Trees have evolved to support a heavy load. Yet the mechanisms by which they do so are fairly unknown.
“Even though the idea of plants sensing their own weight and thickening their stem accordingly sounds intuitive, our study is the first one to address this question in trees,” said Juan Alonso-Serra, from the Faculty of Biological and Environmental Sciences, University of Helsinki, in a statement.
In order to address this question, researchers at the University of Helsinki, University of Cambridge, and the Natural Resources Institute Finland studied a mechanism that balances the radial growth of the stem with an increase in weight. The team tested this response, called “vertical proprioception”, on birch trees.
As detailed in the journal Current Biology, the researchers observed the adjustment in the downy birch's stem diameter when they manipulated its aerial weight. The strength of this response varied along the length of the stem. Other notable findings included the necessity of plants to be free-moving, as opposed to static, in order for the stem to thicken fully in response to weight changes.
One crucial element of the study was the comparison of normal birch to a naturally occurring birch mutant called elimäki. Instead of being “aware” of its size, the elimäki tree grows upright for three months until its stem suddenly bends at the base, causing the tree to collapse.
The failure of these mutant trees to adjust their stem diameter to the increasing weight renders them unstable. According to the researchers, this can be attributed to a single position, known as the ELIMÄKI locus, in the birch genome.
"One main challenge of working with trees is that it takes nearly a decade until they flower, and we needed them to produce a progeny [offspring] fast to segregate the genetic mutations and study them... Birches are among the few species (only ones reported so far) that can be induced to flower within one year," said Alonso-Serra to IFLScience on the importance of using birch trees for their study.
"Understanding vertical proprioceptions in trees can provide a different perspective on plant development," Alonso-Serra continued. "Trees not only are thick because they had sufficient time to grow, but they [are] thick because they are actively 'deciding' how much and where to grow, at the top, at the base and at the center of the trunk."
"Hopefully, this also changes how we all see trees."