POTENTIAL SHIFTS IN OPTIMUM GROWTH AREAS OF SELECTED COMMERCIAL TREE SPECIES AND SUBTROPICAL CROPS IN SOUTHERN AFRICA DUE TO GLOBAL WARMING

The area under agricultural production in southern Africa is declining as a result of industrial expansion, mining as well as formal and inf ormal urbanization. Simultaneously, the increasing demand for timber p roducts will necessitate a doubling of the present area under commerci al afforestation in South Africa over the next 30 years. Potentially a fforestable areas, however, frequently occur in the same climatic belt s as certain subtropical fruit crops. Given the climatic constraints f or optimum growth of commercially afforestable timber species and subt ropical horticultural crops, that southern Africa is a largely semi-ar id region, the fact that the areal extent of land available for agricu lture is shrinking, together with the uncertainties and possible threa ts linked with anticipated global climate change, the identification o f land areas suitable for future expansion of plantations and subtropi cal crops requires careful assessment. The spatial distributions of ar eas suited climatically to optimum growth of two commercial tree speci es, Eucalyptus grandis and Pinus patula, and two subtropical crops, av ocado and pecan nut, were mapped for southern Africa using a climate t hreshold approach in conjunction with detailed gridded climate informa tion bases, for both the present climate and for a possible future cli mate scenario. Results indicate that the possible impacts of a future temperature-increase on the spatial distributions of optimum growth ar eas is species dependent, favouring E. grandis, avocado and pecan nut, but not P. patula. According to the climatic threshold model used, th e timber species and horticultural crops considered produced a westwar d shift to 'new' climatically suitable areas in the future. The result s also indicate that climate change resulting from the augmented green house effect may benefit the horticultural industry to a greater exten t than the timber industry. Hence, competition for land suitable for f uture expansion of certain commercial tree species and subtropical cro ps may become an important consideration in the future.