DOI: 10.5061/dryad.tht76hf6r
title: Data from: Increased biocrust cover and activity in the highlands of Iceland after five growing seasons of experimental warming

description: Background and Aims One of the most important questions of our time is how
 ecosystems will be transformed by climate change. Here, we used a
 five-year field experiment to investigate the effects of climate warming
 on the cover and function of a sub-Arctic alpine ecosystem in the
 highlands of Iceland dominated by biocrust, mosses and vascular plants.
 Methods We used Open Top Chambers (OTCs) to simulate warming; standard
 surface and NDVI analyses to measure plant cover and function; gas
 analyzers to monitor biocrust respiration; and the Tea Bag Index approach
 to estimate mass loss, decomposition and soil carbon stabilization rates.
 Results Contrary to our initial hypothesis of warming accelerating an
 ecological succession of plants growing on biocrust, we observed a
 warming-induced decreased abundance of vascular plants and mosses
 —possibly caused by high temperature summer peaks that resemble heat
 waves— and an increase in the cover of biocrust. The functional responses
 of biocrust to warming, including increased litter mass loss and
 respiration rates and a lower soil carbon stabilization rates, may suggest
 climate-driven depletion of soil nutrients in the future. Conclusion It
 remains to be studied how the effects of warming on biocrusts from high
 northern regions could interact with other drivers of ecosystem change,
 such as grazing; and if in the long-term global change could favor the
 growth of vascular plants on biocrust in the highlands of Iceland and
 similar ecosystems. For the moment, our experiment points to a
 warming-induced increase in the cover and activity of biocrust.
description: <strong>Experimental design and abiotic
 measurements</strong> In June 2018 we set up the
 <em>Climate Research Unit at Subarctic Temperatures</em>
 (CRUST) experiment (Fig. S1b; Salazar et al., 2022) in the highlands of
 Iceland (64°02' N, 19°13' W; 590 m.a.s.l.). The site is
 primarily covered with <em>Anthelia</em>
 <em>juratzkana</em> biocrust, mosses, and vascular plants
 (e.g. <em>Salix herbacea</em>; Fig. S1a), on soils classified
 as andosol Vitricryands (US Soil Taxonomy) in a cryic soil temperature
 regime with an abundance of relatively fresh volcanic glass of basaltic
 and andesitic composition (vitric materials). The area is subjected to
 periodic volcanic activity and intense (0.1-2 mm
 yr<sup>-1</sup>) dust deposition (Arnalds et al., 2015). The
 soils are coarse grained (sandy loam/loamy sand) with coarse sub-surface
 tephra layers (volcanic ash) that negatively affect soil water
 conductivity and storage capacity. Mean annual temperature and
 precipitation (1971-2000; Icelandic Met Office, 2024) at the site are ca
 1.6 °C (Fig. S10) and 1600 mm, respectively. The snow-free season
 generally starts in late June and ends in October. Flooding is common in
 June, when the snow melts and the ice under the surface is still frozen.
 <em>Anthelia</em> <em>juratzkana</em> biocrust is
 often found in this type of late-snow bed ecosystems (Belland, 1983;
 Ottósson et al., 2016; Smáradóttir, 2020). Experimental
 warming was simulated with OTCs (Hollister et al., 2023) that were built
 according to the protocols of the International Tundra Experiment (ITEX;
 Henry and Molau, 1997; Henry et al., 2022). We used a randomized block
 design. First, we visually selected 16 areas of at least 4x4 meters
 covered mainly by biocrust and separated by at least 10 meters. Then we
 randomly selected 8 areas (blocks, n=8). In each of these blocks, we set
 up two 1.5 m x 1.5 m experimental plots and randomly assigned treatments
 to them, an OTC and a control (i.e. ambient temperature) plot. The OTCs
 were left in place all year round. We measured
 temperature, light intensity and moisture in OTCs and control plots at
 different times during five consecutive growing seasons from 2018 to 2022.
 We measured surface temperature and light intensity every 2 hours using
 HOBO pendant temperature/light data loggers UA-002 64 (Onset Computer,
 Bourne, Massachusetts, USA). We used one logger per treatment (OTC and
 control) in 4 of the 8 blocks (n=4). In the growing seasons of 2018, 2019,
 2020 and 2022 we took paired measurements of soil moisture (2-3 cm depth)
 in all blocks (n=8) using an ECH2O EC-5 moisture sensor (Decagon Devices,
 Pullman, WA) attached to a HOBO micro station (Onset Computer, Bourne,
 Massachusetts, USA). <strong>Aboveground cover
 and plant community analysis</strong> We
 estimated percentages of area covered by biocrust, moss and vascular
 plants in the growing seasons of 2018, 2019, 2021 and 2022. For this, we
 placed a 50 x 50 cm quadrant, divided in 16 squares of 12.5 x 12.5 cm, in
 the center of each plot and visually estimated the cover of biocrust, moss
 and vascular plants (example in Fig. S2). In 2022, we complemented our
 cover analysis with: 1) Groundbase (SKYE instrument SpectroSense 2+)
 measurements of Normalised Difference Vegetation Index (NDVI) as proxy for
 primary productivity. 2) A detailed plant community analysis using the
 point intercept method with 25 evenly distributed points within the 50x50
 cm quadrant where all species intercepted (hits) were recorded for each
 point along with visual estimates of total vascular plant, moss and litter
 cover as well as a complete list of vascular plant and moss species
 providing plot species richness. <strong>Litter
 decomposition and stabilization factor</strong>
 To compare the potential fate of organic matter and decomposition
 rates in the soil, we buried green (Lipton, EAN: 87 22700 05552 5) and
 rooibos (Lipton, EAN: 87 22700 18843 8) tea bags in OTC and control plots.
 Tea bags were weighted and buried in the field on July 27th, 2018 and June
 25th 2021. Bags were buried at two depths: at 8 cm, to facilitate
 comparisons with other studies following the protocol proposed by Keuskamp
 et al. (2013); and under the biocrust (2-3 cm), which is more relevant to
 our particular study. We buried 2 subsamples per plot and per depth. The
 bags were collected on June 5th, 2019 (313 d of incubation) and August
 10th, 2022 (411 d incubation), respectively. Notice that the tea bags were
 incubated for ca. 1 yr (similar to other tundra studies, e.g.,
 Björnsdóttir et al., 2021; von Oppen et al. 2024) and not for 90 days (as
 in Keuskamp et al., 2013). From the measured mass loss, we calculated
 litter decomposition rates (k) and stabilization factor (S; an estimate of
 the fraction of labile compounds that stabilize and become recalcitrant
 during decomposition) as in Keuskamp et al., (2013). Our decomposition
 data was submitted to the TBI network: http://www.teatime4science.org/. <strong>Biocrust respiration</strong> We measured biocrust respiration (R<sub>B</sub>) in the growing seasons of 2019 (June and July), 2021 (June and July) and 2022 (June and August). For that, we installed PVC collars (10 cm depth and diameter) in <em>Anthelia</em> <em>juratzkana</em> biocrust patches within the OTCs and control plots, and measured CO<sub>2</sub> fluxes periodically using standard procedures with gas analyzers attached to dark soil chambers. All measurements were done approximately in the middle of the day, between 11:00 and 13:00 UTC. We used different gas analyzers in different years. In 2019, we used a Li-6400XT portable system (Li-COR Inc., Lincoln NE, USA); in 2021 a Senseair K-33 ELG sensor (based on a design by Harmon et al., 2015), calibrated with an EGM-4 (PP Systems, Amesbury, Massachusetts, USA); and in 2022 we used an EGM-5 (PP Systems, Amesbury, Massachusetts, USA). <strong>Statistical analysis</strong> We used a combination of statistical approaches: 1) Linear regression (<em>lm</em> function in the R package) to analyze climate trends near our study site (hourly measurements of air temperature at the Vatnsfell station of the Icelandic Meteorological Office, IMO), and the potential relationships between surface light and OTC-warming. 2) Mixed effect models (<em>lmer</em> function), selected based on the Bayesian information criterion (BIC), to analyze differences in litter mass loss, decomposition rates, stabilization factors, and R<sub>B</sub>. 3) Analysis of variances (<em>aov</em> function) for cover, soil moisture, and NDVI analyses. Surface cover and moisture analyses were further complemented with Tukey Honest Significant Differences (HSD) analyses (<em>TukeyHSD</em> function), to explore potential differences between control and OTC plots in individual years. In the mixed effect models, we analyzed the fixed effects of the OTCs, time and/or tea bag type (depending on the case) and the random effects of the blocks (paired OTC and control plots). To select the mixed effects models with the lowest BICs (i.e. best fit), we used the <em>glmulti</em> package (Calcagno and de Mazancourt, 2010). All statistical work was conducted in R, version 4.1.2 (R Core Team, 2023).
description: # Data from: Increased biocrust cover and activity in the highlands of
 Iceland after five growing seasons of experimental warming
 [https://doi.org/10.5061/dryad.tht76hf6r](https://doi.org/10.5061/dryad.tht76hf6r) Data (csv files) and R scripts used in the analysis of cover (biocrust, moss and vascular plants) and function (respiration decomposition, NDVI) in a sub-Arctic ecosystem in the highlands of Iceland experimentally warmed with Open Top Chambers (OTCs) for five growing seasons. **Author(s)** * Alejandro Salazar (corresponding authors). Faculty of Environmental and Forest Sciences, Agricultural University of Iceland, Reykjavík, Iceland. [alejandro@lbhi.is](mailto:alejandro@lbhi.is) * Eyrún G. Gunnlaugsdóttir. Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Finland. [eyrun.gunnlaugsdottir@helsinki.fi](mailto:eyrun.gunnlaugsdottir@helsinki.fi) * Ingibjörg S. Jónsdóttir. Faculty of Life and Environmental Sciences, University of Iceland, Reykjavık, Iceland.. [isj@hi.is](mailto:isj@hi.is) * Ian Klupar. Faculty of Life and Environmental Sciences, University of Iceland, Reykjavık, Iceland.. [ianklupar@gmail.com](mailto:ianklupar@gmail.com) * Ruth-Phoebe T. Wandji. Faculty of Environmental and Forest Sciences, Agricultural University of Iceland, Reykjavík, Iceland. [ruth@lbhi.is](mailto:ruth@lbhi.is) * Ólafur Arnalds. Faculty of Environmental and Forest Sciences, Agricultural University of Iceland, Reykjavík, Iceland. [oa@lbhi.is](mailto:oa@lbhi.is) * Ólafur Andrésson. Faculty of Life and Environmental Sciences, University of Iceland, Reykjavık, Iceland.. [osa@hi.is](mailto:osa@hi.is) **Dates of data collection:** Between 6/22/2018 and 11/20/2022. **Geographic location of data collection:** approximate center of experiment: 64°02'11.7"N 19°13'08.2"W, near Landmannahellir, Iceland. **Funding sources that supported the collection of the data:** Much of this work was financed with the 2016 Icelandic Research Fund grant no. 163336. **Key abbreviations:** Biocrust: Biological soil crust CRUST: Climate Research Unit at Subarctic Temperatures IMO: Icelandic Meteorological Office. k: decomposition rates NDVI: Normalised Difference Vegetation Index OTC: Open Top Chamber TBI: Tea Bag Index RB: Biocrust respiration S: Stabilization factor ## Description of the data and file structure **File list** * Name: Fig1_S3_data_Salazar_etal_PlantSoil.csv Description: data of figure 1. Surface light, surface temperature and ΔT at the CRUST (Climate Research Unit at Subarctic Temperatures) experiment during five consecutive growing seasons. Data of Figure S3. Positive correlation between surface light and the temperature difference (ΔT) between OTCs and controls (i.e. OTC-warming). * Fig2_data_Salazar_etal_PlantSoil.csv Description: data of figure 2. Cover of biocrust (% area), moss and vascular plants in control and OTC plots, between 2018 and 2022. * Fig3_data_Salazar_etal_PlantSoil.csv Description: data of figure 3. Percentage (%) mass loss of litter in green and rooibos tea bags after ca. 1 yr incubation in control and OTC plots, on 2018-2019 and 2021-2022, at the CRUST experiment. * Fig4_data_Salazar_etal_PlantSoil.csv Description: data of figure 4. Yearly and overall differences in paired measurements of RB (in μmol CO2 m-2 s-1) between control and OTC plots (ΔRB), between 2019 and 2022. * FigS2_data_Salazar_etal_PlantSoil.csv Description: data of figure S2. Air temperature (2 m height; °C) at the IMO Vatnsfell station (64°11.736' N, 19°2.802' W), 20 km northeast from the CRUST experiment. * FigS4_data_Salazar_etal_PlantSoil.csv Description: data of figure S4. Positive correlation between surface light (lux) and the temperature (°C) difference (ΔT) between OTCs and controls (i.e. OTC-warming). ΔT = 0.4 + (2.3 x 10^-5^)*(Surface light) (Table S2). * FigS5_data_Salazar_etal_PlantSoil.csv Description: data of figure S5. Segment of surface light (lux) and ΔT (°C) data, showing that surface temperature was lower in an OTC than in its paired control (i.e. negative ΔT) during the darkest part of the night. * FigS6_data_Salazar_etal_PlantSoil.csv Description: data of figure S6. Soil moisture (2-3 cm depth; % of saturation) in control and OTCs between 2018 and 2022. * FigS7_data_Salazar_etal_PlantSoil.csv Description: data of figure S7. Litter cover (%) in control and OTC plots on August 10th, 2022. * FigS8_data_Salazar_etal_PlantSoil.csv Description: data of figure S8. Number of vascular plant and moss species in control and OTC plots on August 10th, 2023. * FigS9_data_Salazar_etal_PlantSoil.csv Description: data of figure S9. NDVI measurements (unitless) in control and OTC plots on August 10th, 2023. * FigS10_data_Salazar_etal_PlantSoil.csv Description: data of figure S10. Decomposition rates (k) and stabilization factor (S) (unitless) in control and OTC plots, at 2 cm and 8 cm depth, in 2018-2019 and 2021-2022. 
 
 **Methodological information** This data includes measurements of surface
 temperature and light, cover, litter mass loss, decomposition rates, soil
 carbon stabilization rates, biocrust respiration rates, soil moisture,
 number of vascular plant and moss species, NDVI, and air temperature. *
 Data-specific information for Fig1_S3_data_Salazar_etal_PlantSoil.csv:
 Number of variables: 8 Number of measurements/rows: 29406 Variable list: *
 Date_Ctrl: dates of measurements in control (C) plots, from 6/22/2018
 1:14:00 PM to 11/20/2022 10:46:00 PM. Hourly. * Average_Ctrl_T, in °C.
 Average temperature (in °C) in control plots. Average of four (or less, if
 one or more malfunctioned at a particular measuring time) HBO loggers. *
 Average_Ctrl_lux, in lux. Average light intensity (in lux) in control
 plots. Average of four (or less, if one or more malfunctioned at a
 particular measuring time) HBO loggers. * Date_OTC: dates of measurements
 in OTC plots, from 6/22/2018 1:14:00 PM to 11/20/2022 10:46:00 PM. Hourly.
 * Average_OTC_T, in °C. Average temperature (in °C) in OTC plots. Average
 of four (or less, if one or more malfunctioned at a particular measuring
 time) HBO loggers. * Average_OTC_lux, in lux. Average light intensity (in
 lux) in OTC plots. Average of four (or less, if one or more malfunctioned
 at a particular measuring time) HBO loggers. * Warming: Difference (in °C)
 between Average_OTC_T and Average_Ctrl_T. Missing data codes: 848/29406
 NAs in measurements of surface temperature and light in control plots due
 to HOBO logger malfunctioning. Abbreviations used: Ctrl for control, OTC
 for Open Top Chamber, and T for Temperature. Other relevant information:
 None.  * Data-specific information
 for Fig2_data_Salazar_etal_PlantSoil.csv: Number of variables: 6 Number of
 measurements/rows: 24 Variable list: * Year: 2018, 2019, 2020, 2021 and
 2022. * Treatment: Ctrl and OTC. * Cov_type: Cover type: Anthelia
 biocrust, moss and vascular plant * N: sample size, 8. * PercCov_average:
 Averages of percentage cover * PercCov_SE: Standard errors associated to
 percentage cover. Missing data codes: None. Abbreviations used: Ctrl for
 control, OTC for Open Top Chamber, Cov for cover, Perc for percentage, N
 for sample size and SE for standard error. Other relevant information:
 None.  * Data-specific information
 for Fig3_data_Salazar_etal_PlantSoil.csv: Number of variables: 7 Number of
 measurements/rows: 16 Variable list: * Year: 2018-2019 and 2021-2022. *
 Treatment: Ctrl and OTC. * Tea_type: G and R * Depth_cm * N: sample size,
 8. * Perc_mass_loss_average: Averages of percentage mass loss. *
 Perc_mass_loss_SE: Standard errors associated to percentage mass loss.
 Missing data codes: None. Abbreviations used: Ctrl for control, OTC for
 Open Top Chamber, G for green tea (Lipton, EAN: 87 22700 05552 5), R for
 rooibos tea (Lipton, EAN: 87 22700 18843 8), Perc for percentage, N for
 sample size and SE for standard error. Other relevant information: None. 
 * Data-specific information for Fig4_data_Salazar_etal_PlantSoil.csv:
 Number of variables: 11, 6 for RB and 5 for RB diff. Number of
 measurements/rows: 12 for RB, 6 for RB diff Variable list: * Year: 2019,
 2021, 2022 * Month: June, July, August. * Treatment: Ctrl and OTC. * N:
 sample size, 8. * RB_averages: Averages of biocrust respiration rates *
 RB_SE: standard errors of biocrust respiration rates. * N: sample size, 8.
 * Year_diff:  2019, 2021, 2022 * Month_diff: June, July, August. * N_diff:
 sample size, 8. * RB_diff_average: Difference of averages of biocrust
 respiration rates between OTC and Ctrl plots. * RB_diff_SE: standard
 errors associated to RB_diff_average. Missing data codes: None.
 Abbreviations used: Ctrl for control, OTC for Open Top Chamber, RB for
 biocrust respiration, N for sample size and SE for standard error, diff
 for difference (OTC-Ctrl). Other relevant information: None. *
 Data-specific information for FigS4_data_Salazar_etal_PlantSoil.csv:
 Number of variables: 3 Number of measurements/rows: 73 Variable list: *
 Date: from 6/4/2020 12:27:00 AM to 6/7/2020 12:27:00 AM * Temp_diff_C:
 Temperature difference in °C. * Light_lux: Surface light intensity in lux.
 Missing data codes: None. Abbreviations used: Temp for temperature, diff
 for difference (OTC-Ctrl) Other relevant information: Subsection
 of Fig1_S3_data_Salazar_etal_PlantSoil.csv.  * Data-specific
 information for FigS5_data_Salazar_etal_PlantSoil.csv: Number of
 variables: 5 Number of measurements/rows: 8 Variable list: * Year: 2018,
 2019, 2020, 2022. * Treatment: Ctrl and OTC. * N: sample size, 8. *
 Soil_moisture_perc_saturation_average: Average of soil moisture measured
 as percentage of saturation. * Soil_moisture_perc_saturation_SE: Standard
 error associated to Soil_moisture_perc_saturation_average. Missing data
 codes: None. Abbreviations used: Ctrl for control, OTC for Open Top
 Chamber, Perc for percentage, N for sample size and SE for standard error.
 Other relevant information: None * Data-specific information
 for FigS6_data_Salazar_etal_PlantSoil.csv: Number of variables: 5 Number
 of measurements/rows: 2 Variable list: * Year: 2022. * Treatment: Ctrl and
 OTC. * N: sample size, 8. * Litter_cover_average: Average litter cover % *
 Litter_cover_SE: Standard error associated to Litter_cover_average.
 Missing data codes: None. Abbreviations used: Ctrl for control, OTC for
 Open Top Chamber, N for sample size and SE for standard error. Other
 relevant information: None * Data-specific information
 for FigS7_data_Salazar_etal_PlantSoil.csv: Number of variables: 4 Number
 of measurements/rows: 32 Variable list: * Treatment: Ctrl and OTC. * Plot:
 1-8. * Sp: species, Vasc and Moss * No_sp: number of hits per species.
 Missing data codes: None. Abbreviations used: Ctrl for control, OTC for
 Open Top Chamber, Sp for species, Vasc for vascular plants and No for
 number. * Data-specific information
 for FigS8_data_Salazar_etal_PlantSoil.csv: Number of variables: 7 Number
 of measurements/rows: 84 Variable list: * Year: 2018-29 and 2021-2022. *
 Plot: plot number, 1-8. * Depth_cm: Depth in cm, 2 and 8 * Treatment: Ctrl
 and OTC. * Replicate: 1 and 2 * S: Stabilization factor, calculated
 according to the TBI (Tea Bag Index) method. * k: decomposition rates,
 calculated according to the TBI (Tea Bag Index) method. Missing data
 codes: None. Abbreviations used: Ctrl for control, OTC for Open Top
 Chamber, S for stabilization factor, k for decomposition rates. Other
 relevant information: Dataset submitted
 to [http://www.teatime4science.org/](http://www.teatime4science.org/). *
 Data-specific information for FigS9_data_Salazar_etal_PlantSoil.csv:
 Number of variables: 5 Number of measurements/rows: 16 Variable list: *
 Plot: plot number, 1-8. * Treatment: Ctrl and OTC. * time: time of
 measurement on on August 10th, 2022. * date: 8/10/2022 * NDVI: NDVI
 measurements. Missing data codes: None. Abbreviations used: Ctrl for
 control, OTC for Open Top Chamber, NDVI for Normalised Difference
 Vegetation Index. Other relevant information: None. * Data-specific
 information for FigS9_data_Salazar_etal_PlantSoil.csv: Number of
 variables: 2 Number of measurements/rows: 93474 Variable list: *
 Date_time: date and time, from 2011-01-01 01:00:00 UTC to 2021-09-28
 11:00:00 UTC * Air_temp_C: Air temperature in °C. Missing data codes:
 None. Abbreviations used: temp for temperature. Other relevant
 information: Air temperature (2 m height) at the Icelandic Meteorological
 Office (IMO) Vatnsfell station (64°11.736' N, 19°2.802' W), 20
 km northeast from the CRUST experiment. Measurements were taken hourly by
 the IMO, according to the guidelines of the World Meteorological
 Organization (WMO, M., 2021). ## Sharing/Access information The only other
 way to access this data is by contacting the corresponding author. Except
 from the data shown in Figure S10, which is available at the IMO
 website [https://en.vedur.is/, ](https://en.vedur.is/,)all the data was
 collected directly by the authors (i.e. not derived from any source). ##
 Code/Software All statistical work and plotting was conducted in R,
 version 4.1.2 (R Core Team, 2023). Codes available upon request.