Invasive pines rely more on fungi than native trees, countering 'ideal weed' hypothesis

March 30, 2021

Invasive pines need soil fungus to prosper. (Jaime Moyano)

Compared to noninvasive species, researchers have determined that invasive pine trees have a stronger dependence on fungi in the soil, refuting an established hypothesis that asserts invasive species should rely less on these mutualistic relationships.

The study, published March 11 in Ecology, suggests that reliance on mutualism — a symbiotic relationship in which both species benefit — may enhance rather than limit the spread of invasive species, complicating scientists' present understanding of invasive species ecology.

The findings also fly in the face of the "ideal weed hypothesis," which was established nearly 70 years ago.

"The idea is that if you introduce a plant species outside their native range, they are going to do better if they don't need mutualism," said Jaime Moyano, a postdoctoral researcher at Universidad Nacional del Comahue in Argentina. "For example, fungi. If they can live without fungi, they probably do better, because if not, they will be restricted to where they can find this particular fungi."

Plants may have symbiotic relationships with bacteria, insects or even other plants, but fungi are an excellent mutualistic relationship to study in connection with the ideal weed hypothesis because of how ubiquitous these relationships are.

Moyano explained to The Academic Times that about 92% of plants, including all known pine trees, have a mutualistic relationship with fungi, and these relationships can be highly specific, with plants relying on a single species of fungus.

"In this interaction, the fungi help the pines by increasing their absorption of water, nutrients and protecting them from pathogens, and in exchange the pines provide them with carbon compounds," he said.

The ideal weed hypothesis has gone largely unchallenged because it is logical that invasive species should be more successful if they are not forced to rely heavily on host-specific relationships with fungi that may or may not be present in their new environment.

However, Moyano and his colleagues noticed the exact opposite when conducting a prior meta-analysis of existing data on invasive pine trees.

"The big surprise came from the previous work, when we expected the opposite pattern, and we found this," he said. "But then we wanted to test this for ourselves with an experimental study specifically designed to test this hypothesis."

Pine trees are native to the northern hemisphere, but at least 19 species are now considered invasive. The widespread cultivation of pine trees for timber has introduced them into South America, Africa, New Zealand and other regions across the world, where they easily take over and displace native plants by nature of their hardiness.

The team selected five species of pines to test, choosing the plants based on Z-score, an ecological measure of overall invasiveness. Selecting species based on the Z-score rather than incidence in a given area meant their analysis was applicable globally, not limited to a specific region.

The team inoculated seedlings with the mutualistic fungi and compared their growth to that of control plants without fungi. The results confirmed their previous findings: The more invasive plants grew much better with the fungus, whereas less invasive plants saw little difference.

However, this was not the only insight the experiments yielded. They also found that the differences in growth did not kick in until the seedlings were about six months old, indicating that time and seed size may contribute to this effect.

"We introduced two new aspects here. One was the time dynamics, to see that already at six months old, the pines suffer from the absence of their mutualistic partner," Moyano said. "And the other thing is that in this case we could test independently the effect of the seed size. And we found that both factors were important — something that we couldn't do in the meta-analysis."

Moyano explained that trees with larger seeds were able to last longer without the benefit of their mutualistic fungi because large seeds store more energy for the plant to use for growth. However, smaller-seeded plants tend to be more invasive because small seeds can disperse much more easily. This is one possible factor behind the effects found in the study.

In the future, Moyano and his colleagues will continue their work on invasive species, specifically focusing on the question of why some species become invasive when others do not. For now, he maintains that this work provides one clue into how mutualistic fungi factor into the equation.

"Interaction with fungi is not well understood and is really something that we are learning a lot more about," he said. "And I think it could explain a big part of why some plant species are invading."

Moyano also hopes that these findings may be applicable to indirectly stopping the spread of invasive species by targeting the mutualistic fungi they live with, a strategy that wouldn't be viable under the original ideal weed hypothesis.

"I think something really interesting to consider is that if we could at least limit the spread of the fungi, this could really help to prevent the pine invasion," he said. "To prevent the dispersal of microscopic fungi that we don't even see is really difficult, but when introducing to a new region, we should not only focus on the pines that are introduced, but also their mutualistic partners."

The study, "Invasive trees rely more on mycorrhizas, countering the ideal weed hypothesis," published March 11, was authored by Jaime Moyano, Universidad Nacional del Comahue; Mariano A. Rodriguez-Cabal, Universidad Nacional del Comahue and University of Vermont; and Martin A. Nuñez, Universidad Nacional del Comahue and University of Houston.  

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