DOI: 10.5061/dryad.9ghx3ffxx
title: Towards rainy high Arctic winters: how experimental icing and summer warming affect tundra plant phenology, productivity and reproduction

description: The Arctic is warming rapidly, and much faster in winter than in summer.
 Warm spells in winter lead to more frequent extreme rain-on-snow events
 that alter snowpack conditions and can encapsulate plants in ‘basal ice’
 (‘icing’) for months. Yet, how icing affects plant communities, especially
 over multiple winters and under warmer summers, remains largely unstudied.
 We investigated winter icing and summer warming effects on vascular
 plants’ productivity, reproduction and phenology in mesic dwarf-shrub
 heath, an important reindeer habitat in high Arctic Svalbard, where winter
 temperatures have been rising particularly fast. In a full factorial field
 experiment, rain-on-snow and resultant icing were simulated in five
 consecutive winters, and each followed by experimentally increased summer
 temperatures. Vascular plant responses at the community-level, with
 particular attention to the dominant dwarf shrub Salix polaris, were
 assessed throughout each subsequent growing season. Icing alone increased
 community-level primary productivity, but only late in the growing season,
 and reduced inflorescence production. Accordingly, S. polaris showed
 delayed early leaf phenophases, but accelerated subsequent development,
 resulting in smaller, thinner leaves. This compensatory growth response
 apparently occurred at the cost of delayed seed maturation. The
 phenological delay was associated with icing-induced delays in spring soil
 warm-up, possibly favouring resource allocation to primary productivity
 over reproduction. Experimental summer warming (on average 0.8 °C) largely
 counteracted the effects of icing, enhancing community productivity
 throughout the growing season, offsetting S. polaris leaf size reductions
 and turning around its delayed phenophases, including seed dispersal.
 Effect sizes could be larger than under warming alone. Yet, summer warming
 did not negate the reduction in community inflorescence production caused
 by icing. Synthesis. Extreme rain-on-snow events encapsulating plants in
 ice can influence high Arctic plant communities in mesic habitats to
 similar extents as – the better-studied – summer warming. Nevertheless,
 the absence of magnified icing effects over the years indicates community
 resistance to icing, particularly under warmer summers, which contrasts
 with earlier documented ice-induced die-offs in communities dominated by
 evergreen shrubs. As warm spells during winter become the rule rather than
 exception, we call for similar experiments in coordinated circumpolar
 monitoring programmes across the tundra biome.
description: Please refer to the methods of the open access
 publication:  Le Moullec et al. (2025) Towards rainy
 high Arctic winters: how experimental icing and summer warming affect
 tundra plant phenology, productivity and reproduction.
 Published in Journal of Ecology, Accepted on 31 October
 2025.
description: # Towards rainy high Arctic winters: how experimental icing and summer
 warming affect tundra plant phenology, productivity and reproduction
 Dataset DOI:
 [10.5061/dryad.9ghx3ffxx](https://doi.org/10.5061/dryad.9ghx3ffxx) ##
 Description of the data and file structure **The datasets include all raw
 data from the manuscript:** Towards rainy high Arctic winters: how
 experimental icing and summer warming affect tundra plant phenology,
 productivity and reproduction.  Mathilde Le Moullec, Anna-Lena Hende,
 Matteo Petit Bon, Ingibjörg Svala Jónsdóttir, Øystein Varpe, René van der
 Wal, Larissa Teresa Beumer, Kate Layton-Matthews, Ketil Isaksen and Brage
 Bremset Hansen. Published in Journal of Ecology, Accepted on 31 October
 2025. Data contains information measured at the plot level on: soil
 temperature, soil moisture, air temperature, Normalized Digital Vegetation
 Index (NDVI), point intercept biomass quantification, flower count and
 Salix polaris leaf traits and phenology. **Common column names related to
 the study design, included within each datasets:** year: 2016 to 2020
 block: 3 replicated units across the landscape (A1, A3, A4) with 12 plots
 each plot: 36 plots with unique IDs followed throughout the years. Plots
 are named 1-12 in each block, so a unique plot name is e.g. A1-2 IDfull:
 unique information of each plot, combining block_plot_treatment_year info,
 e.g. A3_2_W_2018 treatment: the 4 different categories include C (control
 plots), I (icing plots), W (warming plots), IW (icing + warming plots).
 The icing treatment encased the vegetation into a layer of basal ice for
 several months in winter. The warming treatment consisted of placing an
 Open Top Chamber on the plot for the snow-free season. Each block has 3
 plots with the same treatment. icing: binomial recording of the icing
 treatment on a plot; 0 (no icing), 1 (icing). warming: binomial recording
 of the warming treatment on a plot; 0 (no warming/Open top Chamber), 1
 (warming/Open top Chamber) round: the repetition number of a measurement
 throughout one same summer DOY or day-of-year = The day of year since 1st
 of January that year. Note that year, block, plot, treatment and round
 should be used as factors! None of these variables have a numerical
 meaning. **A R script is available on request:**
 **[mathilde.lemoullec@ntnu.no](mailto:mathilde.lemoullec@ntnu.no)**
 **and** **[malm@natur.gl](mailto:malm@natur.gl)** ### Files and variables
 #### File: JoE_IcingExp_Biomass_PIM.txt **Description:** Community-level
 vascular plant species abundance estimation by Point Intercept Methodology
 (Bråthen & Hagberg, 2004). Measruements were conducted shortly after
 peak growth (end of July) in 2016–2019 at 25 intersections per plots. We
 recording all ‘hits’ of live vascular plant species and dead tissue
 classified as standing dead or litter. ##### Variables * year: 2016–2019 *
 block: 3 replicated units across the landscape (A1, A3, A4) with 12 plots
 each. * plot: 36 plots with unique IDs followed throughout the years.
 Plots are named 1-12 in each block, so a unique plot name is e.g. A1-2. *
 treatment: the 4 different categories include C (control plots), I (icing
 plots), W (warming plots), IW (icing + warming plots). The icing treatment
 encased the vegetation into a layer of basal ice for several months in
 winter. The warming treatment consisted of placing an Open Top Chamber on
 the plot for the snow-free season. Each block has 3 plots with the same
 treatment. * icing: binomial recording of the icing treatment on a plot; 0
 (no icing), 1 (icing). * warming: binomial recording of the warming
 treatment on a plot; 0 (no warming/Open Top Chamber), 1 (warming/Open Top
 Chamber). * species: Corresponding to the species and groups presented in
 Fugure S1. alobor = Alopecurus borealis, bisviv = Bistorta vivipara,
 dryoct = Dryas octopetala, equarv = Equisetum arvens, luzcon = Luzula
 confuse, poaarc = Poa arctica, salpol = Salix polaris, other_vasc = other
 live vascular plants, standing dead = sdead, litter = litter, tot_vasc =
 total live vascular plants, vascular_noSalixAlop = total live vascular
 plants minus salpol and alobor. * Count: summed number of hits per plots.
 #### File: JoE_IcingExp_HOBO.txt **Description:** Daily surface air
 temperature recorded 5 cm above the bryophyte layer, in summer 2016-2018.
 A total of six HOBO loggers were deployed: 2 per per block, one in a
 Warming and one in a Control plot. Data are averaged per treatment.
 Instrument: HOBO loggers (type U23-003/UA-001; ± 0.2°C accuracy) #####
 Variables * DOY: The day of year since 1st of January * treatment: the 4
 different categories include C (control plots), I (icing plots), W
 (warming plots), IW (icing + warming plots). The icing treatment encased
 the vegetation into a layer of basal ice for several months in winter. The
 warming treatment consisted of placing an Open Top Chamber on the plot for
 the snow-free season. * Date: Date corresponding to the DOY * Time: Not
 relevant * Temp: Temperature in degree Celsius  * RH: Relative Humidity in
 %, not used and not explored in this study * year: Year of recording,
 2016, 2017 or 2018.  #### File: JoE_IcingExp_Flower_Count.txt
 **Description:** Flowering counts was assessed in mid-July each year by
 counting total inflorescence numbers within each plot (50 × 50 cm),
 subdivided into 16 sub-squares- Recorded species are S. polaris (males and
 females [catkins]), B. vivipara, A. borealis, L. confusa and P. arctica.
 ##### Variables * year: 2016 to 2020. * block: 3 replicated units across
 the landscape (A1, A3, A4) with 12 plots each. * plot: 36 plots with
 unique IDs followed throughout the years. Plots are named 1-12 in each
 block, so a unique plot name is e.g. A1-2. * square: the sub-square within
 the plot recording frame 1-16. * treatment: the 4 different categories
 include C (control plots), I (icing plots), W (warming plots), IW (icing +
 warming plots). The icing treatment encased the vegetation into a layer of
 basal ice for several months in winter. The warming treatment consisted of
 placing an Open Top Chamber on the plot for the snow-free season. Each
 block has 3 plots with the same treatment. * icing: binomial recording of
 the icing treatment on a plot; 0 (no icing), 1 (icing). *
 warming: binomial recording of the warming treatment on a plot; 0 (no
 warming/Open Top Chamber), 1 (warming/Open Top Chamber). * salix_tot:
 total number of S. polaris (males and females) catkins per plot. *
 Bis_tot: total number of Bistorta vivipara reproductive stems
 (inflorescences and/or bulbils) per plot. * gram: total number of
 graminoid inflorescences, suming species A. borealis, L. confusa and P.
 arctica. * total: total number of inflorescences. #### File:
 JoE_IcingExp_Leaf_trait.txt **Description:**  ##### Variables * year: 2018
 to 2020 * block: 3 replicated units across the landscape (A1, A3, A4) with
 12 plots each. * plot: 36 plots with unique IDs followed throughout the
 years. Plots are named 1-12 in each block, so a unique plot name is e.g.
 A1-2. * Branch_id: Unique branch ID within plot. * Leave_id: Unique leaf
 ID within branch or within sampling sub-square. * Leave_id_br: Unique leaf
 ID within branch ID. * treatment: the 4 different categories include C
 (control plots), I (icing plots), W (warming plots), IW (icing + warming
 plots). The icing treatment encased the vegetation into a layer of basal
 ice for several months in winter. The warming treatment consisted of
 placing an Open Top Chamber on the plot for the snow-free season. Each
 block has 3 plots with the same treatment. * icing: binomial recording of
 the icing treatment on a plot; 0 (no icing), 1 (icing). *
 warming: binomial recording of the warming treatment on a plot; 0 (no
 warming/Open Top Chamber), 1 (warming/Open Top Chamber). *
 Dry_weight_mg: Dry mass per leaf. At the nearest 0.01 mg. *
 SLA_mm2_mg: Specific leaf area (SLA) was calculated as the ratio of leaf
 area to dry mass. * Area_mm2: Leaf area in mm^2. Measured on ImageJ
 software. #### File: JoE_IcingExp_NDVI.txt **Description:** NDVI handheld
 raw measurements at the plot level in summer 2016-2020. Instrument: Skye
 SpectroSense2+ handheld device ##### Variables * Date: Date of sampling,
 corresponding to DOY * Day_of_year: The day of year since 1st of January
 that year * year: 2016 to 2020 * block: 3 replicated units across the
 landscape (A1, A3, A4) with 12 plots each * round: the repetition number
 of a measurement throughout one same summer * plot: 36 plots with unique
 IDs followed throughout the years. Plots are named 1-12 in each block, so
 a unique plot name is e.g. A1-2 * treatment: the 4 different categories
 include C (control plots), I (icing plots), W (warming plots), IW (icing +
 warming plots). The icing treatment encased the vegetation into a layer of
 basal ice for several months in winter. The warming treatment consisted of
 placing an Open Top Chamber on the plot for the snow-free season. Each
 block has 3 plots with the same treatment. * warming: binomial recording
 of the warming treatment on a plot; 0 (no warming/Open top Chamber), 1
 (warming/Open top Chamber) * icing: binomial recording of the icing
 treatment on a plot; 0 (no icing), 1 (icing). * NDVI: the raw value of
 NDVI between 0 and 1, 4 digits. * plotID: Combination of block and plot
 number to give a unique plot ID.  #### File: JoE_IcingExp_NDVI_metrics.txt
 **Description:** NDVI metrics calculated based on the daily NDVI
 predictions from the file 4b. JoE_IcingExp_NDVI_Daily_Prediction.txt .
 Note that 2017 does not has sufficient monitoring rounds to estimates
 accurately these metrics. ##### Variables * year: 2016 to 2020 (but do not
 use 2017). * plot: 36 plots with unique IDs followed throughout the years.
 Plots are named 1-12 in each block, so a unique plot name is e.g. A1-2. *
 block: 3 replicated units across the landscape (A1, A3, A4) with 12 plots
 each. * treatment: the 4 different categories include C (control plots), I
 (icing plots), W (warming plots), IW (icing + warming plots). The icing
 treatment encased the vegetation into a layer of basal ice for several
 months in winter. The warming treatment consisted of placing an Open Top
 Chamber on the plot for the snow-free season. Each block has 3 plots with
 the same treatment. * icing: binomial recording of the icing treatment on
 a plot; 0 (no icing), 1 (icing). * warming: binomial recording of the
 warming treatment on a plot; 0 (no warming/Open top Chamber), 1
 (warming/Open top Chamber). * time_maxNDVI: day-of-the-year of when the
 maximum NDVI is reached. * maxNDVI: maximum NDVI value between 0 and 1,
 use max. 4 digits. * NDVIcumStart: early-season cumulative NDVI
 (‘cumulative start’) obtained by integrating daily predicted NDVI values
 from the first day of measurements that summer to the day when Control
 plots, on average, reached maximum NDVI. Use max. 4 digits. *
 NDVIcumEnd: late-season cumulative NDVI (‘cumulative end’) obtained by
 integrating daily values from the day when Control plots, on average,
 reached the maximum NDVI to the last measurement day of that season. Use
 max. 4 digits. * NDVIcumEntireSeason: cumulative NDVI across the growing
 season (‘cumulative total’) obtained by integrating daily values from the
 first to the last day of measurements that season. Use max. 4 digits. ####
 File: JoE_IcingExp_Moisture.txt **Description:** Volumetric water content
 (i.e. soil moisture) measured at 5-10 cm depth, repeated 3-15 times per
 summer (round) in 2017-2020. Instrument: ML3 ThetaProbe Sensor (HH2 Soil
 Moisture Meter from Delta-T Devices Ltd, UK, % volume with 1% accuracy)
 ##### Variables * Date: Date of measurement, corresponding to DOY. *
 Day_of_year: The day of year since 1st of January that year * year: 2016
 to 2020. * block: 3 replicated units across the landscape (A1, A3, A4)
 with 12 plots each. * round: the repetition number of a measurement
 throughout one same summer. * plot: 36 plots with unique IDs followed
 throughout the years. Plots are named 1-12 in each block, so a unique plot
 name is e.g. A1-2. * treatment: the 4 different categories include C
 (control plots), I (icing plots), W (warming plots), IW (icing + warming
 plots). The icing treatment encased the vegetation into a layer of basal
 ice for several months in winter. The warming treatment consisted of
 placing an Open Top Chamber on the plot for the snow-free season. Each
 block has 3 plots with the same treatment. * square: replicated
 measurement at five points per plot (in each corner [soil_moisture_1;
 soil_moisture_2; soil_moisture_3; soil_moisture_4] and centre
 [soil_moisture_5]). * moisture: soil moisture in %. #### File:
 JoE_IcingExp_NDVI_Daily_Prediction.txt **Description:** Daily estimated
 NDVI values in summer fitting a generalized additive model (GAM) to each
 plot-year combination. We used the R function ‘gam’ from the R-package
 mgvc (Wood et al., 2015). Note that 2017 does not have enough replicated
 measurement rounds across the summer (and do not cover the peak growing
 season) for estimating daily NDVI values with gam. Based on data from file
 JoE_IcingExp_NDVI.txt ##### Variables * ID: specific ID for each plot each
 year, used for the gam fit. * Day_of_year: The day of year since 1st of
 January that year. * NDVI: daily estimated NDVI based on gam for each plot
 seperately. gam(NDVI ~ s(Day_of_year,k=K),data=PI,method="REML")
 with k representing the maximum number of measurement rounds. * year: 2016
 to 2020 * treatment: the 4 different categories include C (control plots),
 I (icing plots), W (warming plots), IW (icing + warming plots). The icing
 treatment encased the vegetation into a layer of basal ice for several
 months in winter. The warming treatment consisted of placing an Open Top
 Chamber on the plot for the snow-free season. Each block has 3 plots with
 the same treatment. * color: or plotting purpose.  #### File:
 JoE_IcingExp_Phenology_Binary.txt **Description:** Phenology of S. polaris
 was recorded for the most advanced vegetative and reproductive
 phenological stages (‘phenophases’) in each of the 16 sub-squares of the
 50 × 50 cm sampling frame. Records are binary, 0 (stage not reached) or 1
 (stage reached). ##### Variables * squareID: unique sub-square ID
 combining Block_Plot_Treatment_Square IDs. * day_of_year: The day of year
 since 1st of January that year. * block: 3 replicated units across the
 landscape (A1, A3, A4) with 12 plots each. * plot: 36 plots with unique
 IDs followed throughout the years. Plots are named 1-12 in each block, so
 a unique plot name is e.g. A1-2. * square: the sub-square within the plot
 recording frame 1-16. * treatment: the 4 different categories include C
 (control plots), I (icing plots), W (warming plots), IW (icing + warming
 plots). The icing treatment encased the vegetation into a layer of basal
 ice for several months in winter. The warming treatment consisted of
 placing an Open Top Chamber on the plot for the snow-free season. Each
 block has 3 plots with the same treatment. * year: 2016 to 2020. *
 PhenoPart: female = female catkins development, male = male catkins
 developments, veg = vegetative leaves development. * icing: binomial
 recording of the icing treatment on a plot; 0 (no icing), 1 (icing). *
 warming: binomial recording of the warming treatment on a plot; 0 (no
 warming/Open Top Chamber), 1 (warming/Open Top Chamber). * phase0: female
 = NA, male = NA, veg = buds not open. * phase1: female = distinct
 inflorescence buds visible, male = distinct inflorescence buds visible,
 veg = leaves starting to unfurl. * phase2: female = buds recognisable as
 female, male = buds recognisable as male, veg = leaves fully expanded. *
 phase3: female = receptive stigmas, male = open anthers releasing pollen,
 veg = start of senescence. * phase4: female = stigmas withered, male =
 anthers withered, veg = leaves fully senesced. * phase5: female = seed
 dispersal, male = NA [No 5th stage], veg = NA [No 5th stage]. #### File:
 JoE_IcingExp_Temperature.txt **Description:** Soil temperature recorded at
 different depth from January to August in each experimental plot, in
 different years (2016-2020). Instrument: iButton loggers (type DS1921G-F5,
 ± 1.0 °C accuracy, 0.5 °C resolution) ##### Variables * datetime: the
 exact recording time of the iButton, each 6 to 12 hours per day. Date
 format year-month-day * temp: Temperature in degree celsius. * block: 3
 replicated units across the landscape (A1, A3, A4) with 12 plots each *
 treatment: the 4 different categories include C (control plots), I (icing
 plots), W (warming plots), IW (icing + warming plots). The icing treatment
 encased the vegetation into a layer of basal ice for several months in
 winter. The warming treatment consisted of placing an Open Top Chamber on
 the plot for the snow-free season. Each block has 3 plots with the same
 treatment. * plot: 36 plots with unique IDs followed throughout the years.
 Plots are named 1-12 in each block, so a unique plot name is e.g. A1-2 *
 depth: specific depth where the iButton logger is placed in the soil; S
 (2cm depth, below moss layer), D (5cm depth, soil subsurface), 10d (10cm
 depth, only in 2018), 20d (20cm depth, only in 2018), NA (in winter 2016,
 2-5cm depth, soil subsurface). * DOY: The day of year since 1st of January
 that year. * year: 2016 to 2020 * month: month of year * day: day of the
 month * IDfull: ID composed of block_plot_treatment  * ID_year: ID
 composed of block_plot_treatment_year  * icing: binomial recording of the
 icing treatment on a plot; 0 (no icing), 1 (icing). * warming: binomial
 recording of the warming treatment on a plot; 0 (no warming/Open top
 Chamber), 1 (warming/Open top Chamber) ## File:
 README_JoE_LeMoullec2025.txt Summary of this README file, to download and
 keep with the datafiles.  ## Code/software A R script is available on
 request: [mathilde.lemoullec@ntnu.no](mailto:mathilde.lemoullec@ntnu.no)
 and [malm@natur.gl](mailto:malm@natur.gl) ## Access information This data
 is associated to the open access publication:  Le Moullec et al. (2025)
 Towards rainy high Arctic winters: how experimental icing and summer
 warming affect tundra plant phenology, productivity and reproduction.
 Published in Journal of Ecology, Accepted on 31 October 2025.