Abstract
Invasive alien plants cause major environmental and economic impacts and preventing the establishment and spread of emerging invaders is crucial. Black Alder (Alnus glutinosa) is well established as a widespread invader in a number of countries, notably the USA and New Zealand, and was recently detected invading riparian ecosystems in South Africa's Western Cape Province. We review the introduction history, current distribution and invasion potential (via species distribution and risk analysis) of Black Alder in South Africa, collate information on its biology, environmental impacts and options for management from its native and invaded range, assess its potential range and management attempts in South Africa, and provide guidelines for effective management. Furthermore, correlative modelling predicted areas of the southern and eastern part of South Africa to have suitable environments for Black Alder establishment. However, water availability will likely limit the species to riparian areas, and areas where the annual rainfall exceeds 500 mm per annum. We estimated control costs to be minimum R 82 000 per month, and the vigorous resprouting nature of Black Alder and its riparian zone preferences presents large management challenges. We propose that Black Alder should be listed as Category 1a under the National Environmental Management: Biodiversity Act (NEMBA) in South Africa. We advise that riparian areas in particular be monitored closely to prevent Black Alder from becoming a widespread invader.
Highlights
Invasive alien plants (IAPs) pose significant threats to the ecosystems they invade; many IAPs have devastating impacts on agricultural and ecosystem services (Pysek et al, 2020)
South Africa has been greatly impacted by IAPs (Le Maitre et al, 2016; van Wilgen et al, 2011), especially invasive alien tree species (Richardson et al, 2020)
To determine the invasion risk that Black Alder poses to South Africa we used currently available information, via extensive literature searches, to conduct a risk analysis according to a newly developed framework for supporting alien species regulation, namely the Risk Analysis for Alien Taxa (RAAT) (Kumschick et al, 2020a, 2020b)
Summary
Invasive alien plants (IAPs) pose significant threats to the ecosystems they invade; many IAPs have devastating impacts on agricultural and ecosystem services (Pysek et al, 2020). There are currently about 2033 alien plant species recorded outside of cultivation in South Africa (van Wilgen and Wilson, 2018), of which 759 are considered naturalized or invasive (Richardson et al, 2020). South Africa has been greatly impacted by IAPs (Le Maitre et al, 2016; van Wilgen et al, 2011), especially invasive alien tree species (Richardson et al, 2020). Thorough assessments of the potential of emerging invasive tree species to extend their ranges and to cause damage is warranted. In this regard, reports of local-scale invasions of an alien tree species (Black Alder; Alnus glutinosa (L.) Gaertn.; Betulaceae) that is known to be highly invasive in certain parts of the world is of concern. This study aimed to: 1) review the characteristics, uses, and environmental impacts of Black Alder in other parts of the world; 2) investigate the introduction history of the species in South Africa; 3) investigate its current distribution and invasion potential in South Africa (using species distribution modelling and risk analysis) and its global invasion history; 4) assess attempts to control invasions of the species in South Africa to date; and 5) provide guidelines for effective management
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