Dead Wood, Insects, and Fire

The study used similar sized log sections laid out and burned or left unburned. Logs were collected over time and placed in bags to capture the insects within. Photo by Michael Ulyshen, USFS.

Dead wood is a secret harbor of biodiversity. About one-third of all insect species are saproxylic – or dependent upon dead wood – at some stage in their life cycle.

The effects of common forest management practices on this important resource and the insects that use it are understudied, especially in subtropical climates.

USDA Forest Service research entomologist Michael Ulyshen with colleagues Andrea Lucky from the University of Florida and Timothy T. Work from University of Quebec investigated how fire, a common forest management tool in southern pine forests, affected insects that use dead wood.

This research was recently published in the journal Scientific Reports.

The location of this research in a subtropical climate and its focus on groups of insects that all use dead wood in different ways – beetles, termites, and ants — make this study unique.

“Many insects may not be directly dependent on dead wood but may be predators of insects in dead wood. A lot of research focuses on beetles, but termites are also an ecologically important species in dead wood. We have limited information on what all of these species are doing, and there are hundreds and hundreds of species,” says Ulyshen.

The study was established on the Noxubee National Wildlife Refuge in Mississippi in frequently burned, mature loblolly pine forests.

Twelve fallen and decaying loblolly pine trees were cut into sixteen logs each, or eight pairs. The scientists placed one member of each pair in an area scheduled for a low intensity burn, and the other in an area not planned for a burn.

The team collected log pairs from burned and unburned sites at set intervals since the burn – immediately after, and at 2, 6, 26, and 52 weeks.

The researchers placed logs in insect emergence bags for six months. They counted beetles and identified both beetles and ants. Termites were recorded as present or absent.

The logs contained a total of 220 species of beetles. Photo by Michael Ulyshen, USFS.

The logs were alive with insects, and the insects seemed to be resilient to fire. The study found nearly 13,000 beetles belonging to 220 species and 38 species of ants. Termites were active in 66 percent of the logs. These patterns didn’t differ between burned and unburned logs.

Total beetle and ant numbers differed little between burned and unburned logs, although significantly more beetle species were collected from burned logs two weeks after the fire. These results suggest some species (possibly predators) rapidly colonize burned wood, but fire otherwise has little overall effect.

Ulyshen and colleagues reached a similar conclusion in a previous study, but they detected more negative effects of fire due to a higher burn intensity.

Even though fire effects were weak in both studies, the composition of both beetle and ant communities differed significantly between treatments in the current study. Some species were associated with burned logs while others showed the opposite response. “That suggests that repeated fires over long periods of time may lead to changes in species assemblages,” Ulyshen says.

Regardless of fire and time interval, the insects were coexisting on the same logs — an unexpected result. Most relationships between beetles, ants, and termites were positive or neutral. Negative interactions were significantly less common.

“The initial expectation was that ants would have more negative effects on other insects because many of them are predators,” adds Ulyshen, “and that termites may have negative effects by taking up space. That there were significantly more positive than negative associations among species was unexpected and provides some insight into how wood supports such diverse insect assemblages.”

This study provides valuable insights into how forest management can change dead wood and insects and begs more questions. “Future studies testing similar questions with more intense fires or in less fire-adapted systems would be of great interest,” says Ulyshen.

Read the full text of the study.

For more information contact Michael Ulyshen at michael.d.ulyshen@usda.gov.

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