Provenance variation, genetic parameters, and potential gains in Eucalyptus viminalis subsp. nobilis tested in two high altitude cool temperate sites in the summer rainfall region of South Africa

Abstract

Over the past 50 years, commercial forestry in South Africa has expanded into mid and high altitude summer rainfall regions, predominantly planting Eucalyptus nitens and Eucalyptus macarthurii. However, the suitability of these sites is increasingly threatened by projected climate changes that are expected to result in warmer temperatures and reduced productivity. This study aimed to evaluate the potential of Eucalyptus viminalis subsp. nobilis (referred to as E. nobilis) as an alternative species at two high-altitude sites, Windy Gap and The Bends, in South Africa. Trials were established using a randomised complete block design with four replicates. Diameter at breast height (DBH) and height were two growth traits measured at 66 months from which basal area (BA) and volume (VOL) were calculated. In singlesite analyses, four Australian provenances, Butter Leaf, Forest Land, Chaelundi and Styx River, consistently ranked among the top performers in both trials. Type B correlations indicated that genotype × environment (GEI) interactions were negligible, justifying across-site analyses of genetic parameters. The narrow-sense heritabilities within provenances were relatively high, with estimates ranging from 0.30 to 0.35 for the three growth traits, suggesting moderate to substantial genetic gains through provenance and family selection. Additionally, the wide range of within provenance additive genetic variance highlights the potential for significant genetic gains in E. nobilis through both within- and among-family selection. Estimated genetic gains for the top ranking provenances ranged from 2% to 10%. The findings suggest that a single, multi-provenance breeding population would be suitable for E. nobilis in South Africa, with DBH serving as a cost-effective and reliable measure of growth in the breeding programme.