Data from: Thermal homogenization of boreal communities in response to climate warming

Globally, rising temperatures are increasingly favoring warm-affiliated
species. Although changes in community composition are typically measured
by the mean temperature affinity of species (the Community Temperature
Index, CTI), they may be driven by different processes and accompanied by
shifts in the diversity of temperature affinities and breadth of species
thermal niches. To resolve the pathways to community warming in Finnish
flora and fauna, we examined multidecadal changes in the dominance and
diversity of temperature affinities among understory forest plant,
freshwater phytoplankton, butterfly, moth, and bird communities. CTI
increased for all animal communities, with no change observed for plants
or phytoplankton. In addition, the diversity of temperature affinities
declined for all groups except butterflies, and this loss was more
pronounced for the fastest warming communities. These changes were driven
in animals mainly by a decrease in cold-affiliated species and an increase
in warm-affiliated species. In plants and phytoplankton the decline of
thermal diversity was driven by declines of both cold- and warm-affiliated
species. Plant and moth communities were increasingly dominated by thermal
specialist species, and birds by thermal generalists. In general, climate
warming outpaced changes in both the mean and diversity of temperature
affinities of communities. Our results highlight the complex dynamics
underpinning the thermal reorganization of communities across a large
spatio-temporal gradient, revealing that extinctions of cold-affiliated
species and colonization by warm-affiliated species lag behind changes in
ambient temperature, while communities become less thermally diverse. Such
changes can have important implications for community structure and
ecosystem functioning under accelerating rates of climate change.