The study links tree genetics to biodiversity patterns

A cottonwood grove nestled along a river near the Grand Canyon. Areas like this are important to wildlife in the Southwest, but development and water consumption demands, along with stress from climate change, threaten this habitat. A new study offers a model for reforestation efforts in these areas, however, which could make aspen groves more resilient to stress in years to come. (Photo by Helen Bothwell)

It’s easy to think of trees as part of the landscape. But if the trees were the landscape?

That’s what a new study by a University of Georgia researcher asks us to imagine. By viewing each tree as a world hosting its own populations of insects and fungi, and observing the genetic variation that sustains these communities, we can better understand the role trees play in the larger ecosystem.

But then the study took the concept a step further, broadening the scope to investigate the influence of tree genetics on communities across a large swath of the North American Southwest. It is the first time that researchers have linked genetic variation in trees to community biodiversity on a continental scale.

Helen Bothwell

Understanding this relationship between tree genotypes and the organisms they support offers a more comprehensive roadmap for reforestation efforts that also support healthy ecosystems, said Helen Bothwell, assistant professor at UGA Warnell School of Forestry and Natural Resources and author principal of the study.

Diversity breeds diversity. We know that different communities congregate on different tree genotypes and we can now show that this relationship increases to affect biodiversity maintenance even on very large scales. Planting diverse afforestation stocks is of paramount importance to conserve the wealth of pollinators and predators which in turn provide valuable services to our agricultural systems and serve as a food source for bird and wildlife populations.

Study focused on poplars

Bothwell and his collaborators collected hundreds of samples from trees at more than 50 sites in California, Nevada, Utah, Arizona and northern Mexico. They focused on poplar trees, a key species of riparian ecosystems in that region. These river corridors are oases, hotspots of biodiversity within the surrounding arid and rocky landscape. But these patches of green are among the most endangered in the United States, accounting for less than 3 percent of their pre-20th-century distribution.

Development demands, water consumption, and climate change stress also threaten poplar habitat. But insights from the study, published in May in the journal Forests, may help inform reforestation efforts and create more resilient aspen groves in the future.

Many previous studies have documented strong relationships between tree genotypes and invertebrate and fungal communities at the scale of individual trees within communal gardens, he said. Common gardens are a valuable tool for studying genetic effects; by growing plants in a common environment, any residual variation is due to genetic effects. But now, the researchers wanted to know if insects and fungi could still detect this variation beyond the sharp boundaries of the garden, where nature’s disorder takes over.

We wanted to see how these insect and fungal communities relate to really large continental-scale species management. For example, do communities still respond to differences between entire populations of trees? Different watersheds? Or even large geographic regions, such as on the scale of the entire North American Southwest? said Bothwell.

After collecting samples of trees, insects and fungi, the researchers began to see patterns emerge, such as certain characteristics of trees matching certain populations of insects or fungi.

And so, we correlated the models of genetics with the models of community members; these relationships were very strong on a local scale, but as we got older, environmental variation was having a greater effect, she said.

The impact of trees on regional biodiversity

But despite the increase in environmental noise, she was still able to detect the influence of tree genetics on communities on very large scales, throughout the US Southwest and Mexico. This was surprising, she said, and highlights the importance of considering the influence of trees when aiming to conserve regional biodiversity.

This knowledge may influence future reforestation and conservation efforts in these endangered aspen ecosystems, he added. Rather than harvesting seeds only from nearby forests, land managers can look to nearby regions to strengthen tree genetics.

With climate change, there’s a recognition that local may no longer be best, trees may now be locally misfit where they settled 50 or 100 years ago, Bothwell said. So, if you’re planning to plant for future climate change, one option is to take a phased approach by harvesting seeds both locally and from nearby regions with, say, temperatures closer to those predicted by climate models 50 years from now. By taking a mixed approach, you will maintain local genetic variation while also including trees that may be better able to withstand future climatic pressures.

Losing a tree like the poplar would be devastating to the Southwest landscape, Bothwell added, due to the wealth of associated plants, fungi, insects and wildlife they support. By better understanding the forces that affect trees, land managers can also better conserve the plants and animals that live around them.

Communities that use trees as habitat pay attention to the variation they host. In short, Bothwell said, planting different trees supports different communities.

It’s a powerful retention model; you can think of it as an umbrella, he added. If you understand forest systems and work to conserve them, we can also provide conservation benefits to entire communities of organisms that we don’t have the time or resources to focus on individually. Gain more conservation benefits by focusing on the genetic conservation management of grassroots species, those species that have a large influence on their ecosystems, such as trees.