An invasive plant thrives on the edge of fire

Normally you’d expect killing something with fire to be a good plan. However, a recent study published in Restoration Ecology by Stuart Schwab and colleagues has revealed that burning may inadvertently assist invasive species like the non-native forb, Oncosiphon pilulifer, to rebound post-fire. Contrary to popular belief, fire doesn’t consume all plant matter evenly; residual “singed” patches end up providing ideal microclimates and seed storage for this invasive species. Prescribed burns, generally thought to curb non-native plant proliferation, could unwittingly aid their spread.

Oncosiphon pilulifer, also known as Globe Chamomile if you want to be polite – or Stinknet if you don’t, is a relative of the daisy, native to South Africa and Lesotho. It looks attractive, but in Mexico, California and Arizona, it’s spreading rapidly and not responding to usual treatments against invasive plants.

The team conducted their study at Lake Perris State Recreation Park in California, where the climate varies between warm, dry summers and cooler, wet winters. The park carries out controlled fires to manage the spread of invasive plants and create better living conditions for the local wildlife, including Stephens’ kangaroo rat.

This opportunistic plant capitalises on the post-fire environment, quickly reestablishing itself within the mosaic of singed areas left behind. These singed regions, where fire has not entirely consumed the vegetation, provide a more hospitable environment for Stinknet by retaining litter (plant debris) that creates a favourable microclimate. Schwab and colleagues found that these litter-rich patches act like shelters, allowing Stinknet and other invasive plants to establish themselves successfully post-fire. This happens because the litter adjusts soil conditions such as moisture and temperature, making it easier for invaders to grow. Plus, invasive species like Stinknet are great at hogging resources quickly, leaving little for native plants.

Moreover, these singed areas become stockpiles of invasive seeds, providing a local source for invader propagation within burnt areas. This combination of microclimate benefits and available seed resources allows the plant to reestablish rapidly, posing a significant challenge for native species trying to regain their foothold.

While the fires might help Stinknet establish, they remain an important part of plant control due to invasive grasses. In an email to Botany One lead author Stuart Schwab said: “Prescribed fires absolutely help clear out invasive grasses and are one of the only effective ways to remove the thick litter layer that grasses can create from previous years of growth at the spatial scale that managers need to implement treatments. I am not a prescribed fire expert, but my understanding is that there are too many logistical barriers and safety issues to make fire effective for Stinknet.”

The problem is that while the fire clears out grass, the forbs aren’t incinerated in the same way. “We did not measure why Stinknet was not consumed, but we think it is because of differences in fuel type compared to grass. Forbs can be woodier than grasses and grow thicker stems that might be holding on to more moisture for longer making them less flammable than the grasses when prescribed burns are performed,” said Schwab.

“Basically you would need a hotter and potentially longer-lasting fire, and we don’t even know how much hotter it would need to be, so I think it’s unlikely we can adapt prescribed fires to work on Stinknet too, simply due to safety.”

“I think the main adaptation to improve the efficacy of prescribed burning for reducing the invasive forbs is to add another treatment afterwards to target remnant stands. The stands we found in the prescribed burn site were substantially smaller than the full treatment area and were really easy to find after the fire because the landscape was so barren otherwise, so I think it should be feasible to engage in secondary treatments most of the time.”

Schwab and colleagues explored whether adding native seeds could help native plant recovery. However, this approach didn’t seem to significantly improve native establishment, pointing to additional barriers beyond just seed availability. These could include competition from invasives, lack of necessary symbiotic relationships, and environmental changes.

The researchers conducted an experiment in a previously burned area to understand the factors influencing plant growth and spread after a fire. They examined the effects of complete or partial burning, the presence of litter, and the availability of native seeds. Using 80 small plots, they tested various combinations of these conditions, measuring soil moisture, temperature, and light levels in each plot as well as documenting the plant species present.

They found 27 different kinds of plants, both native (14 species) and non-native (13 species). Interestingly, the non-native plants covered a much larger portion of the area on average than the native plants.

Schwab and colleagues found that the burning method, litter, and addition of seeds can influence how plants grow and establish themselves in an area. While complete burning seems beneficial for native plants and seed germination, singed areas show higher diversity and richness, with a significant presence of Stinknet. The presence of litter and seeding don’t have a consistently beneficial effect.

“To be honest there is probably not an optimistic ending for stinknet at Lake Perris,” said Schwab. “I’m still happy with the outcome because this information can be useful for managers in areas that are not as invaded. The more we can figure out about new problematic invasives like stinknet the better, and if we can figure out how especially aggressive invaders like stinknet function ecologically it can really help expand our understanding of which strategies can be leveraged to get more native plants established in places that haven’t been hit quite as hard as Lake Perris.”

READ THE ARTICLE

Schwab, S.T., Jenerette, G.D. and Larios, L. (2023) “Prescribed burning may produce refugia for invasive forb, Oncosiphon pilulifer,” Restoration Ecology. Available at: https://doi.org/10.1111/rec.13922.

Cover image: Stinknet, Oncosiphon pilulifer. Image: SAplants / Wikimedia Commons