Bend it Like Betula: the discovery of springy Arctic shrubs
When I was growing up, I loved snow days. As soon as school was canceled, all the neighborhood kids would race outside to our favorite natural trampoline – a giant bush. Its branches would bend under the weight of the snow and spring back slightly when stepped on. We took this a step further and often leapt from the top of the fence into the abyss of the bush, relying on the springy branches to push us back up.
Drs. Peter Ray and M. Syndonia Bret-Harte from Stanford University and the University of Alaska, Fairbanks described this phenomenon in a new paper about why shrubs are so bendy in cold weather.
And now, this springy quality of frozen branches has a name – Cryocampsis.
“Tundra” by Umnak is licensed under CC BY-SA 2.0.
The Strange Growth Habit of Arctic Shrubs
Like the bush near my childhood home, shrubs in the arctic lie with their branches nearly touching the soil surface during the winter. Doing so shelters the plant from harsh winter weather and protects them from grazing animals. As the weather warms and the snow melts, the shrubs pop back up to their erect position. This is a common observation in the Arctic. One June, Ray and Bret-Harte were measuring the bendiness of twigs and a late-June storm dropped the temperature to -6C. This allowed the researchers to measure the bendiness of the same twig above 0C and below 0C and surprisingly, they found that the colder stick was more bendy. This coincidental discovery spurred more research on why colder branches are more bendy and less brittle than warmer branches.
Is Cryocampsis Common?
To determine if this was a widespread phenomenon and not just a fluke in the branches near their field station, the researchers collected branches from nearly 200 plant species around the world and tested their bend and snap under different temperatures. Dr. Ray built an apparatus that tested the strength and bendiness of the branch in a uniform way to avoid introducing differences in human force and strength.
After testing branches from six different biomes (arctic, boreal, temperature, desert, mediterranean, and tropical). The researchers found that even some fair-weather plant species from the temperate, desert, and mediterranean biomes showed evidence of cryocampsis. But, the most springy plants were from the Arctic – especially after the branches were exposed to cold temperatures. This is evidence that cryocampsis is common in arctic habitats around the world and is a specific adaptation to survive in the cold environment. Tropical species were also able to bend, but when the temperature increased, they were unable to spring back to their original shape.
The most bendy branch they sampled was from the leatherleaf shrub (Chamaedaphne calyculata) which bent more than 200% with below freezing temperatures compared to warmer temperatures. Furthermore, they found these bendy branches across multiple groups of plants, like willows, poplars, and birches – suggesting that the ability to become more bendy with freezing temperatures evolved separately in multiple plant families.
The Cause of Cryocampsis
There are several reasons why plants might have developed this adaptation. One reason is to protect the buds on the end of the branches. To test this, the researchers let some shrubs lie down and forced others to remain upright through two winters and then counted the number of surviving buds. And sure enough, the shrubs that were allowed to bend had greater bud survival and were thus able to make more flowers and seeds.
Protection from the elements and herbivores may be the ecological reason for cryocampsis but the researchers still needed to figure out how the plants were able to physically become more bendy under lower temperatures. This is where the research is heading next. Ray and Bret-Harte have several theories including the movement of water between the plant cells and interactions with the ice that forms inside the plant
“Arctic Tundra” by USFWS Headquarters is licensed under CC BY 2.0.
There is still much to be discovered about this phenomenon and how it helped shape the ecosystems and environments that we know today. And importantly, this study shows the value of serendipitous experiments. This study started with an observation of a single branch of a single species and blossomed into a years-long project and the discovery of a new physiological mechanism within plants.
So the next time you are launching yourself from a fence into a snow covered bush or see something interesting in the world around you, you may discover something new with a simple question: “why?”
Peter M Ray, M Syndonia Bret-Harte, Cryocampsis: a biophysical freeze-bending response of shrubs and trees under snow loads, PNAS Nexus, Volume 1, Issue 4, September 2022, pgac131, https://doi.org/10.1093/pnasnexus/pgac131