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.

Guiding restoration of riparian ecosystems degraded by plant invasions: Insights from a complex social-ecological system in the Global South

“…The natural vegetation along the Dwars River riparian zone has largely been replaced by agricultural land and invasive plants, notably Alnus glutinosa (Black Alder), Acacia melanoxylon (Blackwood), Populus alba (White Poplar), Acacia dealbata (Silver Wattle), Acacia saligna (Port Jackson) and Acacia mearnsii (Black Wattle) (Belcher et al 2015 ), (see Supplementary Material S1 for species list and S2 for photographs of invasions). The lower section of the river has the largest known invasive population of A. glutinosa in the Western Cape (Keet et al 2020 ). The Biodiversity Spatial Plan Map indicates that the entire Dwars River valley has been designated an ecological support area, meaning that it plays a vital role in supporting the ecological functioning of Critical Biodiversity Areas (Pool-Stanvliet et al 2017 ).…”

“…From August 2018 to the end of 2019, dense stands of invasive trees were cleared along the lower reaches of the Dwars River by a conservation trust with government funding. However, there are issues with the sustainability of this funding, impacting adequate follow-up maintenance, resulting in coppicing and re-invasion of species such as A. glutinosa and A. saligna (Keet et al 2020 ).…”

The Good, the Bad, and the Useable Microbes within the Common Alder (Alnus glutinosa) Microbiome—Potential Bio-Agents to Combat Alder Dieback

“…Preferable growing sites include riverbanks, ponds, surrounding lakes, streams, marshy waterlogged areas, shaded mountainous regions, wet woodlands, wet soils, and highlands with sufficient moisture content [ 10 , 11 ]. This species tends not to grow well on calcareous soils, acidic peat, arid sandy soils, and areas with stagnant water [ 1 , 12 ]. Alder has been found in and/or introduced to regions of Africa, Australia, Canada, India, Japan, Russia, North America, South America, and New Zealand [ 5 , 12 , 13 , 14 ].…”

“…This species tends not to grow well on calcareous soils, acidic peat, arid sandy soils, and areas with stagnant water [ 1 , 12 ]. Alder has been found in and/or introduced to regions of Africa, Australia, Canada, India, Japan, Russia, North America, South America, and New Zealand [ 5 , 12 , 13 , 14 ].…”