Invasive status of pampas grass in South Africa

Abstract

Two 𝘊𝘰𝘳𝘵𝘢𝘥𝘦𝘳𝘪𝘢 species, 𝘊. 𝘫𝘶𝘣𝘢𝘵𝘢 and 𝘊. 𝘴𝘦𝘭𝘭𝘰𝘢𝘯𝘢 have become invasive outside their native range, including in South Africa. They were introduced to South Africa for ornamental use, erosion control and stabilising mine-dumps. They have long escaped containment and spread throughout the country, invading grasslands, roadsides, wastelands, rivers and seasonally wet habitats. Little is known regarding their distribution, seed viability and whether accurate identifications have been made of the 𝘊𝘰𝘳𝘵𝘢𝘥𝘦𝘳𝘪𝘢 species in South Africa, which may hinder effective management. Although the national regulations forbid the trade of pampas grass, the continuing popularity of their inflorescences for home décor and special events is of concern. While nurseries have stopped the sale of the plants, inflorescences are still being sold through retail and informal trade. This study aimed to assess the invasion risk posed by pampas grass in South Africa and distinguish between the different species across their invaded range using molecular techniques. Leaf and seed samples were collected from 79 populations across South Africa.

Eight microsatellite primers were used to assess the genetic variation across 𝘊𝘰𝘳𝘵𝘢𝘥𝘦𝘳𝘪𝘢a populations. Seed viability and germination success were assessed for seeds from 28 naturalised and 13 traded inflorescences using a triphenyl tetrazolium stain, and germination trials in a growth chamber and greenhouse. Impacts and risks of 𝘊. 𝘫𝘶𝘣𝘢𝘵𝘢 and 𝘊. 𝘴𝘦𝘭𝘭𝘰𝘢𝘯𝘢 in South Africa were assessed using version 1.2 Risk Analysis for Alien Taxa (RAAT) framework. Microsatellite confirmed the presence of 𝘊. 𝘫𝘶𝘣𝘢𝘵𝘢 and 𝘊. 𝘴𝘦𝘭𝘭𝘰𝘢𝘯𝘢 in South Africa and that 𝘊. 𝘴𝘦𝘭𝘭𝘰𝘢𝘯𝘢 populations have higher genetic diversity compared to 𝘊𝘰𝘳𝘵𝘢𝘥𝘦𝘳𝘪𝘢 from other invaded regions. Tetrazolium tests found that 𝘊. 𝘴𝘦𝘭𝘭𝘰𝘢𝘯𝘢 (71.66 %) had higher seed viability than 𝘊. 𝘫𝘶𝘣𝘢𝘵𝘢 (54.28 %). Assessments of germination trials in a growth chamber revealed that 𝘊. 𝘫𝘶𝘣𝘢𝘵𝘢 (68.89%) had a higher proportion of seeds germinate compared to 𝘊. 𝘫𝘶𝘣𝘢𝘵𝘢 (52.62 %) but this difference was not statistically significant. Seed germination under greenhouse experiments also found 𝘊. 𝘴𝘦𝘭𝘭𝘰𝘢𝘯𝘢 (79.15 %) to have higher germination success compared to 𝘊. 𝘫𝘶𝘣𝘢𝘵𝘢 (62.54 %). Seed viability of cultivated and naturalised 𝘊𝘰𝘳𝘵𝘢𝘥𝘦𝘳𝘪𝘢 inflorescences differed significantly to formally traded inflorescences but not significant to informally traded inflorescences. Seeds from naturalised populations of both 𝘊𝘰𝘳𝘵𝘢𝘥𝘦𝘳𝘪𝘢 species had significantly higher germination success (65.69 %, 74.35 %) than the formally (25.00 %, 5.33 %) and informally traded populations (28.75 %, 35.56 %) under growth chamber and greenhouse experiments, respectively. Viability and germination success of seeds from cultivated 𝘊𝘰𝘳𝘵𝘢𝘥𝘦𝘳𝘪𝘢 inflorescences were statistically similar to naturalised inflorescences while those from formally traded inflorescences performed statistically similar to seeds from informally traded inflorescences across triphenyl tetrazolium stain, growth chamber and greenhouse experiments.

The risk analysis assessments determined that both 𝘊. 𝘫𝘶𝘣𝘢𝘵𝘢 and 𝘊. 𝘴𝘦𝘭𝘭𝘰𝘢𝘯𝘢 have major impacts and a high risk of invasion in South Africa with medium ease of management. Determining the invasion risk posed by pampas grass in South Africa has provided important information on managing these species. Biological control is species-specific; therefore, this study has provided information that will guide the biological control programme of pampas grass in South Africa and future research can focus on finding potential biocontrol agents. Seed viability and germination experiments established that when seeds are present on flowers, most of them are viable and that traded inflorescences’ seeds are not sterile and can thus be promoting invasion and further spread of 𝘊𝘰𝘳𝘵𝘢𝘥𝘦𝘳𝘪𝘢 species. Effective management of 𝘊𝘰𝘳𝘵𝘢𝘥𝘦𝘳𝘪𝘢 species must focus on enforcing legislation to stop this pathway of spread.