# Experimental warming interacts with soil moisture to discriminate plant responses in an ombrotrophic peatland

Alexandre Buttler, B.J.M. Robroek, Fatima Laggoun-Défarge, Vincent E. J. Jassey, Cédric Pochelon, Gregory Bernard, Frédéric Delarue, Sébastien Gogo, Pierre Mariotte, Edward A. D. Mitchell, et al.

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## To cite this version:

Alexandre Buttler, B.J.M. Robroek, Fatima Laggoun-Défarge, Vincent E. J. Jassey, Cédric Pochelon, et al.. Experimental warming interacts with soil moisture to discriminate plant responses in an ombrotrophic peatland. Journal of Vegetation Science, 2015, 26 (5), pp.964-974. 10.1111/jvs.12296. insu-01163362

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Experimental warming interacts with soil moisture to discriminate plant responses in an ombrotrophic peatland

<table><tr><td>Journal:</td><td>Journal of Vegetation Science</td></tr><tr><td>Manuscript ID:</td><td>JVS-RA-02979.R1</td></tr><tr><td>Manuscript Type:</td><td>Research article</td></tr><tr><td>Date Submitted by the Author:</td><td>n/a</td></tr><tr><td>Complete List of Authors:</td><td>Buttler, Alexandre; EPFL-WSL, Laboratory of Ecological Systems - ECOS<br>Robroek, Bjorn; Utrecht University, Institute of Environmental Biology; EPFL-WSL, Laboratory of Ecological Systems - ECOS<br>Laggoun, Fatima; CNRS/INSU, ISTO, UMR 7327<br>Jassey, Vincent; EPFL-WSL, Laboratory of Ecological Systems - ECOS<br>Pochelon, Cédric; EPFL-WSL, Laboratory of Ecological Systems - ECOS<br>Bernard, Gregory; Fédération des Conservatoires d'espaces naturels, Pôle-Relais Tourbières<br>Delarue, Frédéric; Muséum National d'Histoire Naturelle, Laboratoire de Minéralogie et de Cosmochimie du Muséum, UMR 7202 CNRS<br>Gogo, Sébastien; CNRS/INSU, ISTO, UMR 7327<br>Mariotte, Pierre; EPFL-WSL, Laboratory of Ecological Systems - ECOS<br>Mitchel, Edward; Université de Neuchâtel, Laboratory of Soil Biology, Institute of Biology<br>Bragazza, Luca; EPFL-WSL, Laboratory of Ecological Systems - ECOS</td></tr><tr><td>Keywords:</td><td>climate change, vegetation, Sphagnum, OTC, temperature, drought, NDVI, point-intercept survey</td></tr></table>
1 Research article

2 Estimated size: 6617 words (8 pages), figures 2 pages, total 10 pages

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4 **Experimental warming interacts with soil moisture to discriminate plant responses in an ombrotrophic peatland**

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7 **Running head: Experimental warming and plant responses**

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9 Alexandre Buttler, Bjorn J.M. Robroek, Fatima Laggoun-Défarge, Vincent E.J. Jassey, Cédric

10 Pochelon, Gregory Bernard, Frédéric Delarue, Sébastien Gogo, Pierre Mariotte, Edward A.D.

11 Mitchell & Luca Bragazza

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13 Buttler, A. (Corresponding author, alexandre.buttler@epfl.ch) ¹, ², ³, ¹²

14 Robroek, B.J.M. (bjorn.robroek@epfl.ch) ¹, ², ¹¹

15 Laggoun-Defarge, F. (Fatima.Laggoun-Defarge@univ-orleans.fr) ⁴, ⁵, ⁶

16 Jassey, V.E.J. (vincent.jassey@epfl.ch) ¹, ², ³

17 Pochelon, C. (cedric.pochelon@bluewin.ch) ¹, ²

18 Bernard, G. (gregory.bernard@reseau-cen.org) ¹, ², ¹⁴

19 Delarue, F. (fdelarue@mnhn.fr) ⁴, ⁵, ⁶, ⁷

20 Gogo, S. (Sebastien.Gogo@univ-orleans.fr) ⁴, ⁵, ⁶

21 Mariotte, P. (pierre.mariotte@hotmail.com) ¹, ², ¹², ¹³

22 Mitchell, E.A.D. (edward.mitchell@unine.ch) ⁹, ¹⁰

23 Bragazza, L. (luca.bragazza@wsl.ch) ¹, ², ⁸

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25 Author contributions: A.B. wrote the manuscript, A.B., G.B and C.P. collected the vegetation data,
26 A.B., F.L.D, S.G. and E.M. conceived the experiment, B.R., V.J. and F.D. helped for vegetation and
27 micrometeorological data analysis and all contributed to the interpretation of results and commented
28 on the manuscript.

29

30 Addresses:

31 <sup>1</sup>Ecole Polytechnique Fédérale de Lausanne EPFL, School of Architecture, Civil and Environmental
32 Engineering ENAC, Laboratory of ecological systems ECOS, Station 2, 1015 Lausanne,
33 Switzerland.

34 <sup>2</sup>Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Site Lausanne, Station 2,
35 1015 Lausanne, Switzerland.

36 <sup>3</sup>Laboratoire de Chrono-Environnement, UMR CNRS 6249, UFR des Sciences et Techniques, 16
37 route de Gray, Université de Franche-Comté, F-25030 Besançon, France.

38 <sup>4</sup>Université d'Orléans, ISTO, UMR 7327, 45071 Orléans, France.

39 <sup>5</sup>CNRS/INSU, ISTO, UMR 7327, 45071 Orléans, France.

40 <sup>6</sup>BRGM, ISTO, UMR 7327, BP 36009, 45060 Orléans, France.

41 <sup>7</sup>Laboratoire de Minéralogie et de Cosmochimie du Muséum, UMR 7202 CNRS, Muséum National
d'Histoire Naturelle, 57 rue Cuvier, 75231 Paris Cedex 05, France.

43 <sup>8</sup>University of Ferrara, Department of Life Science and Biotechnologies, Corso Ercole I d'Este 32,
44 I-44121 Ferrara, Italy.

45 <sup>9</sup>Laboratory of Soil Biology, Institute of Biology, University of Neuchâtel, CH-2000 Neuchâtel,
46 Switzerland.

47 <sup>10</sup>Jardin Botanique de Neuchâtel, Chemin du Perthuis-du-Sault 58, 2000 Neuchâtel, Switzerland.

48 <sup>11</sup>Ecology and Biodiversity, Institute of Environmental Biology, Utrecht University, Padualaan 8,
49 3584 CH Utrecht, The Netherlands.

50 <sup>12</sup>Department of Environmental Science, Policy and Management, Suding Lab, University of
51 California Berkeley, 147 Hilgard Hall, Berkeley, CA 94720.

52 <sup>13</sup>University of Sydney, Centre for Carbon, Water and Food Biogeochemistry Group - 380
53 Werombi Road, Camden NSW 2570.

54 <sup>14</sup>Pôle-relais Tourbières, Fédération des Conservatoires d'espaces naturels ☐7, rue Voirin, 25000
55 Besançon, France.
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Keywords: climate change; vegetation, *Sphagnum*; OTC; temperature; drought; NDVI; point-intercept survey.

ABSTRACT

Question: A better understanding of the response of *Sphagnum* mosses and associated vascular plants to climate warming is relevant for predicting the carbon balance of peatlands in a warmer world. Open-top chambers (OTCs) have been used to investigate the effect on soil biogeochemical processes in peatlands, but little information is available on the effects of OTCs on microclimate conditions and the associated response of plant community. We aimed to understand how simulated warming and differences in soil moisture affect plant species cover.

Methods and location: We used OTCs to measure the effect of a near-ground temperature increase (+ 1.5 °C on average) on vegetation dynamics over five growing seasons (2008-2012) in a *Sphagnum*-dominated peatland (French Jura), in two adjacent microhabitats with different hydrological conditions – wet and dry. Microclimatic conditions and plant species abundance were monitored at peak biomass in years 1, 2, 3 and 5 and monthly during the plant-growing season in year 5.

Results: The response of plants to warming differed between vascular plants and bryophytes as well as among species within these groups and also varied in relation to soil moisture. *Andromeda polifolia* abundance responded positively to warming, while *Vaccinium oxycoccus* responded negatively and *Eriophorum vaginatum* showed a high resistance.
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Conclusion: Depth of rooting of vascular plants appeared to control the response in plant abundance, while moss abundance depended on various other interacting factors, such as shading by the vascular plant community, precipitation and soil moisture.

## INTRODUCTION

Peatlands are long-term sinks of atmospheric carbon (C) due to the accumulation of peat (Yu 2011). These ecosystems develop in areas that are hydrologically and climatically characterized by a net surplus of water. As a consequence, prevailing soil anoxic conditions hamper microbial decomposition of organic matter (Holden 2005).

In northern hemisphere ombrotrophic (= rain fed) peatlands, bryophytes of the genus *Sphagnum* (peat mosses) are generally the dominant plant species. Peat mosses produce litter with antimicrobial properties, which hampers microbial decomposition (Hajek et al. 2011), leading to the build-up of peat. Further, *Sphagnum* mosses can affect the abundance and the performance of associated vascular plants (Dorrepaal et al. 2006) by regulating soil moisture, energy balance, soil temperature, and nutrient availability (Turetsky et al. 2012). Ultimately, the inter-specific competition between *Sphagnum* mosses and vascular plants controls peat accumulation because an increase of vascular plant abundance and in particular their shading effect can reduce *Sphagnum* growth (Heijmans et al. 2002; Bragazza et al. 2013). Changes in the competitive balance between plant species of different growth forms has been shown to alter the C balance in peatlands (Kuiper et al. 2014), potentially by changes in the litter quality, which ultimately affects microbial decomposition (Bragazza et al. 2007; Dorrepaal et al. 2007; Gogo et al. 2011).

Climate warming and associated extreme climatic events can be detrimental to *Sphagnum* mosses as, like all terrestrial bryophytes, these plants cannot actively control their water balance and
therefore rely on atmospheric precipitation to maintain suitable hydrological conditions (Robroek et al. 2007, Bragazza 2008; Nijp et al. 2014). Increased air temperature can increase evapotranspiration with direct negative effects on *Sphagnum* productivity (Gerdol 1995; Weltzin et al. 2001), but with positive effects on vascular plant growth by drying out the top soil and enhancing soil oxygenation (Weltzin et al. 2000; Bragazza et al. 2013), to which plants of different functional types might respond differently (Kuiper et al. 2014). A better understanding of the response of *Sphagnum* mosses and associated vascular plants to climate warming is then relevant for predicting the C balance of peatlands in a warmer world. Indeed, a warmer climate has been reported to alter plant species abundance in peatlands, in particular favoring ericaceous dwarf shrubs (Weltzin et al. 2003, Breeuver et al. 2009), which will affect soil biogeochemical processes (Bragazza et al. 2013).

Under field conditions, simulation of climate warming can be obtained by using passive warming open-top chambers (OTCs) (Marion 1997). Although OTCs are recognized to increase temperature consistent with general circulation models, they also affect a set of microclimatological variables such as moisture, light, snow accumulation and wind speed (Bokhorst et al. 2013). In peatland studies, OTCs have been reliably used to investigate the effect on soil biogeochemical processes (Dorrepaal et al. 2009), but little information is available on the effects of OTCs on microclimate conditions and the associated response of plant species (Weltzin et al. 2003, Keuper et al. 2011; Delarue et al. 2014; Jassey et al. 2013).

In the present study we used OTCs to simulate increased soil and air temperature, and we monitored changes in vegetation cover in a *Sphagnum*-dominated peatland over five years of experimental warming. We aimed to understand how species abundance and plant community composition responded to simulated warming and associated changes in soil moisture. We hypothesized that plant responses will vary among functional types; and more specifically that 1) vascular plants will respond to warming according to their rooting depth, and 2) *Sphagnum* mosses
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will have a more complex response depending on the interaction between micrometeorological conditions and abundance of vascular plants.

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# MATERIAL AND METHODS

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## Study site

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The study was performed on an undisturbed, ombrotrophic *Sphagnum*-dominated peatland situated in the Jura Mountains (The Forbonnet peatland, France, 46°49'35''N, 6°10'20''E, 840 m a.s.l). Climate is characterized by relatively cold winters (average temperature -1.4°C) and mild summers (average temperature 14.6°C), with annual mean temperature of 7.5°C and annual mean precipitation between 1300 and 1500 mm (Laggoun-Défarge et al. 2008).

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Two areas (microhabitats) were selected based on their hydrological conditions. The first microhabitat (hereafter called 'wet') was a *Sphagnum*-dominated relatively flat area bordering a transitional poor fen, characterized by a moss cover dominated by *Sphagnum fallax*. Vascular plants such as *Eriophorum vaginatum* and *Andromeda polifolia* were recorded with low abundance, together with *Scheuchzeria palustris*, *Drosera rotundifolia* and *Vaccinium oxycoccus*. The second microhabitat (hereafter called 'dry') was adjacent to the wet microhabitat, but in a area with generally deeper water tables. The dry microhabitat was characterized by a pattern of hummocks dominated by *Sphagnum magellanicum*, *V. oxycoccus*, *E. vaginatum* and *Calluna vulgaris*, and lawns with *S. fallax*, *Carex rostrata* and *A. polifolia*. Tree (*Pinus rotondata*) encroachment occurs in this dry microhabitat.

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## Experimental design

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In April 2008, in both the dry and the wet microhabitats, six representative plots (3 × 3 m) were selected, which were then randomly allocated to either the control or the warming treatment

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154 (Laggoun-Défarge et al., 2008). Warming treatments were obtained by using open-top chambers
155 (OTCs), which consisted of transparent polycarbonate hexagonal chambers (50 cm high, 1.7 m top-
156 width, 2.4 m base-width). OTCs allow quasi-natural transmittance of visible wavelengths, and
157 minimize the transmittance of re-radiated infrared wavelengths (Marion et al., 1997). All OTCs
158 were raised 10 cm above the peat surface to allow air circulation. The minimal (adjacent plots) and
159 maximal (distant plots) distance between plots were c. 5 and 30 m, respectively.

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161 *Environmental monitoring*

162 From November 2008 to November 2012, peat temperature (7 cm below the moss carpet) and air
163 temperature (at 10 cm above the moss carpet) were monitored at 30-minute intervals using
164 thermocouple probes connected to a data logger (CR-1000 Campbell). Sensors were repositioned as
165 necessary to keep height and depth of measurement constant despite moss carpet accumulation. In
166 2012, these measurements were augmented with combined soil temperature/moisture sensors. In
167 each microhabitat (wet/dry) and treatment (OTC/control) a randomly chosen plot was equipped
168 with soil temperature-moisture probes (Decagon 5TM) at 3 cm and 5 cm depth. Furthermore,
169 precipitation at the study site and water table depth in randomly chosen plot were continuously
170 monitored by a meteorological station and sensors connected to the CR-1000 data logger.

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172 *Plant species abundance*

173 At peak biomass (July or August, depending on the year) in 2008, 2009, 2010 and 2012, plant
174 species abundance was measured using the point-intercept frequency method (Buttler 1992) in 50 ×
175 50 cm subplots located in flat lawns. We used a Plexiglas frame with adjustable legs placed above a
176 permanently marked subplot. A 20 holes ruler was moved along 20 different positions so as to
177 obtain 400 measuring points on a regular grid. A metal pin (1 mm diameter) was lowered through
178 each hole in the ruler and each contact of the pin with living vegetation and litter was recorded by
species until the pin reached the *Sphagnum* surface. This field technique could account for three- dimensional biomass distribution because, at each point, each plant that was intercepted was recorded. The relative frequency of each species was then calculated.

To allow analyses on seasonal dynamics of plant species cover, during the growing season 2012 (April - September), monthly pictures were taken from aforementioned subplots. In order to obtain high-resolution images, the subplots were divided into four 25 × 25 cm quadrats. For the digital image analysis, a grid of 100 points was laid on each 25 × 25 cm picture, after which the four grids were merged into a single 400 points grid-picture. Species overlaying the grid intersects were identified at 200% digital magnification. The relative frequency of each species was then calculated for each sampling date, as for the point-intercept method. Because we could not reliably distinguish *S. fallax* and *S. rubellum* in our point-intercept field measures and in the frequency analysis on photographs, we pooled these two species.

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192 Sphagnum *height growth*

The cranked wire method (Clymo 1970) was used to measure *Sphagnum* growth. At the start of the 2011 and 2012 growing seasons (early April, after snow melt) cranked wires (5-10) were placed in each subplot. At the beginning of November, before snowfall, the length of the cranked wire above the moss surface was measured with a graduated pipe sliding down along the cranked wire and the average height of the *Sphagnum* capitula around the wire was calculated.

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199 Seasonality of plant community

The normalized difference vegetation index (NDVI) was used as a phenological proxy and measured every fortnight between March and November 2012 using a portable spectroradiometer sensor. This measure is also used as an indirect measure of bulk biomass (Soudani et al. 2012). The sensor was maintained at 50 cm above the moss carpet, which allowed obtaining a NDVI signal of
the whole plant community, including the highest plants, while covering only the subplot surface. In each 50 × 50 cm subplot and for each campaign of sampling, 10 consecutive measures were taken according to a grid sampling design and then averaged to have a single NDVI value per plot. The NDVI measurements were performed under direct sunlight and with an umbrella to shadow the subplot.

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## Statistical analysis

Differences in vegetation between years (i.e. 2008, 2009, 2010, 2012) were analyzed using relative species frequency data obtained from the point-intercept method in the field, whereas seasonal trend of plant species cover during the 2012 growing season was analyzed with species relative frequency data obtained from the photographic image analysis. Analysis at species level was done with Linear Mixed Models, with treatment (OTC vs. control) and years (or months for the seasonal data set in 2012) as fixed factors, and years (or months) nested in plots and blocks (wet and dry microhabitats) as random factors. The same model was also used to test the block effect. *Sphagnum* growth was analyzed with Linear Mixed Models, with treatment and block as fixed factors, and location of growth measures within plots given as random nested factors. The model was also tested for each block separately. NDVI was also analyzed with Linear Mixed Model, with treatment and block as fixed factors, and dates nested in plots as random factors. Random effects on the intercept were used to correct the inflation of the residual degrees of freedom that would occur if repeated measurements within sites would have been used as true replicates (Pinheiro & Bates 2000). In addition, for NDVI, separate ANOVA's were performed for each block and date. Pearson's correlation coefficients were calculated and tested between NDVI signals and species frequencies. Differences in monthly means of differences in daily mean air and soil temperature were assessed by ANOVA's. Comparisons of mean water level depth between blocks were done
228 with Student t-tests. Data were log transformed prior to analysis. All statistical analyses were
229 performed in R (R Core Team, 2014).

230 **Results**

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233 *Micrometeorology*

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235 In general, over the period 2009-2012, monthly air temperature in the OTCs was 0.2-1.5 °C
236 higher than in the control plots (*p*<0.05). Interestingly, OTCs rarely affected soil temperature, and
237 when they did (*p*<0.05), the pattern was less clear, varying between a warming effect up to 0.3 °C
238 in some cases, and a cooling effect up to -1 °C in some others (see also Delarue et al. 2011, 2014,
239 2015, Jassey et al. 2011). These effects of OTC on soil temperature were not related to the season,
240 neither to the microhabitat. Soil moisture and temperature measured in 2012 with Decagon sensors
241 in a subset of 4 plots (dry control, dry OTC, wet control and wet OTC) indicate that soil
242 temperature was slightly higher in OTCs this particular year (mean daily differences between OTC
243 and control plots for soil temperature was +0.6 °C in both microhabitat), and that moisture was
244 slightly lower in OTC plots (mean daily differences between OTC and control plots for soil
245 moisture were -0.011 and -0.067 (VWC) in dry and wet microhabitat, respectively).

246 Monthly cumulative rainfall during the period of *Sphagnum* growth measurements was higher in
247 2012 (2108 mm) than in 2011 (1376 mm). Depth of water level fluctuated over time, but highest
248 water levels were recorded during spring 2012. Mean water level during the measurement period of
249 *Sphagnum* growth was significantly higher in 2012 than in 2011 (Student t-test; *p*=0.002 in dry,
250 *p*=0.004 in wet). Mean water level was also higher in the wet than the dry microhabitat (*p*=0.004).

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253 *Annual change in species abundance*

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255 Considering all vascular plants, abundance (relative frequency) increased significantly with the
256 OTC treatment (Figure 1). At species level, it appeared that with warming *A. polifolia* increased its
abundance consistently in wet and dry microhabitats, whereas the abundance of *V. oxycoccus* decreased. *D. rotundifolia* abundance responded marginally to the OTC treatment whereas *E. vaginatum* did not respond. Some other species responded contrastingly to the OTC treatment in the two microhabitats, for example *P. strictum* and *S. fallax+rubellum* increased their abundance in the dry microhabitat, whereas their abundance decreased or did not change in the wet microhabitat (treatment x block *p*=0.0367 and 0.0767, respectively). Litter increased in the wet microhabitat, and decreased in the dry microhabitat (treatment x block *p*=0.0001). Significant changes occurred also over time, with a decrease of abundance for *V. oxycoccus* and *S. fallax+rubellum*, and an increase for *E. vaginatum* and litter.

There was also a strong microhabitat effect related to the dry and wet situations (*p*<0.05), except for *Drosera rotundifolia* for which the microhabitat effect was marginally significant, and for *Carex rostrata*, *C. limosa*, *Calluna vulgaris* and *Pinus rotundata* for which there was no significant microhabitat effect. *Vaccinium oxycoccus*, *D. rotundifolia*, *Scheuchzeria palustris*, and *Sphagnum fallax+rubellum* were more abundant in the wet microhabitat, whereas species such as *Andromeda polifolia*, *Eriophorum vaginatum* and *Polytrichum strictum* were more frequent in the dry microhabitat, or exclusively found in the latter one (*S. magellanicum*).

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274 *Monthly change in species abundance*

During the 2012 season, total vascular plant abundance and, more specifically *Andromeda polifolia* frequency increased significantly in the OTCs (Figure 2). *Vaccinium oxycoccus* abundance decreased in the wet microhabitat (treatment x block *p*=0.0034) whereas that of *Sphagnum fallax+rubellum* declined in the wet microhabitat, but increased in the dry microhabitat (treatment x block *p*=0.008). Litter increased in the wet microhabitat and decreased in the dry microhabitat (treatment x block *p*=0.0001). Significant changes occurred also over the monthly course of 2012,
281 with a decrease in abundance for *S. magellanicum* (present only in the dry microhabitat) and litter,
282 and an increase for *V. oxycoccus* and total vascular plants.

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284 *Sphagnum* growth

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286 The length increment of *Sphagnum* species differed between years, being higher in 2012 as
287 compared to 2011 (Figure 3). In 2011, *Sphagnum* growth was significantly lower in the wet
288 microhabitat, but warming did not affect the length increment. In 2012, there were significant
289 microhabitat and treatment effects as well as a cross effect, with increased moss growth in the
290 warmed plots of the dry microhabitat (*p*=0.017), whereas no-significant changes were observed in
291 the wet microhabitat.

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293 Normalized Difference Vegetation Index

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295 NDVI values taken under direct sunlight and in the shade were strongly correlated (R²=0.83), yet
296 measurements at the start of the season (early May) were less correlated (R²=0.49) than
297 measurements close to the peak of biomass (mid-June; R²=0.82). The precision of the
298 measurements was calculated as the mean difference between each pairwise measurement. It gives
299 an error of 1.6% under sun and 2.0% in the shade. As such, we only used NDVI data from sun
300 measurements in further analyses.

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302 Throughout the season, OTCs increased the vegetation's NDVI signal (*p*=0.001), yet the effect
303 of OTCs differed between wet and dry microhabitats (block x treatment *p*=0.002). In the dry
304 microhabitat, the NDVI signal in the OTC exceeded the signal in control plots throughout the
305 season (Figure 4a). Such difference was mainly significant at the beginning of the growing season,
306 and marginally different in summer. In the wet microhabitat, however, the difference between OTC
307 and control plots was less obvious, and occurred during a more limited time in summer (Figure 4b).
307 NDVI was positively correlated to vascular plant abundance (r=0.85, p<0.001), and negatively with
308 litter (r=-0.60, p<0.001). The correlation between NDVI and *Sphagnum* cover was negative (r=-
309 0.24, p<0.045).

310 **Discussion**

311 *Effect of OTC on microclimate*

312 The observed OTC warming effect on near-ground air temperature was in the range of those
313 found in other studies (e.g., Marion et al., 1997; Hollister & Webber, 2000; Dorrepaal et al. 2003;
314 Sullivan et al., 2005, Walker et al. 2015). Yet, in accordance with Dabros et al. (2010), we observed
315 that OTCs can reduce soil temperature, as a result of increased evapotranspiration (Delarue et al.
316 2011). Daily temperature fluctuations, minimum and maximum values and day or night means have
317 all been shown to be affected by OTC warming (e.g. Marion et al. 1997; Dabros et al. 2010). A
318 previous study at the same site revealed that daily soil temperature amplitudes were smaller but
319 often significantly different between OTC's and control plots in winter months (Delarue et al.
320 2011). In summer months, amplitudes were larger, and even more so in OTCs as compared to
321 control plots in April, May and June in the dry microhabitat (but differences were marginally or not
322 significant), whilst an opposite trend appeared in the wet microhabitat. The additional Decagon
323 sensors installed in 2012, which integrate more soil volume, might be more adapted to the very
324 porous peat soil than tiny thermocouples. Despite the absence of replication, it is reasonable to
325 recognize that our OTCs had a slight warming effect on soils for the largest part of time, yet this
326 effect coincided with a slight reduction in soil volumetric water content as previously reported
327 (Jassey et al. 2011)

328

329 *Effect of OTCs on vegetation*

330 We showed that experimental warming differently influenced plant abundance (or frequency),
depending on the underlying hydrological conditions and on individual species identity. *A. polifolia* increased in abundance while *V. oxycoccus* decreased, and both *S. fallax*+*rubellum* and *P. strictum* responded positively to the OTC treatment in the dry microhabitat, and decreased or remained constant in the wet microhabitat. Among these species, the response of *A. polifolia* is interesting. This species has been shown to be sensitive to changes in environmental conditions such as snow cover and spring/summer warming (Aerts et al. 2006; Jassey et al. 2013; Robroek et al. 2013). It allocates about 98% of the total biomass belowground, concentrated in the 0-15 cm peat layer, with the bulk at 2.5-7.5 cm, and a continuous rooting system from recent vegetation down into the peat (Wallén 1986). Roots of *A. polifolia* may reach a depth of 45 cm (Jaquemart 1998) and fine roots form up to 24% of the total below ground biomass (Wallén 1986). *A. polifolia* might therefore be expected to have a somewhat deeper root system as compared to several other species enabling the species to sustain growth when soil dries out at the surface. Differently, *V. oxycoccus* has a shallow root system hardly penetrating more than a few centimeters below the living parts of the moss layer (Jacquemart 1998). With its shallow roots, this species relies on the water-conducting capacity of the *Sphagnum* mosses for its water supply (Malmer et al. 1994). Indeed, a decrease of *Sphagnum* water content in the OTC treatment has been demonstrated by Jassey et al. (2011). A subsequent lowering of water table will also reduce the growth rate of *V. oxycoccus* through water stress (Malmer et al. 1994, Jacquemart 1998). Coriaceous leaves of *A. polifolia*, their silvery-glaucous lower surface and their revolute margins, could indicate drought resistance. But anatomical features of *V. oxycoccus* such as thick epicuticular waxes and stomata on the abaxial surface only are considered similar to those of desert plant characteristic to a high ability to avoid drought (Jacquemart 1997). Drier soil might also promote ericoid mycorrhizal infection (Cullings 1996) and thus better access to nutrients in the soil, but this might equally apply to other Ericaceous present in the bog, such as *V. oxycoccus*.
An alternative explanation for the decrease of *V. oxycoccus* with OTC warming may come from direct nutrient resource competition (Malmer et al. 1994), in particular with *A. polifolia*. Because, however, OTC warming coincided with a decrease in moisture content, a direct effect of moisture limitation on the shallow rooting *V. oxycoccus* cannot be ruled out. The evidence of moisture limitation is supported by the lack of response to the OTC by *E. vaginatum*. This species is known to root deeply into the peat, up to 1.5 meters, with roots having highest elongation rates at low soil temperatures (Shaver & Billings 1977). Contrary to *A. polifolia* and *V. oxycoccus*, no mycorrhizal association has been reported for *E. vaginatum*. In warming experiment, no significant differences were reported for *E. vaginatum* root production between OTC's and control plots, albeit a tendency to higher production was observed (Sullivan & Welker 2005). We believe that the resistance of *E. vaginatum* is higher as compared to more shallow rooted species, and that an effect of warming has to be expected on a longer term, as soil warms up at lower depth. Weltzin et al. (2003) found that within bog and fen communities, different life forms and species can respond differently to warming and water table manipulation. They showed that warming may lead to an increase in abundance of woody plants, as does lowering water table as a result of changes in rates of evapotranspiration. In the peatlands studied by Weltzin et al. (2003), *Andromeda glaucophylla* responded mostly to the heat treatment, while other dominant shrubs (such as *Vaccinium oxycoccus*) did not, whereas graminoids decreased their cover. As observed by Weltzin et al. (2000), change in belowground biomass can be even more important. Our wet and dry sites showed similar trends for vascular plants, but not for mosses. The better growth of some mosses in warmed plots might be transient since, on the long run, mosses will be out-competed by the concomitant increase of *Andromeda* shrubs. Overall, these observations could point to a different evolution of such contrasted dry and wet situations, as for "bog" and "fen" like facies (Weltzin et al. 2003).

The increase of litter in the wet microhabitat and its decrease in the dry microhabitat under the OTC treatment does not follow the pattern of the abundance of total vascular plants, which could
380 indicate that litter abundance is mainly driven by the differential decomposition rate in the dry and 381 wet microhabitats. Nevertheless, this relationship between warming and decomposition is not 382 straightforward (Aerts 2006) and would deserve a proper litter decomposition experiment.

383 Moss length increment was also higher in the dry microhabitat in 2011, and was promoted by 384 OTC warming in 2012 (Figure 3). In the dryer 2011 year, the higher cover of vascular plants, i.e. 385 *Eriophorum* leaves in the dry microhabitat is the determinant factor for moss growth and might 386 have had a facilitating effect on *S. fallax*+*rubellum* in limiting the drying out of the moss carpet 387 (Grosvernier et al. 1994, Buttler et al. 1998, Walker et al. 2015). Instead, in the wetter year 2012, 388 when water limitation was not critical and engendered an overall higher length increment, the 389 warming effect by OTC's could promote further the moss growth in the dry microhabitat. Adaptive 390 seasonal acclimation has been shown for *S. fallax*, as reflected by the maintenance of relatively high 391 net photosynthetic rates to lower water contents and no response differences when plants were 392 collected from different heights above the water table (Titus et al. 1983). In our study, we pooled *S. 393 fallax* and *S. rubellum*, because we could not reliably distinguish these two species in our point- 394 intercept field measures and in the frequency analysis on photographs. Nevertheless, some field 395 sampling in the plots showed that *S. fallax* is by far the most frequent species and that *S. rubellum* 396 occurs only on some very limited patches. Similarly to *S. fallax*, *S. rubellum* has been shown to 397 adapt and maintain similar height increment at different experimental water table depths (Robroek 398 et al. 2007). Therefore, shading may be critical for the development of these mosses, mediating soil 399 moisture and near ground temperature, which both are direct drivers of growth. Our results show 400 that shading can facilitate growth in dry years, but when wetter years are combined with warmer 401 conditions, in our case the OTC effect, this shading effect on growth is out-ruled by temperature. 402 Nevertheless, a better length increment of *Sphagnum* might also, on the long run, change the 403 structure of the moss carpet and consequently its moisture holding and transport capacity 404 (Dorrepaal et al. 2003). Therefore, when analyzing long term-effect of global change, not only the
change in species composition should be considered (Heijmans et al. 2008), but also the change of vegetation structure might be critical. In their work, Walker et al. (2015) mentioned a mismatch in the growth responses of different plant functional types to simultaneous climate and vegetation change. This was due to the complexity of interacting factors between dwarf-shrubs, bryophytes and graminoids, suggesting that some functional types reflecting a direct response to increased air temperature, some others an indirect one.

The evolution of aboveground biomass over the seasons was well reflected in the NDVI, which was also higher in OTC plots. Interestingly, NDVI was also better correlated to vascular plants than to mosses, which explains the more obvious difference in the dry plot, where vascular plants are more abundant. Indeed, when mosses dry out temporarily in dry periods, which can be observed in the wet microhabitat where the moss carpet is less protected from wind, reflectance is lowered. This differentiation between mosses and higher plants is critical in studying the carbon cycle using airborne remote sensing (Yan et al. 2014).

## Conclusion

Our study on the response of plant cover to experimental warming in two contrasting hydrological peatland microhabitats – wet and dry – shows that experimental warming by OTC interacts with soil wetness, resulting in divergent responses of vascular plants and bryophytes. For vascular plants, the depth of rooting seems the key in determining the response of plant cover, whereas for mosses, the growth is under the dependence of various other interacting factors, such as shading by shrubs and large statured vascular plants, which contributes further to regulate the microclimate conditions in the moss carpet.
428 Acknowledgments

429 This work was funded as part of the PEATWARM initiative through an ANR (French National
430 Agency for Research) grant (ANR-07-VUL-010). We acknowledge support from Labex
431 VOLTAIRE (ANR-10-LABX-100-01) and grant PSPB-013/2010 from Switzerland through the
432 Swiss Contribution to the enlarged European Union. P.M. acknowledges the support of the Swiss
433 National Science Foundation (nº PBELP3 146538). The authors are also indebted to the Regional
434 Scientific Council of Natural Heritage of the Franche-Comté Region for permitting access to Le
435 Forbonnet site. We also thank M.-L- Toussaint for providing the meteorological data, E. Farrer and
436 L. M. Hallet for statistical advices, and B. Corboz for technical assistance.

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586
587
588

589

For Review Only
590 **Captions to figures**

591

592 **Figure 1:** Changes of the relative frequencies of dominant plant species from 2008 to 2012 (no records in 2011) in the Forbonnet peatland (French Jura). Mean values (n=3) and standard error bars are given for dry (solid line) and wet (broken line) microhabitats, and for control (black line) and OTC (grey lines) plots. Significance of Linear Mixed Model is given for treatment effect (T) and for year effect (Y). Species that are not illustrated: *Carex pauciflora* (appeared in 2009 and then was only seen in one plot the years after), *Carex limosa* (appeared only in 3 plots in 2009), *Calluna vulgaris* (only in 2 different plots in different years), *Pinus rotundata* (only in one plot in one year).

599

600 **Figure 2:** Changes of the relative frequencies of species from April to September 2012 in the Forbonnet peatland (French Jura). Mean values (n=3) and standard error bars are given for dry (solid line) and wet (broken line) microhabitats, and for control (black line) and OTC (grey lines) plots. Significance of Linear Mixed Model is given for treatment (T) and month effect (M). Species that are not illustrated; with no significant differences: *Scheuchzeria palustris*, *Eriophorum vaginatum*, *Carex pauciflora*, *Carex rostrata*, *Drosera rotundifolia*, *Polytrichum strictum*; present only in one plot: *Calluna vulgaris*, *Carex pauciflora* and *Polytrichum strictum*.

607

608 **Figure 3:** Length increment (mm) of *Sphagnum* mosses in the different treatments (OTC vs control) and microhabitats (dry vs wet) in 2011 (a: May 13 – November 27, n=60) and 2012 (b: March 30 – November 6, n=120) in the Forbonnet peatland (French Jura). Vertical bars represent standard errors. Scales of both graphs are different. Significance levels of Linear Mixed Model are given for microhabitat (B) and treatment effect (T), and for their interaction (B x T).

613

614 **Figure 4:** Normalised Difference Vegetation Index (NDVI) at plot scale in the dry (a) and wet (b) microhabitats in year 2012 in the Forbonnet peatland (French Jura). Curves represent mean values (n=3) for control (black) and OTC (grey) plots. Significance codes for ANOVA's are (***)
p<0.001; (**) p<0.01; (*) p<0.05; (.) p<0.1.

618

619

620 **Figure 1**
621

622

623

624

625

626 *Figure 2*

627
628

629 *Figure 3*

630

631
632

633

634

635 *Figure 4*

636

637
638 Electronic appendices

639

640 Supporting Information to the paper Buttler et al. 2015. Experimental warming interacts with soil

641 moisture to discriminate plant responses in an ombrotrophic peatland. *Journal of Vegetation*

642 *Science*. Appendix S1. Relationship between point-intercept frequency measures of vegetation and

643 picture frequency analysis from the Forbonnet peatland (French Jura) for a: total vascular plants

644 (Pearson r: 0.733, p<0.01), b: litter (Pearson r=0.873, p<0.001) and c: total *Sphagnum* and

645 *Polytrichum strictum* (Pearson r=0.942, p<0.001).

646

647 The correlation between frequencies obtained by means of field point-intercepts and photographic

648 analyses gives a good linear relation with vascular plants but shows an underestimation of mosses

649 with the picture analysis. This is because leaves of vascular plants might hide the understory of

650 mosses which than cannot be recorded in the one-layer photographic method, whereas in the field,

651 the needle can hit more than one species or individual. This underestimation is mostly noticeable at

652 intermediate moss cover, when vascular plants are most abundant. Conversely, the picture analysis

653 overestimates litter when it is dense.

654

655

656

657
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

Year

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>
Vaccinium
oxycoccus

Total vascular

<figure><img src="image_13.png" /></figure>

Andromeda
polifolia

<figure><img src="image_14.png" /></figure>

Sphagnum
fallax+rubellum

<figure><img src="image_15.png" /></figure>

Sphagnum
magellanicum

<figure><img src="image_16.png" /></figure>

Litter

<figure><img src="image_17.png" /></figure>
a

B: 0.004

T: n.s.

B x T: 0.101

<figure><img src="image_18.png" /></figure>

Length increment (mm)

b

B: 0.027

T: 0.012

B x T: 0.008

<figure><img src="image_19.png" /></figure>
NDVI

<figure><img src="image_20.png" /></figure>

Ordinal Days, 2012

<figure><img src="image_21.png" /></figure>

Ordinal Days, 2012
a

Picture frequency

<figure><img src="image_22.png" /></figure>

Point-intercept frequency

b

Picture frequency

<figure><img src="image_23.png" /></figure>

Point-intercept frequency

c

Picture frequency

<figure><img src="image_24.png" /></figure>

Point-intercept frequency