Abstract
Invasive alien plants (IAPs) pose a serious threat to the already limited water resources in dry countries like South Africa which are facing increasing water shortages. Much of South Africa is expected to get drier in future due to climate change. In addition, the future climatic conditions are also predicted to accelerate the rate at which IAPs will spread, due to favourable growing conditions, further disrupting the provision of goods and services. Previous studies on tree water use in South Africa focused on commercial forests of introduced genera mainly Pinus, Eucalyptus, and Acacia. This study sought to expand these observations by quantifying water use and its drivers in riparian Grey Poplar (Populus canescens) invasions in the Berg River catchment of South Africa. Whole tree hydraulic resistance ranged from ~ 1.4 MPa·h·g-1 for large trees to ~14.3 MPa·h·g-1 for the small ones. These resistances are higher than those found for poplars in temperate climates, suggesting substantial hydraulic constraints to transpiration. Daily peak transpiration varied from 5 to 6 L·tree-1 in small trees (~9.2 cm DBH) to between 35 and 40 L·tree-1 for large trees (~24 cm DBH). Stand-level transpiration peaked at ~4 mm·d-1 in summer (Jan–Feb). However, the annual total transpiration was only 338 mm due to the deciduous nature of the species and also the high hydraulic resistance in the transpiration stream. Daily transpiration was strongly correlated to solar radiation (R2 > 0.81) while the air vapour pressure deficit (VPD) constrained transpiration at high VPD values. We conclude that the water use of the poplar invasions is significantly lower than that of other riparian invasions. The impact of these invasions on the water resources is therefore likely quite low, warranting less priority in alien plant clearing operations aimed at salvaging water.
Highlights
The total area infested by invasive alien plants (IAPs) in South Africa is over 100 000 km2 (Le Maitre et al, 2016; Van Wilgen et al, 2001) which is over 8% of the country’s total area
We investigate the role of various climate driving variables and physiological factors, principally the hydraulic characteristics of the transpiration stream of the invasions
This study shows that P. canescens invasions growing along a river channel in the Western Cape have a fairly conservative impact on the water resources
Summary
The total area infested by invasive alien plants (IAPs) in South Africa is over 100 000 km (Le Maitre et al, 2016; Van Wilgen et al, 2001) which is over 8% of the country’s total area. These invasions are mostly concentrated in the wetter parts of the country which are key sources of water for the country’s major rivers. The majority of tree water-use studies in South Africa focused on single-species commercial plantations of introduced tree genera such as Pinus, Eucalyptus and Acacia (Gush et al, 2002; Dye and Versfeld, 2007; Scott and Prinsloo, 2008; Clulow et al, 2011). The values obtained are subsequently compared with those found in other climatic regions
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