryi and C. maculata, species that performed well in forerunner ICFR coastal Zululand site-species trials (MAT range 21.7–21.9°C), generally showed poor adaptation to the cooler, inland conditions. Bacterial blight (Pantoea ananatis) impeded early growth in members of Eucalyptus subseries Punctatosae (E. punctata, E. biturbinata and E. longirostrata) and E. urophylla at the two coolest sites, Rooipoort (15.7°C MAT) and Rockvale (16.3°C MAT). The cold-tolerant eucalypts E. badjensis and E. smithii averaged good growth across sites, but showed susceptibility to Phytophthora root-rot disease where MAT exceeded 17.0°C. The planting of either species for commercial purpose under similar conditions in the warm temperate zone would likely carry with it a substantial risk. Eucalyptus benthamii and E. dorrigoensis, species that performed well in earlier ICFR high-altitude site-species trials (MAT range 14.2–15.5°C), showed excellent adaptation to the KwaZulu-Natal warm temperate conditions. These two species, as well as E. oreades (the third best-performing new species), appear good candidate replacement species for the commercial controls in the region, namely E. grandis, E. dunnii, E. smithii and E. macarthurii.