Characterising the water use and hydraulic properties of riparian tree invasions: A case study of Populus canescens in South Africa

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.

Modelling possible habitats for poplar invasion in South Africa

“…For windbreaking and decorative purposes, poplar species including P. simonii, P. fremontii, and P. canadensis are planted in homes and streets and are carefully tended to with frequent irrigation. Overall, the results are consistent with Ntshidi et al (2018) and Mtengwana et al (2021), because, from a geographic perspective, invasive species predominate in the wetter parts of the country, which are important water sources for the country's major rivers. Poplar needs enough water and warmer temperatures for better growth and survival (Fischer et al 2018;Kalcsits, Silim & Tanino 2009).…”

A review of the impacts of biological invasions in South Africa

“…Groundwater resources are also reduced in arid areas because some alien tree species draw water from aquifers at a rate in excess of replenishment (Dzikiti et al 2013). In the case of alien deciduous trees, studies have indicated substantially lower increases in evapotranspiration of around 20 mm/yr (Ntshidi et al 2018).…”

Impacts of Plant Invasions on Terrestrial Water Flows in South Africa

“…The impact of specific invasive alien tree species on streamflow has been assessed in field studies, some of which covered extended periods (Table 15.1). These include Acacia mearnsii (Dye and Jarmain 2004;Clulow et al 2011;Everson et al 2014), Eucalyptus camaldulensis (River Red Gum) (Dzikiti et al 2016), Pinus radiata (Monterey Pine) (Dzikiti et al 2013b), Prosopis species (Dzikiti et al 2013a(Dzikiti et al , 2017 and Populus canescens (Grey poplars) (Ntshidi et al 2018). In recent years some studies have also focused on the impacts of the invasive alien plants on groundwater [e.g.…”