What happens when thousands of Christmas trees are deliberately placed beneath the surface of a lake? Researchers in south eastern Finland explored that question through an unusual experiment involving 1,500 spruce trees submerged in Lake Saimaa.
The study began in 2021, when researchers from the Finnish Environment Institute placed groups of spruce trees on the lakebed. The objective was to determine whether the trees could create new underwater habitats and provide shelter and resources for aquatic organisms and fish.
The trees were arranged in groups of six to eight and positioned at a depth of roughly two metres. Bricks were used to prevent them from moving after being placed underwater. Researchers then monitored the surrounding ecosystem to see how aquatic life responded to the additional structure.
Three years later, the trees were retrieved and the areas around them were examined. Researchers found a substantial increase in biological activity, particularly among organisms living on or around the lakebed.
Sharp increase in bottom-dwelling organisms
Kari-Matti Vuori, who led the research, reported a significant rise in benthic production around the submerged trees. Benthic organisms include animals and other living species that inhabit the bottom of aquatic environments and play an important role in aquatic food webs.
Samples collected before and after the experiment indicated that the number of individual benthic organisms increased by approximately ten times in areas containing the trees. Researchers also recorded a substantial increase in species diversity, with diversity rising by as much as fivefold compared with conditions observed before the trees were introduced.
The submerged branches and trunks created additional surfaces and shelter where organisms could establish themselves. Microorganisms and fungi also developed around the decomposing wood, contributing additional sources of food and helping create a more complex habitat.
There were indications that the changes extended beyond organisms living directly on the lakebed. Local fishermen reported seeing more fish in areas where the trees had been submerged. These observations were not part of the controlled scientific measurements, but they provided another indication of possible changes higher up the aquatic food chain.
Why submerged trees can support aquatic life
The experiment demonstrated how natural woody structures can provide useful habitat in aquatic environments. Branches create protected spaces where smaller organisms can shelter, while the gradual decomposition of wood can support microbial and fungal communities.
Those microorganisms can become food for smaller aquatic creatures, which in turn form part of the diet of fish and other animals. In this way, submerged wood can contribute to the development of interconnected food webs rather than simply acting as physical debris.
The findings also raise questions about how natural materials, including discarded Christmas trees, can be reused after the holiday season. Because Christmas trees are organic material, there are several ways they can be handled after use, including recycling, composting and other forms of controlled reuse.
However, the Finnish experiment should not be interpreted as an invitation to dispose of Christmas trees by throwing them into lakes. The trees used in the study were deliberately placed underwater under controlled scientific conditions, allowing researchers to monitor their effects on the surrounding ecosystem.
Instead, the experiment provided an unusual demonstration of how submerged wood can alter aquatic habitats. Within three years, the 1,500 spruce trees had become structures supporting significantly greater numbers and varieties of bottom-dwelling organisms.
The results suggest that introducing suitable natural structures into aquatic environments can create additional habitat and potentially strengthen parts of the food web. What began as an experiment involving used spruce trees ultimately offered researchers a closer look at how wood can become integrated into a freshwater ecosystem over time.
