Data from: The role of bryophytes for tree seedling responses to winter climate change: implications for the stress gradient hypothesis

1.When tree seedlings establish beyond the current tree line due to
climate warming, they encounter existing vegetation, such as bryophytes
that often dominate in arctic and alpine tundra. The stress gradient
hypothesis (SGH) predicts that plant interactions in tundra become
increasingly negative as climate warms and conditions become less harsh.
However, for seedlings climate warming might not result in lower winter
stress, if insulating snow cover is reduced. 2.We aimed to understand to
if bryophytes facilitate seedling survival in a changing winter climate
and if these effects of bryophytes on tree seedlings comply with the SGH
along elevational gradients under contrasting snow conditions. 3.In the
Swedish subarctic, we transplanted intact bryophyte cores covered by each
of three bryophyte species and bryophyte-free control soil from above the
tree line to two field common-garden sites, representing current and
future tree line air temperature conditions (i.e. current tree line
elevation and a lower, warmer, elevation below the tree line). We planted
seedlings of Betula pubescens and Pinus sylvestris into these cores and
subjected them to experimental manipulation of snow cover during one
winter. 4.In agreement with the SGH, milder conditions caused by increased
snow cover enhanced the generally negative or neutral effects of
bryophytes on seedlings immediately after winter. Further, survival of P.
sylvestris seedlings after one full year was higher at lower elevation,
especially when snow cover was thinner. However, in contrast with the SGH,
impacts of bryophytes on over-winter survival of seedlings did not differ
between elevations, and impacts on survival of B. pubescens seedlings
after one year was more negative at lower elevation. Bryophyte species
differed in their effect on seedling survival after winter, but these
differences were not related to their insulating capacity. 5.Synthesis:
Our study demonstrates that interactions from bryophytes can modify the
impacts of winter climate change on tree seedlings, and vice versa. These
responses do not always comply with SGH, but could ultimately have
consequences for large-scale ecological processes such as tree-line
shifts. These new insights need to be taken into account in predictions of
plant species responses to climate change.