Resurvey of boreal earthworm communities in southern and northern Finland

In northern boreal Europe, the Last Glaciation Maximum resulted in a tabula rasa of earthworm species, post-glacial recolonisation of few peregrine species. Boreal forests represent 11.5% of the Earth's land area, and their cold winters represent the harshest season for earthworms. These biotopes are subject to substantial global changes, notably climate change and land use intensification, which might impact earthworm populations. Yet, no study measuring temporal changes in earthworm population or diversity is known in this biotope. Here, we sampled earthworm communities in two northern and two southern areas in Finland forests. In total, 21 sites were surveyed in 2015 and resurveyed 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 ten years, although not significant, earthworm abundance declined (-20%), whereas richness increased (+13%). The cold-tolerant Dendrobaena octaedra decreased, being the only one with a significant change. Temporal community and species dynamics followed a south-north gradient. Southern sites showed a decrease in abundance and D. octaedra populations, with an increase in abundance and occupancy of more heat-tolerant species. Northern sites showed an increase in abundance uniquely due to an increase in D. octaedra populations, but no other species were found 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.