Trees in Rovaniemi exhibit unusual autumn foliage as warm weather delays winter dormancy in Finland

In many areas, trees still display green leaves despite being in mid-October. This phenomenon is particularly evident in central Rovaniemi, where numerous aspens, rowans, and birches retain their foliage. Last autumn, leaves remained on the trees throughout the winter, raising concerns among observers. This year, a similar occurrence may happen, attributed partly to the prolonged warm weather.

Anne Tolvanen, a professor at the Natural Resources Institute Finland, expressed mild surprise at the delay in winter dormancy among northern native trees. She noted that while the warm weather has slowed the process of winter preparation, most of the essential nutrients have already been absorbed by the trees. The abscission layer, responsible for detaching leaves, has not yet fully developed.

Generally, deciduous trees are well-adapted to winter conditions, shedding their leaves timely in response to shorter days and changes in light. Tolvanen added that if trees have successfully absorbed nutrients into their branches, trunks, and roots, retaining leaves does not harm them. Leaves typically fall when enough snow accumulates or during severe storms, as observed last year.

Different species exhibit varying behaviors in autumn. For instance, birch is a traditional summer-green tree that sheds its leaves in the fall after absorbing chlorophyll and nutrients. Nitrogen is a critical nutrient that birches aim to conserve.

In contrast, alders function differently, as symbiotic bacteria in their root nodules allow them to directly absorb nitrogen from the atmosphere. This gives them the luxury to lose their green leaves without concern. Planting trees from southern origins may still show green foliage, but they tend to miss nutrient absorption and lose some resources, which could impact growth in the following year.