Abstract:
In South Africa the invasion of riparian forests by
alien trees has the potential to affect the country’s limited
water resources. Tree water-use measurements have therefore
become an important component of recent hydrological
studies. It is difficult for South African government initiatives,
such as the Working for Water (WfW) alien clearing
programme, to justify alien tree removal and implement rehabilitation
unless hydrological benefits are known.
The objective of this study was to investigate the water
use (transpiration rates) of a selection of introduced and indigenous
tree species and quantify the hydrological benefit
that could be achieved through a suitable rehabilitation
programme. Consequently water use within a riparian forest
in the upper Mgeni catchment of KwaZulu-Natal in South
Africa was monitored over a 2-year period. The site consisted
of an indigenous stand of eastern mistbelt forest that
had been invaded by Acacia mearnsii, Eucalyptus nitens and
Solanum mauritianum. The heat ratio method of the heat
pulse velocity (HPV) sap flow technique and the stem steady
state (SSS) techniques were used to measure the sap flow of a
selection of indigenous and introduced species. The indigenous
trees at New Forest, South Africa, showed clear seasonal
trends in the daily sap flow rates varying from 8 to
25 L day 1 in summer (sap flow being directly proportional
to tree size). In the winter periods this was reduced to between
3 and 6 L day 1 when limited energy flux was available
to drive the transpiration process. The water use in the
A. mearnsii and E. grandis trees showed a slight seasonal trend, with a high flow during the winter months in contrast
to the indigenous species. The water use in the understorey indicated that multi-stemmed species used up to 12 L day 1.
Small alien trees (< 30 mm) A. mearnsii and S. mauritianum
used up to 4 L day 1 each. The total accumulated sap flow
per year for the three individual A. mearnsii and E. grandis
trees was 6548 and 7405 L a 1 respectively. In contrast,
the indigenous species averaged 2934 L a 1, clearly demonstrating
the higher water use of the introduced species. After
spatial upscaling, it was concluded that, at the current state
of invasion (21% of the stand being alien species), the stand
used 40% more water per unit area than if the stand were in
a pristine state. If the stand were to be heavily invaded at the
same stem density of the indigenous forest, a 100% increase
in water use would occur over an average rainfall year.
