Temperature and the Vegetative Growth of Two Desert Acacias

In the Sudan, Acacia nilotica L. Wild ex Del. is restricted to potentially productive aresearch-articlend comparatively moist habitats, while A. torrilis (Forssk.) Hayne is a typical hot-desert plant. To see if temperature itself played any part in limiting these species to their contrasted habitats, seedlings of both species were grown under controlled-environment conditions over a range of different temperatures. Throughout the experiments growth was largely exponential and so, to develop a uniform series of temperature-time response surfaces, families of growth-analytical variates were combined into multiple regression equations of the general form, variate =f(temperature, time). From an inspection of the plotted surfaces it was reasonably clear that, relative to one another, the two species differed sufficiently in their physiology and morphology for these differences to be advantageous in each of their natural habitats: A nilotica had a superior relative growth rate at day temperatures below 18°C and its seed weight and shoot morphology were advantageous to establishment in productive, closed communities, whereas A. tortilis had a superior relative growth rate at higher temperatures and its inherently higher root/shoot ratio and more compact shoot morphology were considered to be advantageous to life in frequently-droughted, open communities. The two species were assessed in relation to J. P. Grime's recently-developed theories on plant ‘strategies’. It was concluded that, within this framework, the distribution and experimental performance of A. nilotica suggested its classification as a ‘stress-tolerant competitor’ and that of A. tortilis, its classification as a ‘stress-tolerant ruderal’.