Drought-tolerance of an invasive alien tree, Acacia mearnsii and two native competitors in fynbos riparian ecotones

Abstract

Invasive alien plants (IAPs) have successfully invaded many riparian zones in South Africa, especially Australian Acacia spp. which are prevalent along riverbanks in the south-western Cape of South Africa. This Mediterranean-type climate region is predicted to endure severe future water shortages under likely scenarios of increased population growth and climate change, and IAPs aggravate this problem due to their profligate water use. Acacia mearnsii competes aggressively with native species, however, it remains unclear what physiological advantage the species has over co-occurring native species under the predicted reduced streamflow scenarios. A mechanistic approach was used to investigate how key native fynbos riparian woody tree species compare in vulnerability to drought-induced cavitation against A. mearnsii by comparing findings from three Mediterranean-type fynbos river systems that differ in streamflow. A. mearnsii showed lower water potential at 50% hydraulic conductivity loss (P 50 values) compared to native species at certain sites, an indication of drought-tolerance. This suggests it is likely to persist under future drier conditions and it therefore remains a top priority for control. The native Brabejum stellatifolium had consistently higher water potentials across all sites than the other studied species, and is a potentially valuable species for restoration of south-western Cape riparian zones. Consistency in the shapes of species vulnerability curves across sites illustrated a species-specific hydraulic response to different water availability, strengthening the argument that this approach to distinguish site-level drought-tolerance between trees is a practical technique, with great application in understanding future geographic distribution under climate change, and potential for use in restoration research. Additionally, streamflow was an inaccurate predictor of species drought-tolerance along these riparian systems.

High Tolerance to Salinity and Herbivory Stresses May Explain the Expansion of Ipomoea Cairica to Salt Marshes

“…Thus, invasive species maintain a higher tolerance to environmental stress factors than native and exotic non-invasive congeners. Other studies also have found that invasive species have a higher tolerance to various stress factors compared with native species [73], [74], [75], [76], [77] and their native conspecifics [24], [78]. Due to their high tolerance to salinity [37] and herbivory stress [35], some invasive species have successfully invaded harsh habitats.…”

Invasive Acacia mearnsii De Wilde in Kunming, Yunnan Province, China: a new biogeographic distribution that Threatens Airport Safety

“…Duke (1983) reported that it can tolerate an annual precipitation of between 660-2280 mm, an annual mean temperature of 14.7-27.8 °C, and a pH of 5.0-7.2. The GISD says that it does not grow well on very dry or poor soils, however, Crous et al (2012) reported that it is highly drought-tolerant, able to withstand low minimum water potentials, and can utilize a large proportion of soil water.…”

Black Wattle (Acacia mearnsiiDe Wild.) in Rwanda’s Forestry: Implications for Nature Conservation

“…A number of traits contribute to A. mearnsii's invasiveness, including rapid growth and the ability to produce of large amounts of seeds within a year (Moyo & Fatunbi, 2010), high capacity of seedling recruitment, drought-tolerance (Crous, Jacobs, & Esler, 2012), deployment of a wide crown spread to shade other plants (Costermans, 1994), and the ability to change soil properties (Boudiaf et al, 2013). However, the species is vulnerable to grass competition at a young age (NAFA, 2013), which may partly explain why it rarely successfully establishes on undisturbed soils.…”