The invasive grass genus Nassella in South Africa: A synthesis

Abstract

Three species of Nassella have naturalized in South Africa. Nassella trichotoma and N. tenuissima are declared weeds under category 1b of the National Environmental Management: Biodiversity Act (NEM:BA) and occur mainly in the montane grasslands of the Western and Eastern Cape provinces. Nassella neesiana is not listed in NEM:BA but is naturalized in the Eastern Cape, Western Cape and Free State provinces. Research conducted in the 1970s and 1980s led to vigorous government-funded awareness and control campaigns which ended in 2000. No research on Nassella distribution or control has been undertaken since then. Despite this hiatus, Nassella remains a dangerous genus in southern Africa, given the serious impacts of these species in similar social-ecological systems in Australia and New Zealand. This paper presents a synthesis of available information about Nassella invasions in South Africa and identifies research gaps. It specifically addresses these questions: What identification issues exist? What is the current spatial distribution of Nassella? What is the autecology of the genus? What are the social-ecological impacts of Nassella? What control measures are currently applied and what are their strengths and limitations? What do we know about Nassella distribution and its response to climate change? This paper highlights many knowledge gaps about Nassella, such as the species' current distribution range, field identification and detection difficulties, and the uncoordinated control efforts that require urgent research to inform an effective management response.

A Comparative Review of Six Invasive Nassella Species in Australia with Implications for Their Management

“…Nassella trichotoma produces panicle inflorescences that have a slight purple tinge, and as with other Nassella species, produce seeds from both allogamy (open) and cleistogamous (closed) flower heads, with the latter being more prominent [ 17 , 18 ]. Nassella trichotoma produces up to 90% cleistogamous flowers, allowing small populations to grow quickly, as fertilization is not reliant on pollen transfer [ 19 , 20 , 21 ].…”

“…Inflorescences emerge between November and March in Australia, and during this time, N. neesiana has a distinct purple colouration. It produces both chasmogamic and cleistogamic flowers, and open flowers are wind-pollinated [ 18 ]. Nassella neesiana seeds are cylindrical and have a sharp, hooked tip.…”

“…The root system is also very similar to other Nassella species, being shallow and fibrous, allowing for effective water uptake in environments with sporadic rainfall. Nassella tenuissima has the capacity to re-sprout if their wiry roots are not fully removed or treated [ 18 , 50 ], making mechanical removal problematic.…”

“…There are similarities between these six Nassella species with regards to their leaf grazing quality, reproduction timing, pollination, and germination cues [ 5 , 9 , 10 , 18 , 42 , 59 ]. While the palatability of these species varies, they are all considerably less palatable when compared to standard pasture grasses [ 10 , 38 ].…”

“…Inflorescences appear in the spring and continue into the summer for all six Nassella species, which suggests that Nassella control programs should be implemented at the same time of year [ 43 , 79 , 80 ]. The available literature suggests that seed production is high, with pollination mostly occurring within cleistogamous flowers [ 17 , 18 ]. This demonstrates that if even a single plant establishes in a grazing system, it can add a significant number of seeds to the seedbank, as it does not require cross-pollination to produce large seed densities [ 40 ].…”