The role of salinity and sodicity in the dieback of Acacia xanthophloea in Ngorongoro Caldera, Tanzania
Dieback of Acacia xanthophloea (Benth.) has opened up the once densely forested Lerai area in Ngorongoro Caldera, Tanzania. Soil samples were taken from profiles in the Ngorongoro Conservation Area and Lake Manyara National Park at sites of dieback and at sites with healthy A. xanthophloea trees. Dieback sites had significantly greater electrical conductivity (EC), water-soluble Na+, K+, Cl-, SO and sodium adsorption ratios (SAR) than healthy sites. The following mean values were recorded: EC (179 versus 70 mS m-1; P < 0.001, Student's t-test, n = 8 and 10, respectively; 40-60 cm); Na+ (99 versus 30 mmolc kg -1, P < 0.001, n = 7 and 8 respectively); K+ (11 versus 3 mmolc kg-1, P < 0.05); Cl- (36 versus 7 mmolc kg-1, P < 0.01); SO (31 versus 5 mmolc kg-1, P < 0.01); and SAR (28 versus 8 mmol l-1/2, P < 0.01). Water-soluble Na+, Cl- and SO concentrations in the Lerai profiles have probably resulted in toxicity and osmotic stress which contributed to dieback. Accumulation of salts may have occurred because of reduced flow of freshwater through Lerai and/or flow of water from Lake Magadi into Lerai. Forest recovery may be possible if salinity is reduced. Management strategies for reducing salinity have been implemented and included re-establishing streams that flow through Lerai. Exclusion of elephants (Loxodonta africana) from Lerai is another management strategy presently under consideration. © 2006 East African Wild Life Society.