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-producin g forests. Recognition early in the 20 th century that demand for timber had exceeded the su pply available from indigenous forests forced South Africa to accelerat e the expansion of its own exotic plantation forest industry. This then resulted in c oncerns about impacts on water resources, and led to regulation of the industry. N umerous local and international studies have subsequently proven that exotic forest plantat ions do consume more water than the grasslands or scrublands they typically replace, an d 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 forestr y while still conserving our scarce water resources. However, data on the water-use of indigenous trees and forests are scarce and indirect. To broaden understanding of both the growth and water-use rates of indigenous tree species, research was undertaken on their water-use efficiency (WUE), defined as mass of utilisable wood produced per uni t of water transpired. Hourly sap flow rates (water use) over a 12-month period were recor ded in a diverse selection of six indigenous tree species, while stem and branch dime nsions were recorded at fixed positions on the trees 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 existing data for exotic plantation species. WUE in the indigenous species studied was lower than for e xotic plantation species, however overall water-use was also low in the indigenous sp ecies. It was concluded that the relatively lower WUE of the indigenous species stud ied was more a consequence of slow growth rates as opposed to high water-use rates.