Invasive weeds and wildfires
Scientists Tim Curran (Lincoln University), Sarah Wyse (University of Auckland) and George Perry (University of Auckland) looked at the impacts of exotic plant species on fires. This is their summary of aspects of that research, republished from The Conversation under Creative Commons licence CC BY-ND 4.0.
Mixed grill
Burning combinations of invasive and native plants helps us understand how invasive plants make fires hotter and more likely. Here, a laser thermometer is used to read the temperature as samples of plants are burned.
Catastrophic wildfires are becoming more and more prevalent in global media reports. One suggested reason for increases in catastrophic wildfires has been human-induced climate change. Higher temperatures, drier weather and windier conditions all increase the impact of fires.
While climate change indeed raises the risk of wildfires, our research shows that another way humans can change patterns of fire activity is by introducing flammable plants to new environments.
Plantations of highly flammable exotic species, such as pines and eucalypts, probably helped to fuel the recent catastrophic fires in Portugal and Chile. In arid regions, such as parts of the US southwest, the introduction of exotic grasses has transformed shrublands, as fires increase in severity.
Invasive plants and fire
How do invasive plants change fire patterns? We burned species mixtures (aka “mixed grills”) on our plant barbecue to help find out.
Invasive plants are responsible for changing the patterns of fire activity in many ecosystems around the world. In particular, invasive species can lead to hotter and more frequent fires.
Invasive plants can also reduce fire frequency and fire intensity, but there are fewer examples of this occurring worldwide.
One of the main ways flammable invasive plants can have long-lasting impacts on an ecosystem comes from fire-vegetation positive feedbacks. Such feedbacks can occur when a flammable weed invades a less fire-prone ecosystem. By changing the available fuel, the invader makes fires more likely and often hotter.
If the invading species has characteristics that allow it to outcompete native species after a fire, then it will further dominate the ecosystem. Such traits include thick bark, the ability to resprout following fire or seeds that survive burning. This invasion will likely lead to more fires, changing the species composition and function of the ecosystem in a “fire begets fire” cycle. Extreme examples of this dynamic are where flammable grasses or shrubs invade forests, leading to loss of the forest ecosystems.
Mixed grills
We wanted to understand how invasive plants interact with other species when burned in combination. To explore the mechanisms underpinning such feedbacks, we examined how invasive plants might change the nature of a fire when burned together with native species.
We collected 70 cm shoots of four globally invasive species (of both high and low flammability) and burned them in pairwise combinations with New Zealand native trees and shrubs to determine which characteristics of a fire could be attributed to the invasive plants.
We found that overall flammability was largely driven by the most flammable species in the mixture, showing how highly flammable weeds could set in motion fire-vegetation feedbacks.
We established that a greater difference in flammability between the two species led to a larger influence of the more flammable species on overall flammability. This outcome suggests weeds that are much more flammable than the invaded community can have larger impacts on fire patterns.
Importantly, we also showed the influence of the highly flammable species was independent of its biomass, meaning highly flammable weeds may impact community flammability even at low abundances.
When we looked closer at the different components of flammability (combustibility, ignitability, consumability and sustainability or how long the fire burns), we found some important nuances in our results.
While the maximum temperature reached in our burns (combustibility) and the ignition speed (ignitability) were both most influenced by the more flammable species, consumability (the amount of biomass burned) and sustainability were equally influenced by both the more-flammable and less-flammable species.
In short, more-flammable weeds will cause a fire to ignite more quickly and burn hotter.
However, less-flammable species can reduce the duration of a fire compared to when a more-flammable species is burned alone. These results could have important ecological implications, as the longer a fire burns, the more likely it is to kill plants: low-flammability plants could reduce this impact.
Managing weeds to reduce fire impacts
Even low abundances of highly flammable invasive weeds could set in motion positive fire-vegetation feedbacks that lead to drastic changes to ecosystems. If this result holds when our shoot-scale experiments are repeated using field trials, land managers should work quickly to remove even small infestations of highly flammable species, such as gorse (Ulex europaeus) and prickly hakea (Hakea sericea).
Conversely, the role of low-flammability plants in extinguishing fires further supports the suggestion that the strategic planting of such species across the landscape as “green firebreaks” could be a useful fire management tool.
In any case, our “mixed grill” study further highlights the role of exotic plants in fuelling hotter wildfires.
Related content
To understand some of the terminology in this article, go to Investigating fire – key terms.
Learn about Fire behaviour in the outdoors and earlier research in New Zealand that looked at Managing fire risk in the outdoors
Climate change is playing a role in the increase in wildfires. To learn more about climate change, we suggest educators start with our interactive Climate change resources – planning pathways.
For professional learning development, look at our Fire unit plan and the case study Cross-curricula user of fire, and if you’re looking for a way to hook students into learning about fire and to get them excited about science, fire offers many opportunities!
Find out more about wildfires and the links to climate change in this Connected article on wildfires.
The article Weeds and native ecosystems – a context for learning provides curriculum and pedagogical ideas for exploring this vexing topic.
Student activity
In the activity Fire risk assessment, students identify and define fire risks, outdoors and indoors, using a provided image of a rural scene and an indoor scene they draw themselves.
Useful links
Two of the authors of this article, Tim Curran and Sarah Wyse, can be heard talking about their research and methods in the Radio New Zealand podcast Firing up the plant barbecue.
Human-induced climate change is contributing to an increase in catastrophic wildfires. To understand how climate change is a threat to the increase in global wildfires, read the Nature article Prepare for larger, longer wildfires.
To learn more about wildfires such as how much of Earth’s surface burns each year, whether the rate of burning is increasing and which plants are most flammable, download a PDF of the report Global land-cover change – wildfires.
Links to supporting research papers can be viewed in the original version of this article published in The Conversation. This article is based on the research paper Shoot-Level Flammability of Species Mixtures is Driven by the Most Flammable Species: Implications for Vegetation-Fire Feedbacks Favouring Invasive Species, which can be viewed here.
Acknowledgement
This article was written by Tim Curran (Senior Lecturer in Ecology, Lincoln University), Sarah Wyse (Early Career Research Fellow, The Royal Botanic Gardens, Kew and Research Fellow, School of Environment, University of Auckland) and George Perry (Professor, School of Environment, University of Auckland).
The article was originally published on The Conversation. Read the original article.
The Conversation
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