DOI: 10.5061/dryad.5tb2rbp9k
title: More is not always better: Peat moss mixtures slightly enhance peatland stability

description: Species-poor peatlands challenge biodiversity–ecosystem function theory,
 which generally links high species diversity to stable ecosystem
 functions. An open question in ecosystem ecology is whether assemblages of
 naturally co-occurring peat mosses contribute to the stability of peatland
 ecosystem processes. We used a replacement series mesocosm experiment with
 mixtures of Sphagnum cuspidatum and S. medium to assess resistance,
 resilience, and recovery rates of net ecosystem CO2 exchange (NEE) under a
 mild and a deep water table drawdown event. Our results show a positive
 effect of mild water table drawdown on NEE with no apparent role for the
 composition of the peat moss assemblage. Our study indicates that the
 carbon uptake capacity by peat moss assemblages is rather resilient to
 mild water table drawdown, but seriously affected by deeper drought
 conditions. Co-occurring peat moss species seem to enhance the resilience
 of the carbon uptake function – i.e. ability of NEE to return to
 pre-perturbation levels – of peat moss mixtures only slightly. These
 findings suggest that assemblages of co-occurring Sphagnum moss species do
 only marginally contribute to the stability of ecosystem functions in
 peatlands under drought conditions. We did find increased recovery with a
 larger share of S. cuspidatum in the mixtures, aiding in the ecosystem’s
 role as carbon sink. Above all, our results highlight that predicted
 severe droughts can gravely affect the sink capacity of peatlands, with
 only a small extenuating role for peat moss mixtures.
description: Early September 2021, we
 collected 40 cores (Æ 22.5 cm, 15 cm depth) of acrotelm peatmoss
 communities from a <em>Sphagnum­</em>-dominated peatland in
 the Store Mosse National Park, Sweden (57°17’54 N, 14°00’39 E). Our
 sampling strategy was set up to create a single-density replacement series
 (Jolliffe,
 2000) with two naturally
 co-occurring <em>Sphagnum</em> mosses: <em>S.
 cuspidatum</em> Ehrh. ex Hoffm and <em>S. medium
 </em>Limpr<em>. </em>These species grow in close
 proximity to each other in the field and make up &gt;70% (cover) of
 the field layer in the field site. Our sampling strategy resulted in cores
 with monocultures (100% cover) of <em>S. cuspidatum</em> or
 <em>S. medium</em>, and three mixtures of <em>S.
 cuspidatum</em> and <em>S. medium </em>– 75%/25%,
 50%/50%, 25%/75% (n = 8). Final cover ratios may differ by
 ± 5%. Vascular plants were carefully
 removed Following and acclimation period, the eight
 mesocosms for the five compositionally distinct
 <em>Sphagnum</em> communities were randomly assigned an
 experimental drought treatment: mild drought or deep drought (n = 4). The
 mild drought consisted of a water table drawdown of 5 cm followed by a
 seven-day drought period in which no precipitation was added. The
 deep water table drawdown was similar except that the initial seven-day
 mild drought was followed by a water table drawdown of 20 cm ­– hence a
 full drainage ­– and an additional two-week drought. Every drawdown event
 was preceded by mimicking a heatwave, which under natural conditions
 precedes a drop in water table­. The heatwave was realized by increasing
 the climate room temperature to <em>c. </em>35°C for 10 hrs.
 using twelve infrared lamps (Philips IR 250 RH IR2 230-250V 250W) that
 were installed 70 cm above the surface of the mesocosms. Both drought
 treatments were trailed by a rewetting period (= recovery period) which
 lasted eight weeks. Carbon
 dioxide (CO<sub>2</sub>) concentrations were measured
 throughout the experiment using an acrylic, cylindrical, transparent
 chamber (ID 29 cm, height 30 cm) that could be placed air-tight on the
 mesocosms using a rubber inner sealing. The chamber was equipped with an
 internal fan. On measurements days – two times per week in the acclimation
 period; four times per week in the drought period, two times per week in
 the recovery period – the transparent chamber was connected in a closed
 loop to a laser-based gas analyser that uses Optical Feedback –
 Cavity-Enhanced Absorption Spectroscopy to measure gases in air (LI-7810
 CH<sub>4</sub>/CO<sub>2</sub>/H<sub>2</sub>O Trace Gas Analyzer, LI-COR Biosciences). Gas measurements were performed with a frequency of 1 hz over a 120 s interval. The chamber was vented between measurements. Measurements with the transparent chamber represent Net Ecosystem Exchange (NEE). CO<sub>2</sub> fluxes were calculated using the <em>R</em> package <em>FluxCalR</em> (Zhao, 2019), which makes use of the change in gas concentration in the chamber over time. Due to instability in the gas flux measurements, we used a dead band of 30 s for the NEE calculations. The ecological sign convention was used for the NEE data; hence, positive flux values indicate CO<sub>2</sub>uptake, while negative flux values indicate CO<sub>2</sub> loss to the atmosphere. We calculated ecosystems stability measures for all individual mesocosms based on the framework described by Isbell et al., 2015 (<em>Nature</em>, 526, 574–577. https://doi.org/10.1038/nature15374). 
description: All data are .txt files and can be opened using the appended R
 script.
description: **More is not always better: Peat moss mixtures slightly enhance peatland
 stability** Early September 2021, we collected 40 cores (diameter 22.5 cm,
 15 cm depth) of acrotelm peatmoss communities from a *Sphagnum­*-dominated
 peatland in the Store Mosse National Park, Sweden (57°17’54 N, 14°00’39
 E). Our sampling strategy was set up to create a single-density
 replacement series (Jolliffe, 2000) with two naturally co-occurring
 *Sphagnum* mosses: *S. cuspidatum* Ehrh. ex Hoffm and *S. medium *Limpr*.
 *These species grow in close proximity to each other in the field and make
 up &gt;70% (cover) of the field layer in the field site. Our sampling
 strategy resulted in cores with monocultures (100% cover) of *S.
 cuspidatum* or *S. medium*, and three mixtures of *S. cuspidatum* and *S.
 medium *– 75%/25%, 50%/50%, 25%/75% (n = 8). Final cover ratios may differ
 by ± 5%. Vascular plants were carefully removed Following and acclimation
 period, the eight mesocosms for the five compositionally distinct
 *Sphagnum* communities were randomly assigned an experimental water table
 drawdown treatment: mild or deep drawdown (n = 4). The mild water table
 drawdown consisted of a water table drawdown of 5 cm followed by a
 seven-day drought period in which no precipitation was added. The deep
 water table drawdown was similar except that the initial seven-day mild
 drought was followed by a water table drawdown of 20 cm ­– hence a full
 drainage ­– and an additional two-week drought. Every drawdown event was
 preceded by mimicking a heatwave, which under natural conditions precedes
 a drop in water table­. The heatwave was realized by increasing the
 climate room temperature to *c. *35°C for 10 hrs. using twelve infrared
 lamps (Philips IR 250 RH IR2 230-250V 250W) that were installed 70 cm
 above the surface of the mesocosms. Both drought treatments were trailed
 by a rewetting period (= recovery period) which lasted eight weeks. Carbon
 dioxide (CO2) concentrations were measured throughout the experiment using
 an acrylic, cylindrical, transparent chamber (ID 29 cm, height 30 cm) that
 could be placed air-tight on the mesocosms using a rubber inner sealing.
 The chamber was equipped with an internal fan. On measurements days – two
 times per week in the acclimation period; four times per week in the
 drought period, two times per week in the recovery period – the
 transparent chamber was connected in a closed loop to a laser-based gas
 analyser that uses Optical Feedback – Cavity-Enhanced Absorption
 Spectroscopy to measure gases in air (LI-7810 CH4/CO2/H2O Trace Gas
 Analyzer, LI-COR Biosciences). Gas measurements were performed with a
 frequency of 1 hz over a 120 s interval. The chamber was vented between
 measurements. Measurements with the transparent chamber represent Net
 Ecosystem Exchange (NEE). CO2 fluxes were calculated using the *R* package
 *FluxCalR* (Zhao, 2019), which makes use of the change in gas
 concentration in the chamber over time. Due to instability in the gas flux
 measurements, we used a dead band of 30 s for the NEE calculations. The
 ecological sign convention was used for the NEE data; hence, positive flux
 values indicate CO2uptake, while negative flux values indicate CO2 loss to
 the atmosphere. We calculated ecosystems stability measures for all
 individual mesocosms based on the framework described by Isbell et al.,
 2015 (*Nature*, 526, 574–577.
 ).  \## Description of the data
 and file structure \- NEE_acc.txt; containing the Net ecosystem CO2
 exchange data from the acclimation period \- NEE_dd.txt; containing the
 Net ecosystem CO2 exchange data from the deep water table drawdown period
 \- NEE_md.txt; containing the Net ecosystem CO2 exchange data from the
 mild water table drawdown period \- NEE_rec.txt; containing the Net
 ecosystem CO2 exchange data from the recovery period \- Stability.txt;
 containing the calculated values day : [numeric] time in days in the water
 table drawdown period which starts at t = 0; negative values indicate the
 acclimation period day_rec : [numeric] [in NEErec only] time in the
 recovery period, baselined for the mild and deep water table drawdown
 treatment id : individual identifier for the mesocosms; used as the error
 variable nested within day in Linear Mixed Effects Model treat : [factor
 w/ 2 levels] MD = mild water table drawdown, DD = deep water table
 drawdown rep : [factor w/ 4 levels] replicates flux : [numeric] Net
 ecosystem exchange in µmol CO2 m-2 s-1 comp: [factor w/ 5 levels]
 *Sphagnum* community composition: # 1 = 100 % *S. cuspidatum;*            
                                                   # 2 = 75% *S.
 cuspidatum* / 25% *S. medium*                                            
                   # 3 = 50% *S. cuspidatum* / 50% *S. medium*            
                                                   # 4 = 25% *S.
 cuspidatum* / 75% *S. medium*                                            
                   # 5 = 100% *S. medium* For the Stability.txt file: id :
 individual identifier for the mesocosms; used as the error variable nested
 within day in Linear Mixed Effects Model treat : [factor w/ 2 levels] MD =
 mild water table drawdown, DD = deep water table drawdown rep : [factor w/
 4 levels] replicates NEEacc : [numeric] Mean net CO2 exchange during the
 acclimation period NEEd : [numeric] Net CO2 exchange at the last day of
 the water table drawdown treatment NEErec : [numeric] Net CO2 exchange at
 the last day of the recovery period resis : [numeric] resistance
 [dimensionless], see main text for calculation resil : [numeric]
 resilience [dimensionless], see main text for calculation rec.rate :
 [numeric] the change in net CO2 exchange per day, during the recovery
 period absresis : [numeric] absolute values of resistance \##
 Code/Software An R script that was used for analysis has been made
 available