DOI: 10.5061/dryad.dz08kps39
title: Data for: Water level drawdown makes boreal peatland vegetation more responsive to weather conditions

description: Climate warming and projected increase in summer droughts puts northern
 peatlands under pressure by subjecting them to a combination of gradual
 drying and extreme weather events. The combined effect of those on
 peatland functions is poorly known. Here, we studied the impact of
 long-term water level drawdown (WLD) and contrasting weather conditions on
 leaf phenology and biomass production of ground level vegetation in boreal
 peatlands. Data was collected during two contrasting growing seasons from
 a WLD experiment including a rich and a poor fen and an ombrotrophic bog.
 Results showed that WLD had a strong effect on both leaf area development
 and biomass production, and these responses differed between peatland
 types. In the poor fen and the bog, WLD increased plant growth, while in
 the rich fen, WLD reduced the growth of ground level vegetation. Plant
 groups differed in their response, as WLD reduced the growth of
 graminoids, while shrubs and tree seedlings benefited from it. In
 addition, the vegetation adjusted to the lower WTs, was more responsive to
 short-term climatic variations. The warmer summer resulted in a greater
 maximum and earlier peaking of leaf area index, and greater biomass
 production by vascular plants and Sphagnum mosses at WLD sites. In
 particular, graminoids benefitted from the warmer conditions. The change
 towards greater production in the WLD sites in general and during the
 warmer weather in particular, was related to the observed transition in
 plant functional type composition towards arboreal vegetation.
description: The study site was located at Lakkasuo in southern Finland
 (61°47’N, 24°18’E). Based on its ecohydrology, Lakkasuo can be divided
 into a mesotrophic fen, an oligotrophic fen and an ombrotrophic bog,
 hereafter called rich fen, poor fen, and bog. In 2000–2001, a water level
 drawdown (WLD) experiment was initiated to mimic the long-term effects of
 lower water tables (WT) on the ecosystems. Within the three sites, an
 intact control area and a WLD area were established, located 20 m apart.
 The drainage treatment lowered the WT by an average 6.4 cm in 2017 and 7.5
 cm in 2021 when compared with the control areas. To disentangle the
 potential effects of short-term changes in growing conditions from the
 long-term influence of peatland drying/drainage, we compared observations
 collected during two growing seasons with contrasting weather
 conditions, i.e., summers of 2017 (cold and wet) and 2021 (warm and
 dry).  The seasonal development of leaf area index
 (LAI: m<sup>2</sup> m<sup>-2</sup>) was estimated
 for each vascular plant species. Sampling was done throughout both growing
 seasons from early May to late August–September at regular intervals, six
 times in 2017, and five times in 2021. Tree seedlings and saplings (up to
 0.5 m in height) were considered part of the ground vegetation cover and
 were included in the LAI estimations. In each sample plot, the number of
 leaves per vascular plant species were counted from five permanent
 representative subplots (0.08 × 0.08 m). However, large plant species with
 infrequent or uneven distribution were counted at the plot-level. For the
 rich fen WLD area, which had smaller circular plots, leaf number was
 counted at the plot-level. Simultaneously, the mean area per leaf was
 measured for each plant species. Representative leaves (varying in age and
 size) were collected adjacent to the plots and scanned with a
 portable leaf area meter (LI-3000, Li-Cor, UK). Leaf area values were
 multiplied by the leaf counts to determine the LAI value per species per
 plot. Finally, the species-wise LAI estimates were summed to provide an
 estimate of the total plot-level LAI estimate of all leaves present in the
 plot, which was used in subsequent analysis. To assess the impact of WLD
 and the two contrasting growing seasons on the phenological development of
 LAI, we used a nonlinear mixed effects model with a log normal unimodal
 function with parameters that described the leaf area phenology in each
 plot.  Growth over the growing season was defined for
 each <em>Sphagnum </em>species using the cranked wire method.
 Cranked wires were installed into intact homogenous patches of each
 species within each area to serve as a fixed reference
 point. <em>Sphagnum </em>growth was measured as the increment
 of individual <em>Sphagnum </em>stems along the wire from
 spring to autumn. In addition, shoot density and the weight (g) of 1 cm of
 stem were defined in <em>Sphagnum </em>patches adjacent to the
 measurement plots in July in both years. To estimate <em>Sphagnum
 </em>biomass production, the increment (cm) was multiplied by
 relative coverage and dry weight per cm stem to calculate the annual
 biomass production of each <em>Sphagnum </em>moss species (g
 per growing season) and were subsequently summed per plot. Leaf biomass
 production of vascular plants was calculated by converting LAI to biomass
 using specific leaf area (dry mass per leaf area, g
 m<sup>-2</sup>). The specific leaf area was defined for five
 replicates per species per site per WLD/control area in July of both
 years. Species-specific biomass production was quantified as the
 difference between minimal biomass at the start of the growing season and
 maximal biomass at peak growing season and was subsequently summed to
 calculate total and PFT-specific leaf biomass production. Specifically,
 the PFTs were graminoids (sedges and grasses), forbs, shrubs, and tree
 seedlings (up to 50 cm height). The effects of WLD and year on biomass
 growth were analysed using linear mixed effects (LME) models, in which we
 included sample plot as a random effect to account for repeated
 measurements. For biomass production we fitted three LME models with
 either the biomass production of the total ground level vegetation, the
 <em>Sphagnum </em>mosses, or vascular plants as the response
 variable, and site, year and WLD treatment and their interactions as fixed
 effects. The composition of vegetation in terms of vascular PFTs was
 highly variable between sites; this was verified by testing biomass
 production of each vascular PFT with an LME model with site as a fixed
 effect. Because of the strong influence of the site, we continued to
 analyse the biomass production of each vascular PFT with site-specific LME
 models, with year, WLD treatment and their interaction as fixed effects.
 Normality and homogeneity of variances were verified with
 Kolmogorov-Smirnov and Levene’s Tests, respectively, and with diagnostic
 plots. Where needed, variance structures were applied (VarIdent) or the
 data were log normal (ln) transformed.