Resurvey of Boreal Earthworm Communities in Southern and Northern Finland

Boreal forests account for 11.5% of the Earth's land area, and, unlike oceanic and mediterranean forests, their cold winters typically represent the harshest season for earthworms. In Fennoscandia, these forests are subject to substantial global changes, notably climate change and land use intensification, which might impact earthworm populations. At the same time, they are also exhibiting a unique, post‐glacial recolonisation of earthworms, as the last glaciation maximum resulted in a tabula rasa of earthworm species. Yet, no study measuring temporal changes in earthworm population or diversity is known there. Here, we sampled earthworm communities in two northern and two southern areas of Finland forests. In total, 21 sites were surveyed in 2015 and re‐surveyed 10 years later in 2025–2026. We aimed at understanding how earthworm diversity and populations changed in the last 10 years, comparing community and species‐specific diversity metrics. In 10 years, although not significant, earthworm showed an abundance decline trend (−20%), driven by one southern area, whereas richness showed an increase trend (+13%). The cold‐tolerant Dendrobaena octaedra decreased, being the only one with a significant change. Community and species temporal dynamics followed a south–north gradient. Southern sites showed a decrease in abundance and D. octaedra populations, with an increase in abundance or occupancy of more heat‐tolerant species, such as Aporrectodea caliginosa , A. rosea or Bimastos rubidus . Northern sites showed an increase in abundance uniquely due to an increase in D. octaedra populations, but no other species were found in the resurvey there. These changes can be linked to climate changes leading to less rigorous winters in both south and north of Finland and more rigorous summers in the south, potentially interacting with intense forest management, but could also be attributed to a still ongoing post‐glacial recolonisation. These results call for more long‐term sampling of earthworm communities in these fragile and rapidly changing environments.