Biodiversity matters in every forest, but even more in wetter ones

(Press-News.org) Biodiversity is important in every ecosystem. Research has shown having a richer diversity of plant and animal species bolsters ecosystem functioning and stability, making habitats more resilient to the threats of things like pests, disease and climate change.

Still, there are varying degrees of importance. Now, research led by the University of Michigan reveals some of the real-world implications of this fact in forests, providing valuable insights for land managers and conservationists working to protect these habitats.

"Diversity matters everywhere we look," said Peter Reich, professor of environment and sustainability and director of the Institute for Global Change Biology at the University of Michigan. "But in our experiments, we see it matters more in the wetter climates." 

Reich was the senior researcher on a project that encompassed 15 tree diversity experiments across the globe and spanning a range of climates. In an analysis that involved 100,000 trees belonging to nearly 130 species, the researchers found that trees with more diverse neighbors in wetter climates grew more. In drier climates, the effect was weaker or nonexistent.

"This provides a more nuanced insight into how biodiversity works in different environments," said Reich, who is also a professor at the University of Minnesota. "If you're thinking of restoring or regrowing, it makes a lot of sense to plant diverse trees, especially in wetter areas."

Reich stressed that the "wetter" designation includes more than just tropical rainforests that likely come to mind. By the study's metrics, for example, Michigan's forests would also qualify as wet.

The project was enabled by the larger Tree Diversity Network, which is the world's largest network of tree diversity experiments consisting of 34 different sites. The resulting study, published in the journal Nature Ecology & Evolution, included authors from nearly a dozen countries representing more than 30 affiliations. 

The work, likewise, had funding from international sources, including the U.S. National Science Foundation, German Research Foundation, São Paulo Research Foundation and University of Chinese Academy of Sciences.

"We find a tree growth boost in diverse tree neighbourhoods with different species and functional traits—for example, specific leaf area and wood density," said study lead author Liting Zheng, who performed the work as a postdoctoral fellow at Michigan's School of Environment and Sustainability and is now a researcher at the German Center for Integrative Biodiversity Research.

"Our results highlight the importance of establishing and maintaining high neighborhood diversity in forest management—especially to maximize the potential of biodiversity as a nature-based climate solution in wetter regions."

Seeing the forest for the trees

Although previous studies have examined the benefits of biodiversity in the context of climate, their results have been inconsistent. The reasons for that aren't entirely clear, Reich said, but many have shared a couple notable limitations.

For one, they often focused on a single experimental site. Then the few that have used more sites for a more comprehensive analysis tended to quantify biodiversity at a broad level, as a sort of average descriptor of an entire community.

The new study adopted a much more granular approach. Researchers at each experimental site characterized tree biodiversity by looking at every individual and their neighbors.

"What's unique about our study is it has 15 long-term experiments with lots and lots and lots of trees," Reich said. "Then, rather than looking at an entire community, we looked at every individual and eight or so of its neighbors and asked, 'Are those neighbors diverse or not?'"

He added that a limitation of their study is that, because of the nature of the experiments, the trees studied were all fairly young, from 4 to 14 years old. But there's not an obvious argument for why the results wouldn't hold for more mature forests. 

This approach enabled the researchers to show the pronounced impact of biodiversity in wetter forests, as well as make another interesting and important finding. While biodiversity appears to matter more in wetter climates, its impact in wetter years in those wetter climates is negligible. 

"Diversity appears to be better suited for the average condition, which matters for management," Reich said. "There's thinking that biodiversity is really going to help you when you have those extreme years and some evidence supporting this idea, but in our data this was not the case."

Kai Zhu, associate professor of geospatial data science, and Inés Ibáñez, professor of ecosystem science and management—both with Michigan's School of Environment and Sustainability—also contributed to the study.

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