What is a weed?
As described in the Australian Capital Territory (ACT) Weeds Strategy, a weed is ‘considered to be a terrestrial or aquatic species of non-native or native plant that is harmful to the natural environment (ecosystems/biodiversity), agriculture and other industries, or public amenity and health’ (Department of Environment, Climate Change, Energy and Water, 2009).
For the purpose of biodiversity conservation in Canberra’s reserves, this means that both native and non-native species can be considered weeds if they are not endemic to the area. Even non-local natives could pose a threat to biodiversity by competing with local native shrubs, small trees and ground flora, and impede their regeneration.
The Australian Government maintain a Weeds of National Significance (WoNS) list which details priority weeds identified as having the potential for spread causing environmental, social and economic impacts. Currently there are thirty two weeds on the WoNS list, of which the following two grassy weeds have been found in the ACT:
- Chilean Needle Grass (Nassella neesiana)
- Serrated Tussock (Nassella trichotoma)
The ACT has its own legislation regarding pest plants and animals (Pest Plants and Animals Act 2005) and also a separate weed strategy which details 77 plants declared as environmental weeds in the ACT, including two additional grassy weeds:
- African Love Grass (Eragrostis curvula)
- Mexican Feather Grass (Nassella tenuissima)
Serrated tussock is widely established in the ACT although it is not a local, it is native to South America. It reaches from the southernmost border in Namadgi National Park to the norther border of the ACT. Surrounding land in NSW is also extensively effected.
It is a perennial C3 tussock-forming grass with a deep fibrous root system making it very difficult to pull from the ground. Mature plants can produce more than 140 000 seeds per plant per year. The seeds are easily carried by the wind, and even after treatment the seed bank in the soil typically requires substantial follow-up treatment. Serrated tussock is also highly unpalatable to both livestock and native mammals (Department of the Environment and Energy (1), 2016).
African lovegrass has spread throughout Canberra, though it is not a local either. African lovegrass originated from southern Africa. It is a highly persistent summer growing (C4), perennial grassy weed which can tolerate sandy soils with low fertility. Seeds are easily spread animals, mowers, vehicles, water and wind. ‘Spread is enhanced by drought conditions and over-grazing. Paddocks with low ground cover are more susceptible to invasion’ (NSW Department of Primary Industries, 2015).
Due to its lightweight structure, African lovegrass is highly flammable and poses a high fire risk in areas of dense infestation. The weed however, is not killed by fire and therefore post-fire treatment is always required to stop re-establishment.
Chilean needle grass
Chilean needle grass is a perennial tussock grass native to South America. It grows in dense clumps and is closely related to serrated tussock. The grass has a persistent seedbank which can survive even after the adult plants are killed and has the potential to produce more than 20,000 seeds per square metre. Chilean needle grass requires bare earth to seed and therefore good ground cover can reduce infestations. It is heavily grazed by Macropus giganteus (Eastern grey kangaroo), making it harder to identify. In areas where grazing is high, this can make spread of the invasive weed easier (Department of the Environment and Energy (2), 2016).
There are a number of disturbance factors including land management practices and climate change which effect a weeds ability to become invasive. Land clearing, intensive agriculture and fire are examples of land management practices which can alter the ecosystem structures and allow for invasive weeds to take hold (Department of the Environment and Energy (3), 2016).
- Wild fire or prescribed fire which causes significant reduction in the canopy allows more sunlight to reach the ground and gives weeds a chance to thrive.
- Land clearing and agricultural practices using fertilisers can provide certain weeds an opportunity to out compete native pastures.
- Climate change can cause flooding which spreads weeds. Droughts may also weaken native plants and allow adaptable weeds to invade.
Climate change and grassy weeds
The latest climate modelling undertaken by CSIRO indicated a high confidence that southern and eastern Australia are projected to experience harsher fire weather. ‘There is very high confidence in continued increases of mean, daily minimum and daily maximum temperatures throughout this century for all regions in Australia’ (CSIRO and Bureau of Meteorology, 2015, p. 8).
This might mean good news for invasive grassy weeds which can tolerate warmer and drier conditions and/or larger variations in climate. However, this may mean bad news for land managers. With climate change comes predicted increases in temperature and also elevated atmospheric CO2. CO2 is known to improve the growth of plants due to increased efficiency in photosynthesis.
Most plants are known as C3 which means they form a three-carbon compound during photosynthesis, however some grasses have now been found to be C4, meaning they form a four- carbon compound during photosynthesis. Generally, C4 grass species originate from warmer climates than C3 species (Scott, et al., 2014, p. 18).
Modelling of weeds of national significance suggests:
‘Chilean needle grass is expected to increase its range because it is highly invasive (longlived, seed dispersed by wind and water) and drought tolerant.
Serrated tussock is expected to retreat southwards and to higher altitudes because it is sensitive to higher temperatures. As a drought-tolerant plant, it should become more invasive in areas where temperature allows’ (Meat & Livestock Australia, 2008, p. 4).
As years progress and the effects of climate change are felt, it will be interesting to see both native and non-native invasive response to increasing temperatures and CO2. With an understanding and appreciation of grassy weed potential responses to climate change, land managers can strategically plan for the future.
CSIRO and Bureau of Meteorology, 2015. Climate Change in Australia Information for Australia’s Natural Resource Management Regions: Technical Report, Australia: CSIRO.
Department of Environment, Climate Change, Energy and Water, 2009. ACT WEEDS STRATEGY 2009 – 2019, Canberra: Department of Environment, Climate Change, Energy and Water.
Department of the Environemnt and Energy (1), 2016. Nassella trichotoma. [Online]
Available at: http://www.environment.gov.au/cgi-bin/biodiversity/invasive/weeds/weeddetails.pl?taxon_id=18884
Department of the Environment and Energy (2), 2016. Nassella neesiana. [Online]
Available at: http://www.environment.gov.au/cgi-bin/biodiversity/invasive/weeds/weeddetails.pl?taxon_id=67699
Department of the Environment and Energy (3), 2016. Factors influencing weeds. [Online] Available at: http://www.environment.gov.au/biodiversity/invasive/weeds/weeds/why/factors.html
Meat & Livestock Australia, 2008. Communicating Climate Change: Climate change impacts on pest animals and weeds. s.l., s.n.
NSW Department of Primary Industries, 2015. African lovegrass (Eragrostis curvula). [Online] Available at: http://weeds.dpi.nsw.gov.au/Weeds/Details/3
Scott, J. et al., 2014. Weeds and climate change: supporting weed, Canberra: AdaptNRM (CSIRO).