Potential Shifts in Optimum Growth Areas of Selected Commercial Tree Species and Subtropical Crops in Southern Africa Due to Global Warming

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Abstract
The area under agricultural production in southern Africa is declining as a result of industrial expansion, mining as well as formal and informal urbanization. Simultaneously, the increasing demand for timber products will necessitate a doubling of the present area under commercial afforestation in South Africa over the next 30 years. Potentially afforestable areas, however, frequently occur in the same climatic belts as certain subtropical fruit crops. Given the climatic constraints for optimum growth of commercially afforestable timber species and subtropical horticultural crops, that southern Africa is a largely semi-arid region, the fact that the areal extent of land available for agriculture is shrinking, together with the uncertainties and possible threats linked with anticipated global climate change, the identification of land areas suitable for future expansion of plantations and subtropical crops requires careful assessment. The spatial distributions of areas suited climatically to optimum growth of two commercial tree species, Eucalyptus grandis and Pinus patula, and two subtropical crops, avocado and pecan nut, were mapped for southern Africa using a climate threshold approach in conjuction with detailed gridded climate information bases, for both the present climate and for a possible future climate scenario. Results indicate that the possible impacts of a future temperature increase on the spatial distributins of optimum growth areas is species dependent, favouring E. grandis, avocado and pecan nut, but not P. patula. According to the climatic threshold model used, the timber species and horticultural crops considered produced a westward shift to `new' climatically suitable areas in the future. The results also indicate that climate change resulting from the augmented greenhouse effect may benefit the horticultural industry to a greater extent than the timber industry. Hence, competition for land suitable for future expansion of certain commerical tree species and subtropical crops may become an important consideration in the future.

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