A comparison of productive and non-productive green water-use efficiency of Podocarpus henkelii and Pinus patula in the KwaZulu-Natal Midlands

Abstract

A number of studies undertaken in South Africa to quantify the green water-use (total evaporation) of introduced commercial forestry species have shown conclusively that green water-use from commercial forest plantations is substantially higher than from the original grasslands or fynbos that were replaced by afforestation. Green water can be categorised into productive (transpiration) and non-productive (canopy and litter interception and soil evaporation) fluxes. There is a widespread perception within South Africa that indigenous tree species, in contrast to commercial forestry genera/species, are water-wise and should thus be planted more extensively in view of their more efficient use of water. However, information on the water-use of indigenous trees and forests is scarce and indirect, and the relative contributions of transpiration, canopy interception and litter interception to total evaporation have until now not been investigated in South Africa. To quantify these fluxes, both field measurements and modelling were undertaken. In this study, green water-use by indigenous Podocarpus henkelii and an exotic species, Pinus patula, were compared. The results from this study showed that the productive green water-use by P. henkelii and P. patula was 41.0% and 95.9% of gross precipitation, respectively, over the 18-month period of this study. The non-productive canopy and litter interception by P. henkelii accounted for 29.8% and 6.2%, respectively, while canopy and litter interception accounted for 22.1% and 10.7%, respectively, for P. patula. The productive green water-use efficiency (WUE) of P. henkelli and P. patula is 7.14 g mm-1 and 25.21 g mm-1, respectively, in comparison with the total green WUE of 3.8 g mm-1 and 18.8 g mm-1. From a water resources management and planning perspective it is important to consider the total green WUE, but also to have a good understanding of the relative contributions of each component of the green water fluxes so that water abstracted from the soil can be differentiated from the water that does not reach the soil due to losses of canopy and litter interception and does not get lumped as one evaporative loss.