DOI: 10.5061/dryad.02v6wwqb8
title: A deepened water table increases the vulnerability of peat mosses to periodic drought

description: 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.
description: 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.<br>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 cm<sup>2</sup>), 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.
description: # A deepened water table increases the vulnerability of peat mosses to
 periodic drought
 [https://doi.org/10.5061/dryad.02v6wwqb8](https://doi.org/10.5061/dryad.02v6wwqb8) ## Description of the data and file structure This dataset consists of four files, each containing different types of data relating to the peatland surface cores ("mesocosms") used for this experiment. The abbreviations and variable names used in the data are listed below the file descriptions. 1. mesocoms_CO2_exchange_trait.csv contains data on mesocosm traits and photosynthesis CO2 fluxes that were used for non-linear mixed-effects models in R. This is the data file used in the R-script included in this article's supplementary material. 2. mesocosm_eventData.xlsx contains information on the mesocosm collection location and time. These data are not used for analyses. 3. mesocosm_CO2_exchange.csv contains data related to the closed-chamber photosynthesis measurements conducted on mesocosms throughout the experiment. This data file is not used in the analysis. 4. mescosm_traits.csv contains the data related to the functional traits of the Sphagnum mosses measured throughout the experiment. This data file is not used in the analysis. Parameters in the data are defined as follows: * ID: unique measurement identifier made of the mesocosm ID and the campaign/round of measurements * ROUND: Measurements were taken five times over the duration of the experiment. This indicates during which of the five campaigns the given measurement was done (Campaign) * DATE: day measurement was taken * TIME: time of day (24h) that measurement was taken * MESO: unique mesocosm identifier (Mesocosm label) * SITE: type of peatland ecosystem that the mesocosm originated from (Ecosystem) * fen: minerotrophic rich * fen bog: ombrotrophic bog * WT_HIST: water table history that the mesocosm originated from (History, water table) * wet: naturally wet * wld: water level drawdown * dry: naturally dry * SPEC: Sphagnum species that dominates the mesocosm surface in an 8-letter genus species code (Species) * sphaangu: *Sphagnum angustifolium* * sphabalt: *Sphagnum balticum* * sphadivi: *Sphagnum divinum* * spsharecu: *Sphagum recurvum* collective containing *S. fallax* and *S. flexuosum* * spharube: *Sphagnum rubellum* * sphatene: *Sphagnum tenellum* * sphatere: *Sphagnum teres* * TREAT: drought treatment (Treatment) * ctr: no drought treatment (WT maintained at -10cm) * dry: drought treatment applied (WT dropped to -30cm for 43 days) * BLOCK: Treatment block that the mesocosm was located in (1 to 4). Physical experimental setup divided into four blocks, each containing a full set of replicates * ROW: Row that the mesocosm was located in (A to D). Physical experimental setup had four rows, each containing a full set of replicates * PAR: photosynthetically active radiation, measured as photon flux density (μmol m-2 s-1). Each measurement included four PAR levels (high, medium, low, and dark); light levels measured with a light sensor at the moss surface. (Radiation, photosynthetically active) * T_IN: Temperature (°C) of the chamber air during the measurement (Temperature, chamber air) * T_SURF: Temperature (°C) of the moss surface at a neighbouring mesocosm during the measurement (Surface temperature) * T_AIR: Temperature (°C) of the greenhouse air at the time of the measurement (Temperature, air) * C_N: Carbon to Nitrogen ratio * CYL_V: volume (cm^3) of air in closed cylinder during measurement (Chamber volume) * FLUX: measured carbon dioxide flux (mg CO2 m−2 d−1) in closed chamber at the given light level (PAR) during the measurement (Carbon dioxide, flux, in mass carbon) * DIA1: capitulum diameter (mm) during the first measurement campaign (Capitulum width) * DIA2: capitulum diameter (mm) during the second measurement campaign (Capitulum width) * DIA3: capitulum diameter (mm) during the fifth measurement campaign (Capitulum width) * COUNT1: the count of capitula in a given area, i.e. growth density (number cm-2) during the first measurement campaign (Capitulum density) * COUNT2: the count of capitula in a given area, i.e. growth density (number cm-2) during the second measurement campaign (Capitulum density) * COUNT3: the count of capitula in a given area, i.e. growth density (number cm-2) during the third measurement campaign (Capitulum density) * COUNT4: the count of capitula in a given area, i.e. growth density (number cm-2) during the fourth measurement campaign (Capitulum density) * COUNT5: the count of capitula in a given area, i.e. growth density (number cm-2) during the fifth measurement campaign (Capitulum density) * COLOUR1: colour, visually assessed during the first measurement campaign (Color description) * G: green * Y: yellow * R: red * B: brown * W: white * two letters indicates a combination of colours observed * COLOUR2: as above, but for the second measurement campaign (Color description) * COLOUR3: as above, but for the fifth measurement campaign (Color description) * MASS: mass (g) of an individual capitula; an average of 10 capitula (Capitulum, dry weight) * GROW: length growth (cm) of moss stem over the experimental period (Sphagnum stem length growth) * MOIST1: moisture content (% mass) in the given mesocosm at the end of the drought period as measured by cellulose acetate filters (Moisture) * MOIST2: moisture content (% mass) in the given mesocosm at the end of the recovery period as measured by cellulose acetate filters (Moisture) * DRY_R: the relative moisture content (%) of the drought-treated mesocosms of a given species x WT_hist x type to the control mesocosms from the same species x WT_hist x type at the end of the drought period (MOIST1). Each mesocosm from the same species x WT_hist x type shares the same value. Calculated as MOIST1[for species A x WT_hist B x type C, treatment = drought]/MOIST1[species A x WT_hist B x type C, treatment = control] (Moisture, relative to control, at peak drought) * REC_R: the relative moisture content (%) of the drought-treated mesocosms of a given species x WT_hist x type to the control mesocosms from the same species x WT_hist x type at the end of the recovery period (MOIST2). Each mesocosm from the same species x WT_hist x type shares the same value. Calculated as MOIST2[for species A x WT_hist B x type C, treatment = drought]/MOIST2[species A x WT_hist B x type C, treatment = control] (Moisture, relative to control, at end of recovery) * N: Nitrogen content (mg/g) of moss capitula at end of experiment (Nitrogen) * P_AREA1: photosynthetic area in mesocosm surface (cm2 cm-2) calculated as (Pi*(diameter/2)^2) x count, for the first campaign (Area, photosynthetically active) * P_AREA2: photosynthetic area in mesocosm surface (cm2 cm-2) calculated as (Pi*(diameter/2)^2) x count, for the second campaign (Area, photosynthetically active) * P_AREA3: photosynthetic area in mesocosm surface (cm2 cm-2) calculated as (Pi*(diameter/2)^2) x count, for the fifth campaign (Area, photosynthetically active) * P_BIO1: photosynthetic biomass in mesocosm surface (g cm-2) calculated as the mass x count, for the first campaign (Biomass, photosynthetically active) * P_BIO2: photosynthetic biomass in mesocosm surface (g cm-2) calculated as the mass x count, for the second campaign (Biomass, photosynthetically active) * P_BIO3: photosynthetic biomass in mesocosm surface (g cm-2) calculated as the mass x count, for the fifth campaign (Biomass, photosynthetically active) ## Code/Software The analyses for this article were done using R software. The script used for non-linear mixed-effects models is included in the article's supplementary information.