Seed Size & Predation

Longleaf Pine Woodlands and Encroaching Species

tree trunks in the mist
A healthy pine woodland, consisting of longleaf pine and no encroaching hardwoods. USDA Forest Service photo by John Willis.

Among their many benefits, prescribed fires can protect southeastern pine woodlands from encroachment – the process of fire-sensitive species expanding into fire-maintained woodlands.

Because fire is important for longleaf pine regeneration, forest researchers have studied the effects of flammability on the pine woodlands. In a recent study published in in Applied Vegetation Science, USDA Forest Service scientist John Willis investigated the relationship between seed availability and encroachment.

The southeastern pine woodlands stretch from Louisiana to Florida and Virginia. Many of these woodlands are dominated by longleaf pine (Pinus palustris), which is fire-adapted, meaning they have traits that help them survive or utilize wildfire.

“Longleaf pine is a keystone species in the ecosystem because it has flammable needles and promotes frequent fires,” says Willis. “Fires allow the longleaf pine to regenerate by opening up its seedlings so that they can establish.”

In the past 80 years, however, fewer fires have been occurring in the pine woodlands. Willis explains that there are two main causes of fire suppression: people putting out surface fires and the encroachment of hardwood trees.

This lack of fire has been a cause of forest degradation, allowing hardwood species that are less adapted to fire to encroach into the Southeastern pine woodlands.

Willis explains that when these hardwoods encroach, it fundamentally changes the structure of the pine woodlands, which can further affect flammability of the woodland:

“Imagine a park with one layer of trees as an overstory – that’s a pine woodland. When fire suppression happens, there are hardwoods that encroach, and you end up with a midstory and an understory too”, says Willis. In other words, encroachment turns a structurally simple forest into a structurally complex forest with a wider collection of tree species.

When this midstory and understory are non-fire adapted hardwoods, this can lead to further fire suppression.

small brown and white bird
Many bird species eat longleaf pine seeds, including the brown-headed nuthatch. Photo by Totodu74, courtesy of Wikimedia Commons.

Willis explains that before encroachment, most seeds in the woodlands belonged to longleaf pine. However, as new species encroach, there are now seeds available from many different species. This led Willis to ask the question “Given the choice, do seed predators still prefer longleaf pine, or will they begin eating seeds from encroaching species?”

These seed predators, Willis explains, are called granivores. These granivores – often birds, small mammals, and insects – feed on seeds, thus controlling which seeds can germinate and grow into mature trees.

Willis and his team found that granivores in the pine woodlands preferred to eat small seeds rather than large ones.

This may be good news for the longleaf pine: its relatively large seed size may keep it safe from granivores. However, this may not always be the case, as different seed predators have different seed preferences. On the other hand, loblolly pine, an encroaching species, also has larger seeds and may benefit from this same protection.

Willis explains that forest managers and others previously believed that granivores preferred to eat longleaf pine over other seeds, but his novel research shows that that this not necessarily the case. “Encroaching species will potentially take some seed pressure off of longleaf pine.”

Read the full text of the article.

For more information, email John Willis at john.willis@usda.gov.

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