Replanted rainforests may benefit from termite transplants

Termites — infamous for their ability to destroy wood — are rarely welcomed into rainforests that have been painstakingly replanted. But a new paper suggests that termite transplants may be necessary to help regenerating forests to thrive.  Published May 6 in the Journal of Applied Ecology and led by scientists from Cary Institute of Ecosystem Studies, the study found that termites are not thriving in replanted rainforests in Australia. Because decomposers like termites are essential for recycling nutrients and carbon, the researchers worry that the insect’s slow recovery could hinder the growth and health of the young forests.

“People tend to think that by just planting a diversity of trees, these rainforests will regenerate,” said Baptiste Wijas, a postdoctoral fellow at Cary Institute and visiting academic at the University of Queensland in Australia. “But it's worth thinking about, should we actually be putting in other organisms as well, to restore other ecosystem processes that help the forest function? In the context of rainforest regeneration, no one really thinks about it at all.” 

These questions are important, as restored forests make up an increasing proportion of total rainforest area. Reforestation — the process of transforming agricultural or developed land back into rainforest — is a popular strategy to preserve biodiversity and pull carbon from the atmosphere.

Wijas and Cary senior scientist Amy Zanne wanted to find out how well decomposers like termites and fungi function in replanted forests, to help predict forest health and carbon sequestration rates. 

To check how well the decomposers were bouncing back in regenerated forests, the team placed blocks of wood in three forested areas. One site was in an old growth forest in  Australia’s Daintree Rainforest, at theJames Cook University Observatory. The other two nearby sites had been replanted with rainforest trees 4 and 8 years prior to the start of the study. The sites were originally lowland rainforest until around 1900, when they were converted into plantations to grow crops such as pineapple, banana, and oil palm. They were later abandoned in the 2000’s, before being replanted by a nonprofit called Rainforest Rescue in 2010 and 2014.

For four years, the team checked the wooden blocks at the three sites every six months to see whether they had been discovered by fungi, termites, or both, and they measured how quickly the blocks were decomposing.  

The field work could be grueling. “You’re sweating all the time, and there are plants that want to attack you everywhere,” said Wijas. In one year, the team experienced drought, flooding, fires, temperatures of 113F, and “a zombie cyclone that caught us twice,” said Zanne. “It was an epic, biblical year.”

Based on previous studies in South American rainforests, the team expected termite activity to be similar in the replanted and old growth forests. They expected fungal decay rates would be lower in the younger forests. 

Instead, it turned out to be the opposite. Fungi were fairly resilient, functioning similarly in both the old growth and replanted forests, but a bit slower in the youngest forest. Termites were not so resilient. Although they were present at all three sites, they were slower to decay the wood blocks in the replanted forests than in the old growth forest, even 12 years after reforestation. The slower rate of decay may be due to the size, number, diversity, or maturity of termite colonies at the recovering sites, according to the researchers. 

Lower rates of termite-driven decay can mean a slower return of carbon and nutrients back to the soil, which could harm forest health and future growth. 

To avoid these negative impacts, the scientists suggest a novel solution: transplanting deadwood logs from old growth rainforests to newer forests. These logs could bring in decomposers from elsewhere, helping to boost abundance and diversity, while also providing a food source for decomposers that are already present.

“A young, regenerating forest doesn’t have a lot of deadwood in it,” Zanne explained. “So if you bring in these logs, you’re giving them some food to tide them over while they wait for parts of trees to start falling down.”

The team is also considering transplanting termite mounds straight into the forests along with the deadwood, “which is something that no one's really thought about either,” said Wijas. 

Neither management approach has been thoroughly studied yet, and will likely be the subject of future research by the authors. 

Wijas and Zanne acknowledge that it may be challenging to convince forest managers to transplant termites into their forests. “Many people — forest managers included — don’t really like termites,” Wijas noted. “But they play an important role in having a healthy forest.”

Only about 3% of termites are known to damage human homes, Zanne pointed out, while very little is known about the other 97%. She and Wijas are working to better understand the roles that termites have in rainforests and other ecosystems. 

“We think termites might be locking up carbon in their nests,” said Wijas. “When they eat wood, they’re not able to digest all of it, and so the feces they use to build their nests could be quite carbon-rich. They may even lock up more carbon than they emit, but we just don’t know yet.”

Termites are also associated with nitrogen-fixing bacteria, which provide a form of nitrogen that’s vital to tree growth and function. 

“Termites and fungi are absolutely critical to forest function,” said Zanne. “It would be interesting to see who else returns to the regenerating forests if the termites are there — perhaps ants, lizards, and gliders that eat termites. Right now we just don't have any idea whether these organisms are coming back in these systems.”

 

Coauthors

Coauthors on the study also included Habacuc Flores-Moreno (Commonwealth Scientific and Industrial Research Organisation in Brisbane, Australia), Steven D. Allison (University of California, Irvine), Lucas A. Cernusak and Alexander W. Cheesman (James Cook University), Paul Eggleton (Natural History Museum, London), Robert M. Kooyman (Macquarie University and Royal Botanic Gardens and Domain Trust in Sydney, Australia), and Jeff R. Powell (Western Sydney University).

Funding

This research was funded by the US National Science Foundation, Ecosystem Studies Cluster, under awards DEB-1655759 and DEB-2149151 to A.E.Z. and DEB-1655340 to S.D.A., as well as UK NERC grant NE/K01613X/1 to P.E.

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