Trees have different organs, each with specific roles. As an example, through photosynthesis, leaves use light to produce sugars that a tree needs to grow. “As plant ecologists, we study how these organs fulfil their roles by looking at their traits: e.g., trees with small and thick leaves generally grow slower than trees with large and thin leaves because their rates of photosynthesis to produce sugar are generally slower,” explained Monique Weemstra, first author of a paper published in the Journal of Ecology, June 12, 2021. “Besides leaves, roots also take up important resources: underground, they capture water and nutrients that are essential for trees to grow. Yet while we occasionally trip over them, most tree roots remain hidden in the soil, and how roots help trees grow remains largely unknown.”

Weemstra is a postdoctoral fellow in the lab of coauthor María Natalia Umaña. The paper is titled “Tree growth increases through opposing above- and below ground resource strategies.” Additional coauthors are David Allen (U-M EEB Ph.D. 2011), assistant professor in biology, Middlebury College and Jenny Zambrano, assistant professor of plant community ecology, Washington State University.

“Due to their invisibility, our assumptions on how roots drive tree growth have long been inferred from leaves,” Weemstra continued. “So, similar to having large and thin leaves, trees with long and thin roots would be better at capturing water and nutrients and therefore grow faster than trees with short, thick roots. However, if this assumption holds is not clear, because relatively few studies look at roots, and almost none of them link roots to the growth of forest trees. At the same time, looking only at roots might not offer a complete picture of tree growth either. The different organs of trees strongly depend on each other: the process of leaf photosynthesis heavily relies on water and nutrients supplied by the roots, and the production of roots to capture soil resources requires sugars produced by the leaves. Therefore, we examined which combinations of leaf and root traits lead to the fastest growth of trees.”

With the help of students, they collected and measured leaves and roots from different trees in the “Big Woods” plot in southeast Michigan, U.S. This plot is a part of the E.S. George Reserve, a research and education site owned and managed by the University of Michigan and it is one of the 73 plots of the ForesGEO network, a forest global network dedicated to monitor and understand long-term dynamics of forest communities. They used information about tree growth: at Big Woods, every 5 – 7 years, a group of researchers and students measures and records the stem diameter of all trees that shows how fast each tree has grown (in diameter). Then, they used statistical models to see which leaf and root trait combinations best characterized the growth rates of trees.

Tree roots at the E.S. George Reserve. Image credit: Hans Reijnen

By doing these censuses at regular intervals, they are able to monitor the growth and survival of trees, and thus the state of, and changes in these forests. Previous censuses were in 2003, 2008, 2014 and this one, in 2021.

“It turns out that at Big Woods, leaf and root traits together better explained the growth of trees than the traits of leaves and roots alone. Also, the role of leaf traits in tree growth depended on the roots of trees, and vice versa. More specifically, some of the fastest-growing trees had thick roots. At a first glance, this contradicts our general expectations, but it makes more sense when you consider that tree roots almost never work alone: they form close partnerships with mycorrhizal fungi that colonize the roots, and transfer water and nutrients to the tree in exchange for sugars made by the leaves. Thick roots might be able to host more mycorrhizal fungi through which they acquire water and nutrients that helps trees grow faster. However, having thick roots was only helpful for the growth of trees that also had small, thick leaves; trees with large, thin leaves grew faster (as generally expected), but only when they also had thin roots.

“We are not yet certain what drives this distinction between trees with large and small leaves, but it suggests that – unlike just making large, thin leaves and long, thin roots – trees can combine different leaf and root traits to improve their growth. Our study also stresses that to understand why some trees grow faster than others we should not only look above- but also underground, and not only to roots but also to their fungal friends. By studying the above- and belowground we can better unravel the full story behind the growth of trees in our forests.”

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