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Why forests with more tree types grow better, faster

A study by University of Minnesota researchers has found out why diverse forests grow better and faster than single-species plantations. It has to do with shapes and how the trees fit in with each other as they seek sunlight. News Tribune file photo

It's been known for years that forests with lots of different tree species grow better and faster than forests with just one kind of tree.

Now, for the first time, scientists say they know why.

It's shapes.

It turns out trees of different species find a way to get along with their neighbors by spreading branches out to fill in gaps where sunlight is available — they play off each other's shape. And that maximizes their combined ability to soak up the sun falling on a particular plot of land.

The new information could not just help foresters provide more fiber for the region's wood products industry, it could also help reduce climate-changing effects of greenhouse gases.

A study published this week in the scientific journal Nature Ecology & Evolution by scientists from the University of Minnesota and Université du Québec à Montréal looked at 37 plots of trees that had been planted in Montreal four years previously, ranging from a monoculture (only one tree species across the entire plot) to a plot with 12 different tree species commonly found in northern forests.

They used basic tools such as measuring tapes and height poles to differentiate the vertical growth of branches and leaves as well as the amount of trunk biomass that trees produced under the various combinations of species.

They found that in plots with multiple species, the different natural growth forms and light requirements of the various species, combined with their ability to modify their growth to their neighbors, made it possible for the trees to send branches into places where they could better use the available light, growing better together than in single-species plots.

The researchers found that the better-equipped the particular combination of species was to use the range of sunlight within the forest canopy, the more biomass the plot supported.

"This study shows how we can think of forests as communities made up of trees that fit together ... and react to their neighbors in ways that affect how the entire ecosystem functions," said Laura Williams, lead author of the study and a graduate student in Ecology, Evolution and Behavior with the College of Biological Sciences at the University of Minnesota. "In helping to answer the long-unresolved question of why more diverse mixtures grow more, we've improved our understanding of how to sustain and improve the functioning of forests in ways that contribute to the well-being of humans and our planet."

It's not just interesting to know how these tree species play off each other. The data gives foresters better information to manage forests to get more production — and that's good for providing more trees for the Northland's wood products industry. As more private land is taken out of timber production, and as government land managers seek to reserve more big trees as old growth or animal habitat, providing more trees from the same amount of land could be critical to supply industry.

Different species also provide habitat for more birds and wildlife, and even for microbes in the soil that affect nutrients for the trees..

"It might be easier to plant a single species, especially species for the forest industry. But if we want more total biomass, and we want the whole ecosystem to improve in health, than planting a diverse mixture of species clearly has benefits," Jeannine Cavender-Bares, a University of Minnesota professor of plant ecology, told the News Tribune.

Having more trees growing on the same amount of land is also a way to help battle climate change, because more woody biomass growing means more carbon dioxide is getting pulled out of the atmosphere and stored in the trees.

Most of the Earth's human-planted forests, or plantations, are monocultures with just a single dominant species; think of the red pine plantations once common in the Northland or European forests managed for only a specific tree type. Only about 1 percent of all planted forests on Earth fit the diverse forest description needed to produce more fiber, the authors noted in their report.