A deepened water table increases the vulnerability of peat mosses to periodic drought

Here we address the combined impact of multiple stressors that are
becoming more common with climate change. To study the combined effects of
a lower water table (WT) and increased frequency of drought periods on the
resistance and resilience of peatlands, we conducted a mesocosm
experiment. This study evaluated how the photosynthesis of lawn Sphagnum
mosses responds to and recovers from an experimental periodic drought
after exposure to the stresses of a deep or deepened WT (naturally dry and
17-year-long water level drawdown in fen and bog environments. We aimed to
quantify if deep WTs 1) support acclimation to drought, or 2) increase the
base-level physiological stress of mosses, or 3) exacerbate the impact of
periodic drought. There was no evidence of acclimation in mosses from
drier environments; periodic drought decreased the photosynthesis of all
Sphagnum mosses. Water level drawdown decreased the photosynthesis of
bog-originating mosses before periodic drought, indicating that these
mosses were stressed by the hydrological change. Deep WTs exacerbated
Sphagnum vulnerability to periodic drought, indicating that the
combination of drying habitats and increasing frequency of periodic
drought will lead to a rapid transition in lawn vegetation.
Water-retaining traits may increase Sphagnum resilience to periodic
drought. Large capitula size was associated with a higher resistance; the
bog-originating species studied here lacked large capitula or dense carpet
structure and were more vulnerable to drought than the larger
fen-originating species. Consequently, lawns in bogs may become
threatened. Recovery after rewetting was significant for all mosses, but
none completely recovered within three weeks. The most drought-resilient
species had fen origin, indicating that fens are less likely to undergo a
sudden transition due to periodic drought. Synthesis: Water level drawdown
associated with climate change increases the sensitivity of Sphagnum
mosses to periods of drought and moves them closer to their tipping point
as species on the edge of their ecological envelope rapidly shut down
photosynthesis and recover poorly.