1. Winter is a period of dormancy for plants of cold environments. However, winter climate is
changing, leading to an increasing frequency of stochastic warm periods (winter warming
events) and concomitant reductions in snow cover. These conditions can break dormancy for
some plants and expose them to freeze-and-thaw stress. Mosses are a major component of
high-latitude ecosystems, yet the longer-term impacts of such winter warming events on mosses
remain unknown.
2. In order to determine the longer-term legacy effects of winter warming events on mosses, we
undertook a simulation of these events over three consecutive winters in a sub-Arctic dwarf
shrub-dominated open woodland. The mat-forming feather moss, Hylocomium splendens (the
most abundant cryptogam in this system), is one of the most widespread Arctic and boreal
mosses and plays a key functional role in ecosystems. We studied the ecophysiological performance
of this moss during the summers of the experimental period (2007–2009) and in the
following years (2010–2013).
3. We show that the previously reported warming-induced reduction in segment growth and
photosynthesis during the experimental years was persistent. Four years after the last event,
photosynthesis and segment growth were still 30 and 36% lower than control levels, which was
only a slight improvement from 44 and 43% 4 years earlier. Winter warming did not affect
segment symmetry. During the years after the last simulated event, in both warmed and control
plots, chlorophyll fluorescence and segment growth, but not net photosynthesis, increased
slightly. The increases were probably driven by increased summer rainfall over the study years,
highlighting the sensitivity of this moss to rainfall change.
4. Overall, the legacy effects shown here demonstrate that this widespread and important moss
is likely to be significantly disadvantaged in a future sub-Arctic climate where frequent winter
warming events may become the norm. Given the key importance of mosses for soil insulation,
shelter and carbon sequestration in high-latitude regions, such persistent impacts may ultimately
affect important ecosystem functions.
Key-words: Arctic, bryophyte, climate change, extreme events, feather moss, Hylocomium,
snowmelt, warmer winters
