Posts Tagged ‘plant invasions’

Research uncovers what helps weeds invade mānuka and kānuka shrublands

Monday, September 19th, 2022

The edge of kānuka shrubland that has been invaded by weeds

Mānuka and kānuka shrublands are spread across Aotearoa New Zealand and provide important economic, environmental, and cultural benefits. They are a source of high-value honey and traditional medicines, and they provide a refuge for native birds and a nursing ground for native plant regeneration.

Unfortunately, mānuka and kānuka shrublands are also under threat from many factors, including incursions by weeds. To better protect shrublands across the country, postgraduate fellow Dr. Laureline Rossignaud is conducting research that will help us understand what factors help mānuka and kānuka shrublands resist weed invasion, and to better understand the dynamics of weed invasions through time.

This research is part of Nuku-a-rangi, Pou 3 for Bioprotection Aotearoa.  Laureline’s project 3.2 is led by Dist. Prof. Philip Hulme which aims to determine the role of landscape attributes, ecosystem characteristics, and species traits on weed incursion in mānuka and kānuka shrublands. Laureline is using large datasets to investigate the drivers for weed invasions at the landscape scale.

The main database, hosted by Manaaki Whenua Landcare Research, is part of the national vegetation survey databank which provides plant information from permanent plots in mānuka and kānuka shrublands.

The drivers that Laureline is investigating include:

  • the proportion of exotic lands surrounding the plots, which can host weeds and facilitate their incursion in the shrublands
  • the distance to the nearest roads and rivers
  • The climatic conditions at the plot (mean annual rainfall and temperature)
  • the plant community already in the plot, including the structure of the vegetation with native species richness and native plant cover across different heights.

Laureline’s research so far has found that native tree species richness is very important in helping shrublands resist weed invasions because exotic species must compete with native trees for resources. In addition, most exotic weed species are adapted to open habitats, so tall native trees also make the ecosystem resilient against invasion by limiting access to light and space.

On the other hand, Laureline’s research also confirms that factors that help weeds invade shrublands include the proximity of exotic grasslands and the fragmentation of habitats.

Laureline is now creating maps to investigate the relationship between climate and weed invasions. These maps show that shrublands on the West Coast and in the southern South Island have fewer weed invasions than shrublands in other parts of the country. This is probably because these areas are largely in national parks and they are wetter and colder than other parts of the country. Climate is significantly correlated with weed invasion in the models, and warmer climates generally help exotic species establish.

Laureline says, “I have really enjoyed making maps and looking at the distribution of exotic species. When I was building the maps, I discovered some exotic species are spreading and others aren’t, which is quite interesting. I’m now having a more detailed look at why this is happening.”

Laureline’s research over the next year and a half will work to identify which weed species are extending their ranges and invading more plots, what factors are driving this spread, and what threats the weeds pose to mānuka and kānuka shrublands.

Laureline is enjoying being part of Bioprotection Aotearoa and says, “It’s a great opportunity because such a large number of scientists are working in this space. It’s a huge network and I’m going to learn a lot from all of the other cool projects. I feel excited and lucky.”

Indigenous fungus may help to control wilding pines

Wednesday, June 16th, 2021

An indigenous New Zealand fungus may help to control wilding pines – one of the country’s most ecologically damaging weed species – a student’s research project shows.

​Wilding pine control costs New Zealand millions of dollars a year, and involves the costly and time-consuming methods of cutting down the trees and spraying herbicide from the air. Control seldom totally eradicates the pines, which often reinvade sites some years later.

Armillaria novae-zealandiae, also known by Māori as harore, is a fungus that feeds on decaying wood. It is common in native forests, where it is a natural part of the ecosystem, helping to decay fallen trees. But if it gets into pine plantations it is seriously destructive, killing seedlings and reducing growth.

In a Bio-Protection Research Centre student research programme, biology student Genevieve Early, investigated how well A. novae-zealandiae and two closely related species established on wilding pine species.

Supervised by BPRC principal investigator and University of Canterbury Professor Ian Dickie and his colleague Dr John Pirker, she tested what age of wood it grew best on (ranging from live and freshly harvested wood to old and decayed wood).

“The research aimed to address knowledge gaps in our understanding of Amillaria, and eventually investigate whether we could use it as a biological control of invasive pines,” says Genevieve.

“Some of the questions we have about using it, for example, are whether we can introduce it to grassland areas that are susceptible to wilding pine invasions, where it doesn’t currently exist, and whether introducing it at the same time as pines are felled would prevent reinvasion.”

Her results were promising. “Armillaria novae-zelandiae showed the best growth,” she says. “We tested several isolates of this species and all of them grew larger than the other Armillaria species. It also consistently grew most vigorously on live or freshly-felled pine wood.”

Armillaria’s strong growth on live or fresh pine wood is important,” Genevieve says. “It’s really promising that all the fungi grew best on live or fresh wood, as this implies that we could potentially design a way to inoculate wilding pine sites with Armillaria at the same time we are manually clearing trees and using herbicide. That could be practical and economical if we don’t have to plan more site visits to use the fungi.

“We also want to find out if this will accelerate decomposition and reduce wildfire risks.”

Genevieve said A. novae-zelandiae has been used as a food source by Māori, who should be involved in continuing research. “Using it as a biological control may be of particular interest to iwi in areas badly affected by wilding pines, as a way of protecting landscapes and ecosystem values.”

Prof Dickie said his group was seeking funding to continue the research, particularly looking at how Armillaria affected native seedlings, to test whether it could be used to clear pines in areas where ecological restoration was planned.

“Until now, we’ve been good at killing pines, but not at restoring ecosystems,” says Ian. “We are winning the battles, but losing the war. This fungus may be the key to not just killing pine, but to keeping it from reinvading, and to restoring ecosystems.”

You can view a video of Genevieve Early presenting her research here.

Study challenges assumptions on why exotic plants thrive

Tuesday, June 15th, 2021

​Invasion ecology has long suggested exotic species can become successful by escaping their natural enemies. A long-running Bio-Protection Research Centre experiment challenges this, showing that exotic plants dominate their communities, despite accumulating and sharing herbivores more than co-occurring native plants.

The Enemy Release Hypothesis predicts that exotic plants become successful because they escape from natural enemies, compared with native species. However, studies testing whether exotic species escape (known as community enemy release) or attract (biotic resistance) natural enemies have produced mixed results.

We set out to test whether plant-herbivore interactions systematically favour exotic plants. We established 160 experimental plant communities, each containing eight plant species selected from a pool of 39 (19 natives, 20 exotics) that co-occur in Aotearoa New Zealand grasslands. We added 20 species of invertebrate herbivores to half the pots, all of which were enclosed in 2.2 m tall mesh cages. The herbivores were a mixture of seven native and 13 exotic species, including New Zealand grass grub (Costelytra giveni), three species of leafroller caterpillar, several exotic aphid species, and the native grasshopper Paprides nitidus. Most of the herbivores were polyphagous, meaning they feed on several hosts.

Setting up this experiment was a massive logistical exercise. We literally applied New Zealand’s ‘no. 8 wire’ approach, using more than 1 km of the eponymous fencing material to support the cages, and 4 km of thread to sew them all together. All of this work required a small army of helpers, including graduate and undergraduate students, support staff, and friends and family.

We surveyed plants for herbivores eight times during the one-year experiment, counting the number of individuals of each species we saw feeding on each plant and calculating their dry biomass. For species that were highly mobile or lived below ground, we extracted DNA from herbivore regurgitate (vomit) and frass (faeces) samples, and used a molecular technique named restriction fragment length polymorphism (RFLP) to identify the host plant.

Not what we expected

The results surprised us: rather than exotic plants suffering less herbivory than their native neighbours, they supported higher native and exotic herbivore diversity and biomass, and were more damaged. We also took the crucial step of measuring how herbivory affected plant performance, finding that exotic plant biomass was 30% lower in mesocosms with herbivores than those without herbivores, while the biomass of native plants was unaffected.

Despite suffering such strong herbivory, exotic plants consistently dominated the biomass of mesocosm communities, potentially because of their indirect interactions with neighbouring plants. Many herbivores that attacked the exotic plants also fed on other species, indicating that exotic plants may be successful because they support polyphagous herbivores that affect neighbouring plants. However, we found no evidence that these indirect interactions affected neighbours’ biomass. Therefore, it is likely that the exotic plants’ faster growth rates simply allowed them to overcome the high levels of herbivory and still dominate the communities.

Our study represents one of the most comprehensive tests of community enemy release and biotic resistance to date. We conclude that polyphagous invertebrate herbivores are unlikely to play significant direct or indirect roles in mediating plant invasions, especially for fast-growing exotic plants. You can read the full paper in Nature Communications.