Effects of periodic drought on Acacia mangium Willd. and Acacia auriculiformis A. Cunn ex Benth growing on sand tailings in Malaysia
This study aims: 1) To quantify the factors affecting the changes of site water status of the high sand tailings, 2) To investigate the growth and physiological responses of Acacia mangium and A. auriculiformis to the fluctuations of site water status on high sand dunes, and 3) To identify characteristics which may allow the trees to adapt to and survive on high sand tailings. In both shade and open plots on sand tailings, diurnal air water deficits would be anticipated particularly in sunny days and late afternoon in cloudy days. The results from dipwell showed that the main source of soil water supply to the sand tailings is solely dependent on the rainfall and not from surrounding mining pools. Furthermore, at 6 d after rainfall <37 mm, sand tailings at 0-15 cm depth had low sand suction showing soil water deficit to plant uptake. Hence, the rainfall distribution would determine the growth of acacias in sand tailings. Dry periods significantly reduced (P<0.05) the shoot length and stem diameter growth rates and leaf area of both acacias. Dry periods also significantly increased leaf and branch number shedding rates of both acacias. Generally, shading did not improve the growth rates of both acacias in dry periods but significantly delayed leaf shedding of the two acacias in dry periods. A. auriculiformis had significantly higher shoot length and stem diameter growth rates as the result of higher leaf and shoot numbers. The superiority of A. auriculiformis compared to A. mangium in adapting to high sand tailings was explained by its growth patterns that had characteristics of drought avoidance, and/or tolerance. Greater vegetative growth was observed in A. auriculiformis than A. mangium in wet periods. The effects of atmospheric and soil water fluctuations on some physiological responses of the two species were also examined.