Written by: James Daly
Invasive plants have a habit of dominating local ecosystems, dialing biodiversity back and reducing productivity on farm and forest land. And yet despite that, a number of them in New Zealand have distinct uses, including helping to slow the effects of climate change.
Invasive plants are really just opportunists who got given passport entry without much of a background check. Their success usually has to do with their requirements for growth which are usually pretty tolerant of dismal conditions. Many of them now cover a large portion of land in New Zealand, with Gorse alone covering up to 900,000 ha (Magesan 2012).
Did you know that Gorse, or Ulex europaeus can improve soil fertility by fixing up to 200 kg ha–1 N annually? When you consider that 25% of the world’s carbon is removed by Earth’s soils and that the rate of carbon sequestration or carbon storage is improved by soil fertility (fig 1), you see how important our soils really are. Coupled with how gorse can foster native bush regeneration growth if left to its own devices for long enough (Williams 2002), Gorse might not be all that bad.
Figure 1: The relationship between clay content and the organic carbon content of 220 soils in a 10 hectare area of a paddock under cereal-legume rotation in the central agricultural region of Western Australia. Photo credit.
Other invasive species have niche uses, Wandering Willy has been known to help encourage skink populations by providing shelter (Wooley, et al. 2019). Willow trees stabilise river banks efficiently and are often used by councils to do so (Daly 2019). The amount of carbon the Monterey Pine sequesters is higher than other trees because of how quickly it grows, covering up to 210,000ha in NZ (Gibson 2020). From a climate perspective this is huge.
I know that I’d struggle to find many ecological benefits for Didymo, Morning Glory or Old Man’s Beard. But from a climate perspective some of these invasive plants are doing the job of soil improvement that is not occuring as well in pastures. Farmland isn’t exactly nature’s plan just like bare, exposed soil isn’t either.
Based on data from typical perennial grasslands and mature forests in Australia, forests are typically more than 10 times as effective as grasslands at storing carbon on a hectare per hectare basis (Mackey 2009). Allowing certain invasive plants to survive will help to slow the effects of climate change, based on how some can improve bush regeneration.
The answer to whether or not invasive plants deserve their bad name depends on who you ask and in which context you are talking about. Farmers would certainly say yes as they have the most to lose, while climatologists might say no. What would you say? Invasive species often bring a loss of biodiversity with them, but from a climate point of view, a number of them are a big help.
1 – Magesen 2009 Nitrogen cycling in gorse-dominated ecosystems in New Zealand. (n.d.). Retrieved from https://www.researchgate.net/publication/240383631_Nitrogen_cycling_in_gorse-dominated_ecosystems_in_New_Zealand
2 – Williams, P., & Karl, B. (2002). Birds and small mammals in kanuka (Kunzea ericoides) and gorse (Ulex europaeus) scrub and the resulting seed rain and seedling dynamics. New Zealand Journal of Ecology, 26(1), 31-41. Retrieved March 15, 2020, from www.jstor.org/stable/24056282
3 – Woolley, C. K., Hartley, S., Hitchmough, R. A., Innes, J. G., Heezik, Y. van, Wilson, D. J., & Nelson, N. J. (2019, September 17). Reviewing the past, present and potential lizard faunas of New Zealand cities. Retrieved from https://www.sciencedirect.com/science/article/pii/S0169204619304396?via=ihub
4 – Daly 2019 Willows or natives for stream bank control: a survey of usage in New Zealand regional councils . (n.d.). Retrieved from https://icm.landcareresearch.co.nz/knowledgebase/publications/public/Willows_survey_report_2008.pdf
5 – Gibson, E., Gibson, E., Hickey, B., Hancock, F., Daalder, M., Newsroom, & New Zealand Herald. (2020, January 20). The unpopular tree sucking carbon from our air. Retrieved from https://www.newsroom.co.nz/2019/09/09/788817/nobody-loves-radiata
6 – Which plants store more carbon in Australia: forests or grasses? (n.d.). Retrieved from https://www.chiefscientist.gov.au/2009/12/which-plants-store-more-carbon-in-australia-forests-or-grasses