Termites and Dead Wood in Pine Plantations

Ecosystem engineers aid with forest decomposition but not neccessarily with tree growth

termites
Their appetite for decaying wood makes them extremely unwelcome in homes. But in forests, termites play a critical role in decomposition. Photo by Keisotyo.

A handful of the world’s 3,100 known termite species damage homes. In forests, however, termites are valuable.

“Termites recycle dead wood,” says U.S. Forest Service research entomologist Michael Ulyshen. Termites consume as much as 20 percent of the dead wood in forests, as Ulyshen showed in 2014.

“Dead wood exists at the interface between below ground systems and above ground systems,” says Ulyshen. Woody debris links these zones, but its role in nutrient cycles is largely unknown.

Researchers do know that a tremendous number of insect species – hundreds of thousands of them – depend on dead wood. “The vast majority of these species are benign and many are likely to provide important benefits,” says Ulyshen.

Ulyshen recently led a study on subterranean termites, dead wood, and the nutrient cycle. The study took place in Mississippi pine plantations, and the results were published in the journal Ecosphere.

“Our goal was to see how termites affect the release of nutrients from dead wood and how this, in turn, affects tree growth,” says Ulyshen.

The scientists fed the termites freshly cut pine logs, placed at the base of loblolly pine saplings. As a control, some of the logs were encased in fine mesh that excluded termites.

study site
The study took place in Mississippi pine plantations. Study sites were not nitrogen-limited, and termite action did not spur tree growth. Photo by Michael Ulyshen, USFS.

After setting up the logs, the researchers waited more than four years. “It was tempting to roll the logs over and peek underneath,” says Ulyshen. “But we didn’t want to disturb the site, and previous studies had given us a pretty good idea about how much decomposition to expect.”

When the scientists returned, they measured the effects of decomposition, including tree growth and soil nitrogen concentrations.

Nitrate concentrations were higher near saplings with decomposing logs at their bases. Soil near decomposing logs also had higher potential net nitrification rates. However, there were no significant effects on tree growth or nitrogen concentrations in the pine needles.

“Soils in our study site already had enough nitrogen for tree growth,” says Ulyshen. “If the site had been nitrogen-limited, the dead wood additions and termite action could have spurred tree growth.”

Some of the nutrients from the decomposing logs may have left the site. “We think the termites used the nutrients for their own growth and biomass – and then flew away,” says Ulyshen.

Colonies may live for years before producing winged reproductive termites. But once the biological transformation begins, non-reproductive workers grow wings, become reproductive, and fly away.

Termites have such important roles in decomposition that they are considered ecosystem engineers.

Ulyshen and his colleagues found that termites create healthy variations in soil properties. Dead wood also contributes to the spatial heterogeneity of soil.

It is common to remove dead branches, stumps, and other woody debris after forest management operations like harvesting or thinning. “However, dead wood is a legacy structure,” says Ulyshen. “It can provide many potential benefits, including the release of plant-available nitrogen.”

termite swarm
Periodically, colonies produce winged termites that emerge in huge swarms. Photo by Ganesh Subramaniam.

Dead wood is also important for forest biodiversity. As many as 30 percent of all forest insect species are saproxylic, which means that they depend on dead wood. Almost all of these insects are non-pest species, and many are beneficial.

“For example, many predatory beetles, flies and parasitoid wasps are only found in dying or dead wood,” says Ulyshen. “These insects have been shown to significantly reduce populations of forest pests, including bark beetles. This can be a great help to forest managers and landowners.”

Read the full text of the study.

For more information, email Michael Ulyshen at mulyshen@fs.fed.us.

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