Alnus glutinosa (Betulaceae) in South Africa : invasive potential and management options

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.