In 2021, scientists threw 1,500 Christmas trees into a lake; three years later, they had completely transformed the ecosystem

What happens when thousands of Christmas trees are thrown into a lake? In southeastern Finland, scientists set out to find the answer by deliberately submerging 1,500 spruce trees in Lake Saimaa as part of an unusual experiment designed to study their impact on aquatic life.The project began in 2021, when researchers from the Finnish Environment Institute (SYKE) placed groups of spruce trees on the bottom of the lake. The aim was to determine whether the trees could create an underwater habitat for organisms and fish, effectively turning parts of the lakebed into a kind of submerged forest.Three years later, researchers retrieved the trees and examined the areas where they had been placed. What they found was a dramatic increase in life around the submerged trees, particularly among organisms living on or near the lakebed.The trees were placed in groups of six to eight and submerged at a depth of about two metres. Bricks were used to keep them in position underwater. The experiment was designed to examine whether the natural structure of the trees could provide shelter and food for aquatic organisms.

Benthic animals surge

Kari-Matti Vuori, the scientist leading the research, said the experiment resulted in a major increase in benthic production in areas where the trees had been submerged. Benthic organisms are animals and other living organisms that live on, in or close to the bottom of aquatic ecosystems. They form an important part of the food chain, providing food for fish and other species.According to Vuori, the number of individual benthic animals increased tenfold in areas containing the submerged trees. Species diversity also increased dramatically, rising by as much as five times compared with the conditions recorded before the experiment. The difference was identified by comparing benthic samples collected from the lake before and after the trees were introduced.The submerged trees appeared to provide a new physical structure for organisms to colonise. Microbial and fungal communities developed around the wood, creating an additional source of food and helping establish a more complex underwater habitat.The effects were also noticed higher up the food chain. Local fishermen told Vuori that they had observed a considerable increase in the number of fish in areas where the trees had been placed. While the observations from fishermen were not the same as controlled scientific measurements, they added to the researchers’ interest in how the submerged trees were influencing the ecosystem.

Christmas trees as underwater habitats

The experiment also highlighted how natural materials such as discarded Christmas trees can potentially be repurposed rather than simply thrown away. Once submerged, the trees provide branches and surfaces that can be used as shelter by aquatic organisms. As the wood gradually breaks down, it can also support microbial and fungal growth, creating food sources for smaller organisms that are themselves eaten by fish.The Finnish experiment was therefore not simply about disposing of old Christmas trees. It was designed to investigate whether the trees could contribute to aquatic biodiversity by creating additional habitat in the lake. The findings also connect to a broader environmental question: what happens to Christmas trees after the holiday season?Christmas trees are organic material and contain carbon that was absorbed while the trees were growing. Environmental experts have warned against sending them to landfill because decomposing organic waste can contribute to methane emissions. John Kazer, a footprint certification expert at the Carbon Trust, told the BBC that trees should be prevented from ending up in landfills where they can contribute to greenhouse gas emissions.However, the Finnish experiment does not mean Christmas trees should simply be dumped into lakes. The trees in the Lake Saimaa study were deliberately placed under controlled conditions as part of scientific research. Instead, the experiment offered an unusual glimpse into how a familiar seasonal object can become part of an aquatic ecosystem.Three years after 1,500 spruce trees were submerged in Lake Saimaa, researchers found that the underwater structures had become hotspots for benthic life, with the number of individual organisms increasing tenfold and species diversity rising as much as fivefold. What began as an experiment involving discarded Christmas trees had effectively demonstrated how submerged wood can provide new habitat and support aquatic food webs.