Deadwood in Longleaf Pine Forests

Efforts to restore longleaf pine ecosystems are currently underway. Photo by Michael Ulyshen, USFS.

Longleaf pine forests are important ecosystems for a broad array of wildlife and understory plant species, such as the red-cockaded woodpecker, the southeastern fox squirrel, and the venus fly trap. Over time, the forests have dwindled due to replacement by other land uses and the suppression of fires with which they evolved. Now, they are a focus of restoration efforts.

USDA Forest Service research entomologist Michael Ulyshen is interested in studying the insect communities of longleaf pine forests, an area which has received relatively little attention.

“Many insects, especially beetles, depend on deadwood provided by forests,” says Ulyshen. “Because about a third of all forest insects require deadwood, either standing or fallen on the ground, research on the amount and variety of deadwood in all forest types is important.”

Existing research suggests beetle diversity is higher in old growth forests than in younger ones, possibly due to larger amounts of deadwood in older forests. As part of a larger effort to understand the diversity and habitat associations of insects in this ecosystem, Ulyshen wanted to see if old growth longleaf pine forests also have higher deadwood volume.

Deadwood volume is determined by a variety of factors, with moisture and species composition playing a large role. Temperate rainforests generally contain a lot of deadwood, while dry conifer forests often have much less.

Ulyshen and entomologist Scott Horn, along with collaborators from Tall Timbers Research Station and the University of Georgia, published their research in Forest Ecology and Management. They analyzed variations in deadwood across seven longleaf pine forests.

Sapwood, the outer layer of a tree, decomposes relatively quickly with help from termites and fungus. The inner heartwood is slower to decompose. Photo by Michael Ulyshen, USFS.

“We looked at how deadwood volumes vary by site and tested to see if the woody debris could provide any indication – by volume or state of decay – of old growth conditions,” says Ulyshen.

They looked at three old growth sites and two nearby secondary growth sites in Georgia and western Florida.

“We measured living and dead trees and classified deadwood by its decay class. We differentiated between multiple stages of decomposition, paying particular attention to the presence and status of heartwood,” says Ulyshen.

Heartwood is the inner core of a tree which takes a long time to form. Insects, fungi, and microbes are unable to use it, and it decays very slowly.

Deadwood volumes varied greatly between study plots, showing few consistent trends — and the numbers were quite low compared to typical deadwood measurements. The results came as a surprise to Ulyshen.

“The forests we studied are some of the best examples of undisturbed longleaf pine. Finding such low volumes of deadwood within old growth forests complicates the accepted trend that deadwood volumes are higher in the oldest forests,” says Ulyshen.

There are several factors that could explain the low volumes of deadwood in the study sites. Longleaf pine forests generally have a low basal area to begin with and have historically been exposed to frequent fires, which could limit debris accumulation. The climate in the area – hot and humid – supports microbes and fungi that expedite wood decomposition. Additionally, inputs of deadwood from wind storms can result in high variability that complicates efforts to establish baseline deadwood amounts.

Another surprising finding was the amount of heartwood, a key characteristic of advanced decay, present in the observed woody debris.

Woody debris, deadwood found on the forest floor, can provide habitat and nutrients for a variety of wildlife and insects. Photo by Michael Ulyshen, USFS.

“We did not expect heartwood to make up such a large fraction of downed woody debris, but it makes sense because longleaf pine is known to produce this core of extremely decay-resistant wood as the trees age,” explains Ulyshen. “In fact, one key result of this study is the potential use of heartwood as an indicator for old growth conditions. Large amounts of this material don’t necessarily indicate that a forest has never been logged or otherwise disturbed, but do suggest that mature longleaf pine trees have been present for a long time, perhaps hundreds of years.”

For Ulyshen’s research on beetle diversity, this study suggests that more surprises await. What will beetle communities look like in old growth longleaf pine forests that contain variable amounts of deadwood, characterized by heartwood that beetles cannot use?

As he and his collaborators work to find out, Ulyshen urges caution in drawing conclusions too quickly.

“We do not yet have enough information to reach any conclusions about the importance of deadwood in these forests. Deadwood, in general, is extremely important for biodiversity. Future research on the diversity in these ecosystems will help shed more light on the topic,” says Ulyshen.

Read the full text of the article.

For more information, email Mike Ulyshen at michael.d.ulyshen@usda.gov.

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