It is worth immersing a used Christmas tree in a lake, as it attracts fish and binds carbon

Christmas trees after use, they usually end up in an incineration plant via waste management, but in recent years they have also been carted into lakes in various parts of Finland.

The idea is not new: for the old people, sunken trees, or turos, were secret fishing spots, the location of which was not leaked to the neighbors.

An underwater forest of thousands of Christmas trees was established in Saimaa in a project by the Finnish Environment Agency. Development manager Kari-Matti Vuori was involved in investigating whether there is a mind in the hustle and bustle. He tells the Tiede magazine exam why there are Christmas trees in the lakes.

Does the underwater six attract fish?

Yes. Immersing turkeys in water is a thousand-year-old tradition in different parts of the world. Through experience, people have noticed that there are fish around the sunken forest.

In the past, it has not been fully understood why the fish have come there. It has been thought that, for example, perches like to lay their spawning bands on a sunken tree and that the fry thrive in its shelter.

It is now known that sunken wood has a dining table effect: it multiplies the surface area on which biofilm can grow, i.e. the slimy surface formed by bacteria, algae and fungal strands. Snails and aquatic insect larvae arrive to graze on it, which in turn are food for fish and waterfowl.

Does submerged wood affect water quality?

Even the large amount of trees has not weakened the water quality in natural lakes and rivers. Obviously, the top growth on the wood surfaces grabs nutrients from the water. Next, they move to snails and aquatic insects and further to fish and waterfowl. Nutrients are removed from the water when, for example, a butterfly or daylily takes flight after the larval stage and is caught by a bat or bird living on the shore.

In lakes, it is difficult to study the effect on water quality due to changing currents. In international studies, I have seen test setups where it was possible to compare samples of the water flowing into and leaving the water area. In many cases, the submerged tree has improved the water quality.

Where is Turo most needed?

A dead tree is part of any aquatic ecosystem. On barren shores where trees have been cleared, crab and fish populations want to have fun. There, by submerging wood, you can increase the bottom animal production and thus the catches as well.

Good places are also the mouths of ditches, from which nutrients enter the lake. In eutrophic waters, submerged wood is an additional tool to collect nutrients for use by animals.

In addition to fish, feeding sites benefit waterfowl in their nesting areas and resting places on migration routes.

Does the carapace also work in the sea?

The research should be extended to marine areas, as the results of inland waters cannot be generalized to saltwater species.

There is international literature on the subject, according to which the mouths and estuaries of large rivers used to be completely full of wood, which has also drifted into the sea. For example, schools of tuna have in the past dined around driftwood and used them as shelters.

We have started experimenting with submersible wood in a brackish water wetland in the Gulf of Finland, and the first impression is that the wood material brought a huge buzz to the benthic life. So, in brackish water environment, the method at least works.

Even in the Baltic Sea, there are many silted bottoms, river estuaries and anoxic areas that lack natural nutrient traps. Submerged trees could restore the natural habitat that prevailed centuries ago in the coastal waters.

On built-up shores, spruce groves could serve as spawning grounds for herring. The indigenous peoples have attracted herrings with good success even in the spruce crowns embedded in the kudu.

What happens to the carbon footprint of the Christmas tree?

At the bottom of the lake, wood carbon, especially lignin, breaks down very slowly, if ever. Up to 8,000 years old wood has been found in Saimaa. Even finger-thick branches can serve as carbon stores for hundreds of years.

We calculated that when buried, 50 pruned spruce trunks bind as much carbon as is released on a round-trip flight from Helsinki to Rome. We assumed the diameter of the spruce base to be seven centimeters and the length of the tree to be three to four meters.

What should be taken into account when submerging?

If you sink a fresh tree, the weight is pretty absolute so that it doesn’t drift. In Saimaa, we have tied fir trees in bundles to bricks. The most favorable time for immersion is in the winter, when the turon can be transported along the ice.

The ideal immersion depth is a couple of meters, then the bottom is still in the illuminated zone. You should choose a place so that it does not interfere with swimming or boating and that it has a good fishing rod or holds the sea. The quality of the base determines the effect: in awkward places, you should use a wooden frame under the studs so that they don’t get buried in mud.

If the beach is not your own, you must ask the permission of the owner of the water area. In Saimaa, we submerged thousands of Christmas trees, for which we typically asked the consent of the cooperative society of the water area and the Ely center. It is also necessary to mark the places in case of seiners.

Even a single Christmas tree is beneficial. My katiskani has also caught better next to the turo. The same is said by the icebreakers who submerged the big Christmas tree.

Published in Science in Nature 8/2024