Data from: Bryophyte traits explain climate-warming effects on tree seedling establishment

Above the alpine tree line, bryophytes cover much of the tundra soil
surface in dense, often monospecific carpets. Therefore, when climate
warming enables tree seedling establishment above the tree line,
interaction with the bryophyte layer is inevitable. Bryophytes are known
to modify their environment in various ways. However, little is known
about to which extent and by which mechanisms bryophytes affect the
response of tree seedlings to climate warming. We aimed to assess and
understand the importance of bryophyte species identity and traits for
tree seedling performance at tree line temperatures and their response to
warmer conditions. Seedlings of two common, tree line-forming tree species
(Betula pubescens and Pinus sylvestris) were planted into intact cushions
of eight common tundra bryophyte species and bryophyte-free soil and grown
for 18 weeks at current (7·0 °C) and near-future (30–50 years; 9·2 °C)
tree line average growing-season temperatures. Seedling performance
(biomass increase and N-uptake) was measured and related to bryophyte
species identity and traits indicative of their impact on the environment.
Tree seedlings performed equally well or better in the presence of
bryophytes than in bryophyte-free soil, which contrasts to their usually
negative effects in milder climates. In addition, seedling performance and
their response to higher temperatures depended on bryophyte species and
seedlings of both species grew largest in the pan-boreal and subarctic
bryophyte Hylocomium splendens. However, B. pubescens seedlings showed
much stronger responses to higher temperatures when grown in bryophytes
than in bryophyte-free soil, while the opposite was true for P. sylvestris
seedlings. For B. pubescens, but not for P. sylvestris, available organic
nitrogen of the bryophyte species was the trait that best predicted
seedling responses to higher temperatures, likely because these seedlings
had increased N-demands. Synthesis. Climatically driven changes in
bryophyte species distribution may not only have knock-on effects on
vascular plant establishment, but temperature effects on seedling
performance are themselves moderated by bryophytes in a species-specific
way. Bryophyte traits can serve as a useful tool for understanding and
predicting these complex interactions.