Water-use efficiency within a selection of indigenous and exotic tree species in South Africa as determined using sap flow and biomass measurements

Abstract

South Africa has limited indigenous timber-producing forests. Recognition early in the 20th century that demand for timber had exceeded the supply available from indigenous forests forced South Africa to accelerate the expansion of its own exotic plantation forest industry. This then resulted in concerns about impacts on water resources, and led to regulation of the industry. Numerous local and international studies have subsequently proven that exotic forest plantations do consume more water than the grasslands or scrublands they typically replace, and hence reduce water yield (streamflow) from afforested catchments. Conversely, there is a widespread perception that indigenous tree species, in contrast to exotic plantation species, are water-wise and deserve to be planted more widely to expand forestry while still conserving our scarce water resources. However, data on the water-use of indigenous trees and forests is scarce and indirect. Research was undertaken to gain a broader perspective of the water-use efficiency (WUE) of a selection of indigenous tree species yielding potentially useful wood. Hourly sap flow rates (water use) over a 12-month period were recorded in a selection of indigenous tree species. Stem and branch dimensions were recorded at fixed positions at the start and end of the monitoring period, to permit whole-tree volume growth increments to be recorded. Rates of growth and water-use were used to calculate WUE and were compared to data for exotic plantation species. WUE in the indigenous species studied was comparatively low, however overall water-use was also low, making them an attractive option in water-constrained catchments.