Warm range margin of boreal bryophytes and lichens not directly limited by temperatures

1. Species at their warm range margin are potentially threatened by higher
temperatures, but may persist in microrefugia. Whether such microsites
occur due to more suitable microclimate or due to lower biotic pressure
from e.g. competitive species, is still not fully resolved. 2. We examined
whether boreal bryophytes and lichens show signs of direct climate
limitation, i.e. whether they perform better in cold and/or humid
microclimates at their warm range margin. We transplanted a moss, a
liverwort, and a lichen to 58 boreal forest sites with different
microclimates at the species’ southern range margin in central Sweden.
Species were grown in garden soil patches to control effects of
competitive exclusion and soil quality. We followed the transplanted
species over three growing seasons (2016-2018) and modelled growth and
vitality for each species as a function of sub-canopy temperature, soil
moisture, air humidity, and forest type. In 2018, we also recorded cover
of other plants having re-colonized the garden soil patches and modelled
this potential future competition with the same environmental variables
plus litter. 3. Species performance increased with warmer temperatures,
which was often conditional on high soil moisture, and at sites with more
conifers. Soil moisture had a positive effect, especially on the moss in
the last year 2018, when the growing season was exceptionally hot and dry.
The lichen was mostly affected by gastropod grazing. Recolonization of
other plants was also faster at warmer and moister sites. The results
indicate that competition, herbivory, shading leaf litter, and water
scarcity might be more important than direct temperature for performance
at the species’ warm range margin. 4. Synthesis. In a transplant
experiment with three boreal understory species we did not find signs of
direct temperature limitation towards the south. Forest microrefugia, i.e.
habitats where these species could persist regional warming, may instead
be sites with fewer competitors and enemies, and with sufficient moisture
and more conifers in the overstory.