The story of the understorey
New Zealand is home to thousands of native plant species, of which a whopping 80% of these trees, flowering plants and, ferns are endemic – found only in Aotearoa and nowhere else!
A thousand years ago the forests of NZ were comprised of only natives. Since then exotic plants have made their way here – crossing the great seas on waka and sailing ships and riding the air in jets to beautify the human gardens of tomorrow. Sometimes these exotics do more than beautifying modern gardens, they expand and conquer, alongside the human colonists.
Plant invasions pose one of the most large-scale threats to the conservation of biodiversity worldwide. This issue is only getting worse with climate issues allowing for more habitats and niches to be exploited by plant invaders that were previously unavailable.
Acacia is a plant genus that is notorious for enacting the invade and conquer regime, often considered a problematic and invasive weed, with wanted posters in multiple regions of the world. Acacia are also well known by their street name: wattles.
But why is acacia considered an invasive weed?
Wattle is a grade-A problem maker, changing ecosystem structures and soiling the soil. It achieves these feats by changing the composition of the forest, altering the variety of species present in the forest and changing the microbiome of the soil (all the little bacteria and microscopic life we can’t see with the human eye). It also likes to participate in, and encourage, arson for its own benefit.
You can read more about how acacia messes with ecosystems and soils soil here:
Ecosystems -> https://repositorio.uchile.cl/bitstream/handle/2250/154809/art04.pdf?sequence=3
(Disclaimer – Requires translation from Spanish unable to provide pre-translated copy)
Soiled soils -> https://doi.org/10.1371/journal.pone.0086560
Acacia and fire
So, why does acacia love fire so much?
As Acacia benefits from fire. Wattles are light loving, early successional plants. They survive after fires through regeneration from seed. This means that after a fire, when there is lots of available light, the Acacia species is one of the first woody plants to start growing and can dominate the scene.
The Acacia genus is also a major fire hazard due to its leaf litter and twig structure increasing the flammability of the fuel giving the genus a moderate/high flammability. This means they catch fire relatively quickly and are able to sustain it as well.
This flammability creates a lovely little loop of increasing fire risk – fire burns acacia, acacia seedlings profit from the fire that opens up the canopy and grows like crazy, Acacia then dominate, increasing risk of fire once more.
Wildfires are an increasing risk as climate issues intensify, making wildfire management a more pressing topic. So, if we think of wildfire management in forests in New Zealand, we could expect that forests with high proportions of acacia to be at a high risk for wildfire.
The burning question
So, will Acacia remain present once it’s entered a forest scene, especially if its dominant there? Will it keep coming back, continuing to invade and conquer the forest?
Well…
Some scientists from Lincoln University (Tim Curran, Jon Sullivan and Azhar Alam) wanted to know exactly that. Heading off to investigate the story of the understorey of an wattle dominated forest.
The hot and heavy science:
The study was done in sunny Picton, Marlborough, in a forest found in Picton Bay. The forest was a mixture of native and exotic woody plants where the Acacia species were the most common.
Tim, Jon and Azhar devised 24 random 10×10 m plots that were about 3 km apart. These plots had two main acacia species invaders: Acacia dealbata and Acacia melanoxylon, with A. dealbata being the main invader and common canopy species in these plots. The plots were sampled by measuring the abundance of each species present in different height categories:
<0.3m, 0.3-2m, 2-5m, 5-12m, 12-25m.
To measure flammability, 70 cm offcuts were abducted from 8 mature and healthy acacia trees between the two species in the plots and sealed in black plastic bags and chilled.
Offcuts were then subjected to burning where their flammability was measured in four categories:
Ignitability: How many of the samples ignited per species,
Combustibility: The max temperature reached during burning,
Sustainability: How long the offcut burnt for after the source of the fire was removed,
Consumability: How much burnt after the flame stopped.
You can read more detailed versions of the methods and stats-> https://doi.org/10.1016/j.foreco.2023.121671
Methods for flammability -> https://doi.org/10.1071/WF15047
What was hot:
So, what did they find? How does a forest respond when it’s dominated by acacia? Do Acacia species continue to invade and conquer or…
Enter the underdogs of the acacia forest: the NATIVES!
The study found that in an Acacia dominated forest where disturbances, like landslides and fires, were not common that the Acacia seedlings did not survive beneath their own canopy. Instead the understorey was populated and dominated by native forest species! But why?
Well, the Acacia species is a light loving genus of plant, sunbathing is a favorite hobby. When there is a forest with a dense canopy you get shade. Shade is something that the Acacia species do not cope with.
But what does cope with shade you ask?
Well, none other than NZ natives of courses.
That doesn’t mean that Acacia species were absent in the understorey. The Acacia species dominated the height category of <0.3 m, making up most of the seedlings present. Native plant species dominated every other height category.
If the forest environment was to remain undisturbed then the next generation of forest species would be dominated by NZ native plants. The abundance of Acacia would also slowly decline over time.
This is great as we regain native forest. Another win is that a majority of Aotearoa’s native plants, such as broadleaf (Griselinia littoralis), have low flammability, meaning that they don’t catch fire very easily. Overall, this would likely lower the forest’s wildfire risk – YAY.
However, that is the future. For now, the forest still has an increased fire risk. Acacia dealbata was the 5th most flammable plant species in the plot and the most dominant.
A single fire could still change the entire game, causing a great forest reset due to this high flammability from the presence of the Acacia species. This is also a long-term risk as once A. dealbata is present in an ecosystem they tend to persist, even under native canopies.
All it would take is a fire, or another large-scale disturbance, for the Acacia species to dominate once more, due to the existing canopy being cleared and the light-loving Acacia seedlings skyrocketing up to form the new canopy. Like a good villain, they can always come back.
The author, Kayley Wiffen, is a postgraduate student in the Postgraduate Diploma of Applied Science at Te Whare Wānaka o Aoraki Lincoln University. This article was written as an assessment for ECOL 608 Research Methods in Ecology.
Paper Reference: Alam, M. A., Curran, T. J., & Sullivan, J. J. (2024). Variation in understorey floristic composition regeneration and fire hazard under Acacia invaded forest canopy in New Zealand. Forest Ecology and Management, 554, Article 121671. https://doi.org/10.1016/j.foreco.2023.121671
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