Yield potential of selected Eucalyptus genetic stock on the sandy soils of the South African Cape West Coast
Land available for afforestation in the Western Cape Region has largely been subordinate to agriculture. Eucalypt species have never been tested intensively along the West Coast. Climatic matching suggests planting of species from West Australia as native habitat introductions, or from seed sources in the Mediterranean region where eucalypt species have been tried successfully, such as Morocco and Israel. Promising growth of eucalypt hybrids on marginal sites in the summer rainfall areas of South Africa prompted the inclusion of local as well as some hybrid seedlots obtained from Morocco. The terrain of the trial site is an almost flat old dune area (slopes < 3%) with a westerly aspect and 7 km from the sea. The mean annual rainfall is about 400 mm, with most rain falling during winter. The other months are usually windy and dry, and the summer months are hot (maximum temperature > 35°C). The sandy soils have a coarse texture within the normal rooting depth. During winter water tables were observed on all soils within 2-3 m from the soil surface. The trial was planted during the winter of 1991. It consisted of ten local hybrid families of different species involving Eucalyptus grandis, 14 E. camaldulensis seedlots from Australia, Morocco and Israel, 7 hybrid lots from Morocco, 4 E. grandis ex Morocco and 11 other species including E. gomphocephala and E. cladocalyx. The results indicate the potential for correct species/provenance selection. Potential yields of 8 to 10 m3/annum/ha are predicted on the assumption that good genetic stock and correct silvicultural procedures are used. Most notable was the expected performance of the well known West Australian species (E. gomphocephala, E. cladocalyx). The group of 14 E. camaldulensis seedlots can be regarded as representing a series of provenances from Australia, except for three from Israel and one from Morocco. As expected, the different provenances survived well but performed variably, generally ranking in the bottom 50 %. However, two seedlots - the provenance from Lake Albacutya and the seedlot from Morocco - are worth noting as they actually ranked among the top ten for index score. Generally, the provenances from Queensland (e.g. Petford) and New South Wales that normally do well in summer rainfall regions did not do well. As the relevant parent species originate in summer rainfall regions, the phenomenal growth rate of many of the hybrid families came as a surprise in this trial. The yield of the best E. grandis x E. camaldulensis (GXC) family would have been equivalent to that of E. gomphocephala if the survival was equally good. The E. grandis x E. tereticornis (GXT) family did survive and performed equally well. The second best GXC family would have yielded even better than the GXT if it had a better survival. The GXC and GXT families were all within the top 50 % of the trial.