The ecology of ultramafic areas in Zimbabwe

Abstract

Studies of the vegetation of ultramafic soils in Zimbabwe have concentrated on and around the Great Dyke, one of the world’s largest outcrops of ultramafic rocks. This area occurs under a wet-dry tropical climate with annual rainfalls ranging from about 450 to 880 mm. The ultramafic vegetation studied occurs mainly at altitudes from about 1000–1500 m and some is subject to frost. The soils have fairly high nickel concentrations and rarely have exceptionally low calcium concentrations. The vegetation of these soils is generally grassland or open shrubland and is often in marked contrast with well-developed woodland on adjacent non-ultramafic soils. Along river banks on ultramafic soils there is a well-developed riverine forest. Since these forest soils show no substantial chemical differences from those under more open vegetation it is suggested that hydrological factors may be very important in determining physiognomy. The flora of the ultramafic soils is relatively species-poor but includes about twenty important endemics. There have been many chemical analyses of native plants and these have shown that although there are few nickel accumulators, one species Pearsonia metallifera (Leguminosae) had a recorded 14,100 µg g nickel in its leaf dry matter. Experimental work on crop species has apparently demonstrated the existence of acute nickel toxicity on several Zimbabwean ultramafic soils. A very important study made on Zea mays has clarified many aspects of the interactions between nickel, magnesium and calcium supplied in water culture. Zimbabwean work on native species has shown tolerance of nickel in two species and in the case of the non-tolerant tree, Brachystegia spiciformis has given clues about the precise toxic effects of this element.