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.

Methods
96 cores of peatland surface cores (mesocosms) covered by Sphagnum mosses were harvested from the Lakkasuo mire complex in Southern Finland in the spring of 2018 and transported to a greenhouse where an experimental periodic drought was applied. A 17-year water level drawdown experiment (established in 2001) was applied in Lakkasuo in a rich fen and bog. In each peatland type, mesocosms were harvested from intermediately wet surfaces (lawns) from three areas with different water table histories: wet pristine, experimental water level drawdown, and naturally dry, making a total of six subsites where mesocosms originated from. Two Sphagnum species were selected from each subsite, with one species being characteristic of the subsite it was selected from and the other species common across all subsites from a given peatland type. After transporting to a greenhouse where the mesocosm water table and environmental conditions were controlled, mesocosms were allowed to acclimate to conditions for 6 weeks, then a 43-day ecohydrological drought was applied. Measurements were taken before the drought, at the end of the drought, and three times after the drought. The total replicates (96) are distributed as follows: 2 peatland types (fen, bog) x 3 water table histories (naturally wet, water level drawdown, naturally dry) x 2 species x 2 treatments (drought, control) x 4 replicates.

For net photosynthesis measurements measured before, during, and after the drought, we used the closed chamber method with four controlled light levels applied in each measurement campaign. Using net photosynthesis measured at a range of light levels (high, medium, low, and dark) allowed us to calculate gross photosynthesis using the hyperbolic light response curve. EGM-5 (PP Systems) infrared gas analyzer with an attached light sensor was used to measure carbon dioxide concentrations in the sealed chamber.
Functional traits of each mesocosm were also measured either alongside photosynthesis (for non-destructive sampling) or after the experimental period (destructive sampling). Measured traits (non-destructive) were: capitula diameter (mm), capitula density (count per cm2), moss colour, and moisture content (% mass) of cellulose acetate filters embedded in the mesocosm. Destructively sampled traits were nitrogen content and capitula mass. Additionally, the height growth of the moss carpet over the duration was measured using the cranked-wire method.

These data were collected to quantify the impact of periodic drought on Sphagnum mosses from different peatland types as regulated by the dryness of their original environment and functional traits. Non-linear mixed-effects models were used to quantify the impacts of these variables on the photosynthetic response of Sphagnum mosses to periodic drought. The resistance and resilience of different moss species were also analysed using maximum photosynthesis values as calculated using the hyperbolic light response curve.