When Does Biodiversity Make a Difference?

by Stephanie Worley Firley
Tree species that are more distinct in terms of their evolutionary past may have uniquely important ecosystem functions in a forest community. Pictured is a diverse oak-hickory forest in North Carolina. Photo by Kevin Potter.

Tree species that are more distinct in terms of their evolutionary past may have uniquely important ecosystem functions in a forest community. Pictured is a diverse oak-hickory forest in North Carolina. Photo by Kevin Potter.

Biodiversity can be like a forest’s insurance policy. The more and varied the tree species that live there, the better the chance that the forest can remain healthy, stable, and resilient through times of disturbance. But as climate change prompts new forest management approaches intended to maximize growth and productivity for carbon storage, bioenergy, and other benefits, U.S. Forest Service and North Carolina State University (NCSU) researchers are wondering: when exactly does biodiversity make a difference?

“Considering biodiversity in policy and management decision making is essential, but there are likely to be times when biodiversity is more important to how forest ecosystems function as a whole,” says Kevin Potter, an NCSU scientist working with the Eastern Forest Environmental Threat Assessment Center. To examine the central research question, Potter collaborated with research forester Christopher Woodall from the Forest Service Northern Research Station. They studied the dynamics at play between tree biodiversity and live aboveground biomass across the contiguous United States and published the results in the journal Forest Ecology and Management.

Biodiversity can be measured in many ways, though. If one thinks of biodiversity only in terms of the numbers of tree species in an ecosystem (species richness), then all species are assumed to have equally important roles in the ecosystem. Potter therefore uses measures of biodiversity that incorporate relationships between tree species on the evolutionary tree of life. Species that are more distinct in terms of their evolutionary past may have uniquely important attributes; evolutionary diversity can therefore predict the diversity of different ecosystem functions present in a forest community, which may translate into enhanced biomass production. For this study, the researchers used four measures of evolutionary diversity as additional indicators of tree species biodiversity and function in order to understand the connection between forest biodiversity and biomass.

The researchers analyzed data from about 79,000 permanent Forest Service Forest Inventory and Analysis (FIA) monitoring plots. Taking into account the environmental conditions present at each plot as well as the stocking of live trees (a rough measure of the development of a forest stand), the researchers saw a pattern: biodiversity was a better predictor of biomass for forest stands located in more difficult growing conditions. “When the tree species growing together in a more stressful environment are more widely distributed across the evolutionary tree of life, they are more likely to use available resources, such as light, water, and nutrients, differently. These different functional attributes allow the trees to make the most efficient use of available resources and therefore be more productive overall,” says Potter. “This doesn’t seem to be the case in more friendly forest environments, though. In those places, fewer species seem to be able to use the resources more efficiently, so the most dominant and highly productive species are able to outcompete other species in those habitats.”

The researchers say that more studies are needed as FIA monitoring plots are remeasured over time, but this research is already providing forest managers with important information. “As managers make decisions about how to help forests adapt to and mitigate impacts of climate change, they’ll need to consider the interactions between tree biodiversity, site conditions, and the stocking of live trees,” says Potter. “The biodiversity measures used in our research can help managers understand forest function and allow them to maximize biomass to maintain healthy forests and meet societal needs.”

For more information, contact Kevin Potter at kevinpotter@fs.fed.us.

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Posted in Climate Change, Forest Inventory & Analysis

Bringing Fire Back to the Kisatchie Sandstone Hills

by Sarah Farmer, Science Delivery Group
The Kisatchie Sandstone Hills of Louisiana provide habitat for many rare plants and animals, such as red cockaded woodpeckers and Louisiana pine snakes. Photo by Andy Scott.

The Kisatchie Sandstone Hills of Louisiana provide habitat for many rare plants and animals such as red-cockaded woodpeckers and Louisiana pine snakes. Photo by Andy Scott.

The hillside bogs, sandstone glades, and woodlands of the Kisatchie Sandstone Hills in Louisiana are potential homes to a number of rare and endangered animals such as the red-cockaded woodpecker and the Louisiana pine snake. However, in much of the Kisatchie Hills, the open woodlands these animals need have vanished amid a dense midstory of shrubs and small trees.

“Restoration of the Kisatchie Hills requires reintroduction of fire,” says U.S. Forest Service scientist Andy Scott. “Past fires, though, have resulted in high erosion rates and soil loss.” Scott is a research soil scientist at the Forest Service Southern Research Station (SRS) Southern Pine Ecology unit, and author of a paper about erosion and prescribed fire on a study area established in the Kisatchie Hills. The paper was recently published in the Southeastern Naturalist.

The Kisatchie Hills are part of the Kisatchie National Forest in west-central Louisiana and feature a number of fire-dependent ecosystems. Historically, the area was logged and open to cattle for grazing, but now it’s been about 65 years since the 4,000-acre study area was logged, and the last burn or cattle grazing was more than 20 years ago. Kisatchie Ranger District personnel conducted two prescribed burns – one in the growing season and one in the dormant season – on half the study area and compared vegetation, fuels, and erosion potential in similar areas of the burned and unburned sites.

After the fires, many of the shrubs and small trees in the midstory were top-killed, while grasses and wildflowers began to sprout and spread. Scott and his colleagues found that in the burned areas, 40 percent of the forest floor was covered with grasses and wildflowers, while only 7 percent of forest floor in unburned areas had herbaceous vegetation cover. Both red-cockaded woodpeckers and Louisiana pine snakes need a healthy herbaceous understory.

“Red-cockaded woodpeckers require old, widely spaced canopy pines for roosting and nesting, and open pine habitat for foraging,” says Scott. “Louisiana pine snakes prey on animals such as Baird’s pocket gophers, which in turn eat the roots of herbaceous plants. The presence of herbaceous vegetation is of prime importance for both red-cockaded woodpeckers and Louisiana pine snakes.”

Attempting to restore the Kisatchie Hills with prescribed fire is challenging because of the soil and geology of the area. “Soils on the study site are quite infertile,” says Scott. “They are also highly susceptible to erosion.” Much of the topsoil has already washed away, and the soil underneath it is very finely textured and prone to runoff. The prescribed fires reduced the thickness and density of the protective forest-floor layer, and elevated the erosion risk in the burned sites. However, the increased erosion risk may have been countered by the protective effects of a shorter midstory, and the recovery of the herbaceous community.

“Future management should consider erosion prevention,” says Scott. “The timing and intensity of additional burns should also be used to maximize plant cover on the forest floor and to improve the habitat by converting the woody understory to an herbaceous understory.”

Read the full text of the article.

For more information, email Andy Scott at andyscott@fs.fed.us

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Posted in Fire, Fish & Wildlife, Restoration, Southern Pines

Interrrupting an Invasional Meltdown

Research shows removing privet allows native earthworm communities to recover

by Zoё Hoyle, SRS Science Delivery Group
Southeastern forest infested with Chinese privet. Photo by David Moorhead, courtesy of bugwood.org.

Southeastern forest infested with Chinese privet. Photo by David Moorhead, courtesy of bugwood.org.

Earthworms have been described as “ecosystem engineers” because they can transform soil environments in ways – physical, chemical, and biological – that in turn lead to aboveground ecological changes. Most of the 8,000 species of the world’s earthworms stay in areas where they evolved, some occupying very narrow niches, but about 120 “cosmopolitan” or “peregrine” species have spread throughout the world, some invading and displacing native species.

A recent article by U.S. Forest Service and University of Georgia (UGA) researchers reports the effects of removing invasive Chinese privet on soil properties and earthworm communities in floodplain forests of the U.S. Southeast. Scientists found that removing privet disrupts the “invasional meltdown” that can occur when changes to soil due to privet infestation make it easier for exotic earthworms to invade — and allows native earthworm communities to recover.   

The Asian jumping worm (Amynthas agrestis) was one of the invasive earthworm species found in the experimental plots. Photo by National Park Service.

The Asian jumping worm (Amynthas agrestis) was one of the invasive earthworm species found in the experimental plots. Photo by National Park Service.

The researchers — Mac Callaham and Jim Hanula from the Forest Service Southern Research Station (SRS) with UGA graduate student and lead author Joshua Lobe and UGA professor emeritus Paul Hendrix – used the experimental forest plots established by Hanula for his long-term study on the effects of privet removal on aboveground native plant and pollinator communities. The team sampled every three months for a year, comparing soil characteristics and earthworm communities in plots with privet, those where privet had been felled, and those where privet had not yet invaded. 

“Other than a couple of studies on root biomass and soil properties, there have been surprisingly few studies on the effects of privet on belowground biotic communities,” said Callaham, research ecologist for the SRS Center for Forest Disturbance Science. “This is the first study to look at the effects of removing an invasive plant on soil properties and earthworm communities in these southeastern riparian forests.” 

Researchers found 14 different species of earthworms in the privet experiment plots, five native to North America, the others originating from Europe and Asia. Overall, they found the lowest abundance of native earthworms in the plots with privet, greater abundances in reference plots and in plots where privet was felled and cleared. Analysis showed that the soil pH was significantly higher (i.e., less acidic) in the privet plots than that in reference plots and those where privet was felled.

“More research is needed, of course, but we speculate the presence of privet caused pH to rise, favoring some exotic earthworm species and potentially leading to an invasional meltdown,” said Callaham. “We found that where privet was removed, pH was as low as reference plots, and exotic earthworms seemed to lose their competitive advantage and so native earthworm communities began to recover.”

“The study supports the idea that by removing a key invasive species like privet, land managers could decrease interactions with other exotics and short-circuit the invasional meltdown process,” added Callaham. “It also shows that native earthworm species have the potential to recover after the removal of an invasive plant despite the continued presence of exotic earthworm species.”

 Read the full text of the article, published online in April 2014.

For more information, email Mac Callaham at mcallaham@fs.fed.us.

Read more about recent SRS research on privet removal effects

Read more about about Callaham’s research on invasive earthworms.

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Posted in Bottomland Hardwoods, Forest Operations, Invasive Plants, Restoration, Threats