Forests: Spatial elements of biodiversity, threats to forest ecosystems from homogeneity

The relevance of spatial dimensions of biodiversity for the good functioning of naturally occurring forests is highlighted in a study from the Missouri Ozarks.

Forests: Spatial elements of biodiversity, threats to forest ecosystems from homogeneity
Forests: Spatial elements of biodiversity, threats to forest ecosystems from homogeneity

Tree beta diversity, a measure of site-to-site variation in the composition of species present within a particular region, is more important for ecosystem functioning than other components of biodiversity at broader scales, according to biologists at Washington University in St. Louis. 

The study also found that when the spatial scale (the size of a region) increases, the association between beta diversity and tree biomass grows, which has implications for conservation planning. Ecology was the journal that published the research.

As part of her honors thesis in biology, Jacqueline Reu, who graduated from Washington University in 2019 with a double major in environmental biology and physics in Arts & Sciences, led the investigation. Christopher P. 

Catano, a Washington University Ph.D. graduate who is currently a postdoctoral research associate at Michigan State University, and Jonathan A. Myers, an associate professor of biology at Washington University's Arts & Sciences department, were Reu's mentors.

The data for Reu's thesis was obtained at Tyson Research Center, Washington University's environmental field station, as part of a large-scale forest ecology project directed by Myers' research team. For the experiment, over 60 undergraduate students, high school students, and research technicians surveyed over 30,000 trees.

"When it comes to biodiversity and ecosystem functioning, a lot of research has only looked at small scales," said Reu, the study's first author. "As you raise scale in a natural system, our study is one of the first to look at numerous different metrics of biodiversity, as well as direct and indirect effects of the environment on ecosystem functioning."

"Our findings support the hypothesis that at bigger scales, beta diversity, or variance in species composition over space, is the best biodiversity metric," she added. "It outperforms the other diversity indicators we looked at, such as local and regional diversity. Its significance grows as the spatial scale increases."

Landscape research in the Ozarks

Researchers identified 14 oak-hickory forest landscapes for this study, each of which contained at least three major habitat types common in Ozarks forests, including west- or south-facing slopes, which are sunnier, drier, and nutrient-poor; valleys and bottom lands, which are often shady, nutrient-rich, and have small streams running through them; and east- and north-facing slopes, which are the most productive in terms of tree cover.

Each landscape had a nutrient and moisture availability gradient, as well as light availability and diversity of topographic circumstances.

The Tyson Research Center at Washington University, located 20 miles southwest of the Danforth Campus, offers rich, collaborative field activities for both scientists and students. (Washington University/Thomas Malkowicz)

The researchers calculated the strength of the relationship between each diversity component and aboveground tree biomass, a property of forests related to ecosystem functioning, by quantifying the direct effects of three different diversity components: beta diversity, local diversity (the average number of species present in a small area), and regional diversity (the total number of species present in the larger landscape).

Finally, they looked at the strength of these correlations on 11 different spatial scales within each landscape, ranging from 20m to 120m. (For context, that's a size difference of around 2.6 football fields, or about the size of an end zone in American football.)

"Trying to define the scale that we should use to investigate a system has been a tricky thing in ecology — and most sciences, really," said Catano, who co-led the current study. "It causes a lot of debate and a lot of misunderstanding."

Other research has found comparable links between beta diversity and ecosystem functioning, but those previous studies tended to be based on small-scale comparisons.

Understanding how and why the relationship between beta diversity and ecosystem health scales up is a high-stakes investigation, in part because scientists are still grappling with the various ramifications of recent biodiversity loss.

"It's not simply the number of species gained or lost that matters; it's also changed in their distribution and where they might be found in an ecosystem," Catano added. "In the sense that they're showing its (species) turnover or variation across space, that seems to be really critical, this study and a few others recently have been pretty consistent in the sense that they're showing its (species) turnover or variation across space, that seems to be really critical."

"As you scale up from tiny, local plot-based ecosystems to huge, heterogeneous landscapes, (beta diversity) is driving a range of different functions, as well as the stability of these functions over time," he added.

"This experiment demonstrates the value of large-scale, long-term field research," stated Myers. "We had set up the 14 big forest plots over the Tyson region for a different investigation, so the study was mostly coincidental." However, the way we built them up was ideal for answering questions regarding how environmental variation and species diversity across space affect ecosystem function.

"One conclusion is that if environmental conditions are homogenized, it may have a deleterious influence on ecosystem functioning," Myers said. "The loss of habitat or the homogenization of the number of nutrients and other limiting resources available to organisms in an ecosystem could have cascade consequences on ecosystem functioning and services. These are indirect impacts that are difficult to predict unless both the environment and diversity are taken into account."

Monitoring ecosystem functioning

Reu, the study's first author, has worked as an intern at the Smithsonian Marine Station in Fort Pierce, Fla., and at the Smithsonian Environmental Research Center's Marine Invasions Lab in Tiburon, Calif., since graduating from Washington University. Her work at Tyson Research Center, however, continues to inspire her.

"My favorite ecosystem to be in is the forest," Reu stated. "Nothing else in the food chain would work without plants: it wouldn't exist. And it's for this reason that I find studying them so fascinating."

Reu tramped through the forest with a small group of Tyson research fellows during her undergraduate summers, identifying trees, measuring their diameters, and tagging and mapping them as part of a long-term forest monitoring program. Seeds gathered from seed traps were recognized, and seedlings were cleaned, weighed, and cataloged by Reu. She learned herself a statistical programming language at night to assist her in sorting through the data she was collecting.

"The numerical aspect of things has always appealed to me," Reu remarked. "I basically trained myself to program in R utilizing YouTube videos for this project."

"It wasn't the first computer language I'd learned, but I believe it was the most comprehensive," she remarked. "After that, I used GIS to create some visualizations."

While preparing to go to graduate school, Reu will be working on a project with rare butterflies in New Hampshire this summer.

"I've always been interested in ecology," Reu stated. "When I was looking at numerous choices, Jonathan's lab jumped out to me. He's quite creative, and they do a great job.

"And I adore trees," she added, "so that definitely helped." "Because Tyson is part of the Smithsonian Forest Global Earth Observatory (ForestGEO) network, the work there was particularly interesting. That just offers up a lot of possibilities."

This research was made possible by a grant from the National Science Foundation (DEB 1557094).


Washington University in St. Louis provides the materials. Talia Ogliore wrote the original. Please keep in mind that content may be altered for style and length.

Journal Reference:

Jacqueline C. Reu, Christopher P. Catano, Marko J. Spasojevic, Jonathan A. Myers. Beta diversity as a driver of forest biomass across spatial scales. Ecology, 2022; DOI: 10.1002/ecy.3774


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