Distribution and abundance of invasive Tamarix genotypes in South Africa

Abstract

The exotic Tamarix chinensis and T. ramosissima, believed to have been introduced into South Africa in the early 1900s to control erosion on mine dumps, are invading riparian zones and have been proven to hybridise with T. usneoides, which is native to southern Africa. In this study, we document the abundance of invasive Tamarix genotypes in South Africa. Eleven riparian zones from the Northern, Eastern and Western Cape Provinces were surveyed. Three quadrats of 600 m 2 each were selected per site. Plant density, canopy cover and tree height were recorded to quantify invasiveness. Leaf samples were randomly collected from an average of eight individuals per site to record genotypes of the invaders. Tamarix density and canopy cover were significantly greater than those of co-occurring trees and shrubs in Olifants River in De Rust (Western Cape Province). A linear correlation between percentage Tamarix spp. cover and other cooccurring tree and shrub species showed a strong negative relationship (R 2 = 0.78). Genetic analysis showed that the Western and Eastern Cape Provinces have the highest proportion of the exotic Tamarix species and their hybrids. This suggests that these two provinces require urgent management intervention to contain the spread of the weed. The distinctions made between the native and the exotic Tamarix species and their hybrids should also facilitate the testing and future release of potential biological control agents.

The Impact of Exotic Tamarix Species on Riparian Plant Biodiversity

“…The study was conducted at three riparian sites in the Western Cape Province, South Africa, where Tamarix invasion had been identified as most prevalent [13]. Both Leeu River (S32,76794 and E21,97958), near the town of Leeu Gamka and the Swart River (S33,16627 and E21,97994) near the town of Prince Albert were in the Central Karoo District of Western Cape.…”

“…Exotic Tamarix species have been identified as riparian invaders and are well studied in the United States of America (USA) [9]. Several studies conducted in South Africa have identified riparian zones as regions highly invaded by exotic Tamarix species [10][11][12][13]. Invasive alien plants (IAPs) not only change the plant species composition and distribution patterns but they alter litter quantity and quality.…”

“…The decline in native plant populations and extirpations due to exotic Tamarix invasion in South Africa is yet to be quantified. Newete et al [13] compared the density of exotic Tamarix species to the co-occurring plants and found that the Tamarix density was greater in seven of the 11 sites investigated.…”

“…Although the country has an indigenous Tamarix species (T. usneoides), the two exotic Tamarix species (Tamarix ramosissima L. and Tamarix chinensis L.) have been in the country close to a century after their first introduction reportedly as garden plants for ornamental purposes [19]. The two species, along with their hybrids, are currently threatening many of the riparian ecosystems in the country and they are listed under the National Environmental Management: Biodiversity Act 2014 (NEM: BA) as category 1b invasive weed requiring urgent management intervention [13,20]). Alien Tamarix species are also considered as one of the 100 worst invasive species globally, under the International Union for Conservation of Nature (IUCN) list of Global Invasive Species Database [21].…”

“…Tamarix ramosissima and T. chinensis have been found to hybridise between themselves and with the native congeneric, T. usneoides, further extending their distribution range across the country [22]. This has accelerated the species' invasive potential, and complicated control of the exotics as the species are cryptic [13,[22][23][24][25]. Exotic Tamarix species are well adapted to survive adverse conditions, i.e., frost, floods, or drought, and they are also able to re-sprout, regenerate, and establish post-natural disasters [20,26].…”