When Does Biodiversity Make a Difference?

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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