Increased biocrust cover and activity in the highlands of Iceland after five growing seasons of experimental warming

doi:10.21203/rs.3.rs-4373194/v1

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Research Article

Increased biocrust cover and activity in the highlands of Iceland after five growing seasons of experimental warming

https://doi.org/10.21203/rs.3.rs-4373194/v1

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Journal Publication

published 21 Aug, 2024

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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.

Biocrust (biological soil crust)

surface cover

stabilization factor

decomposition

biocrust respiration

warming

Biological soil crust, often termed biocrust, is a system composed of soil particles held together by diverse biological communities, including bryophytes, algae, lichens, fungi and bacteria (Belnap et al., 2016; Bowker et al., 2018; Weber et al., 2022). Biocrust surfaces cover ca. 12% of the terrestrial surface, and are projected to decrease under all Representative Concentration Pathways (RCPs) warming scenarios: by ca. -25% in the most optimistic RCP2.6, and by as much as -40% in the severe RCP8.5 (Rodriguez-Caballero, et al., 2018). The obscurity of this surface system, with a thickness of only a few millimeters to centimeters (Weber et al., 2022), belies the magnitude and scope of the ecosystem functions that it provides. Biocrusts fix carbon (C) and nitrogen (N) (Elbert et al., 2012), play a key role in regulating global dust emissions (Rodriguez-Caballero et al., 2022), mediate vertical and horizontal flows of water (Eldridge et al., 2022), can facilitate the establishment and growth of vascular plants (Aradottir and Halldorsson, 2018; Zhang et al., 2016) and are in themselves hotspots of microbial abundance (Yoshitake et al., 2018) and diversity (Steven et al., 2013).

Much of what is known about the ways biocrusts respond to environmental change comes from long-term field experiments, such as those carried out with open top chambers (OTCs; Hollister et al. 2023). These experiments have shown that OTC-simulated warming, which often causes drying (Salazar et al., 2020), can affect both biocrust cover (Ladrón de Guevara et al., 2018; Lafuente et al., 2018; Maestre et al., 2015) and function (García-Palacios et al., 2018; Hu et al., 2020; Ladrón de Guevara et al., 2014; Raggio et al., 2023). Most of this research has been concentrated in mid-latitude drylands (e.g., García‐Palacios et al., 2018; Hu et al., 2020; Ladrón de Guevara et al., 2014). Vast areas at high latitudes covered with biocrust (Pushkareva et al., 2016), including polar deserts (Pushkareva et al., 2018; Steven et al., 2013), and wetter environments in alpine and Arctic regions (Salazar et al., 2022), have in comparison been overlooked for various reasons, including logistic challenges.

Most in situ warming experiments at high northern latitudes have been conducted in ecosystems predominantly covered by non-crusted surfaces, such as mosses (e.g. Racomitrium lanuginosum; Jónsdóttir et al., 2005) or lichens (e.g. Alectoria ochroleuca; Biasi et al., 2008), or by vascular plants (e.g. Betula nana; Jónsdóttir et al., 2005). Only a handful have been conducted involving ecosystems that are at least partially covered with biocrust (Chapin et al., 1991; Rousk et al., 2018; Sullivan et al., 2008; Wahren et al., 2005). In our study, we investigated the potential effects of warming on biogeochemical and ecological properties of a sub-Arctic alpine biocrust characterized by the liverwort Anthelia jurkatzkana (Salazar et al., 2022) —a type of biocrust common in Icelandic highlands with heavy snow cover (Aradottir and Halldorsson, 2018; Arnalds et al., 2015; Ortiz-Rivero et al., 2023) and in other high latitude regions (Bakalin et al., 2022; Belland, 1983; Konstantinova and Savchenko, 2008; Smith, 1917; Talbot et al., 2018).

We hypothesized that warming would increase the growth rate of biocrust-forming organisms, but that ultimately it would decrease biocrust cover by accelerating the succession of mosses and vascular plants. Additionally, unless limited by moisture, we expected warming to increase decomposition and biocrust respiration rates. Overall, we can expect that warming within the ranges projected for the sub-Arctic over the next decades (IPCC, 2023) will transform the extent and function of biocrust-dominated subarctic ecosystems. Our five growing seasons study aims at showing the potential path of such transformations.

2.1. Experimental design and abiotic measurements

In June 2018 we set up the Climate Research Unit at Subarctic Temperatures (CRUST) experiment (Fig. S1b; Salazar et al., 2022) in the highlands of Iceland (64°02'11.7"N 19°13'08.2"W; 590 m.a.s.l.). The site is primarily covered with Anthelia jurkatzkana biocrust, mosses, and vascular plants (e.g. Salix herbacea; 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^− 1) 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. Anthelia jurkatzkana 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).

2.2. Aboveground cover and plant community analysis

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.

2.3. Litter decomposition and stabilization factor

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 year (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/.

2.4. Biocrust respiration

We measured biocrust respiration (R_B) 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 Anthelia jurkatzkana biocrust patches within the OTCs and control plots, and measured CO₂ 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).

2.5. Statistical analysis

We used a combination of statistical approaches: 1) Linear regression (lm 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 (lmer function), selected based on the Bayesian information criterion (BIC), to analyze differences in litter mass loss, decomposition rates, stabilization factors, and R_B. 3) Analysis of variances (aov function) for cover, soil moisture, and NDVI analyses. Surface cover and moisture analyses were further complemented with Tukey Honest Significant Differences (HSD) analyses (TukeyHSD 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 glmulti package (Calcagno and de Mazancourt, 2010). All statistical work was conducted in R, version 4.1.2 (R Core Team, 2023).

3.1. Simulated warming

In addition to an average warming of 0.83 ± 0.01°C, OTCs created heat wave-like warming events of up to + 16°C (Fig. 1c). The highest recorded mean (n = 3) surface temperature in our experiment was 50.2 ± 2.0°C, in the OTC treatment at 15:00 UTC July 19th, 2021 (Fig. 1b). At the same time, surface temperature in control plots was ca. 10°C lower (40.5 ± 0.4°C). Maximum surface temperature in control plots was 41.4 ± 0.5°C (13:00 UTC July 9th, 2021).

Even though OTC-warming increased with surface light (Fig. S3), which is expected given that OTCs operate like greenhouses, surface temperatures in OTCs remained higher than in control plots even at the beginning of the winter, when surface light went down to zero (likely because of snow cover; Fig. 1). Although OTCs warmed the surface in general, we observed an occasional night-time surface cooling (Figs. 1 and S4) that results from the complex energy balance within the chambers (Hollister et al. 2023; Marion et al., 1997). Finally, as expected, the OTCs had a drying effect. Overall, the soil (2–3 cm depth) in OTCs was 9% drier than in the controls (Fig. S5).

3.2. Surface cover

In the summer of 2022, after five growing seasons of OTC-warming, the biocrust, moss, and vascular plant covers were 15% higher, 20% lower and 49% lower in the OTC than in the control plots, respectively (Fig. 2). Vascular plant cover changed substantially between years, both in ambient and OTC-warmed conditions (Table S5-6). Also, in 2022, OTC plots had a higher cover of litter (mostly dead leaves from vascular plants; Fig. S6; Table S7).

The reduction in moss and vascular plant cover within the OTCs after five consecutive years of simulated warming was not, however, accompanied by a reduction in the number of moss and vascular plant species (Fig. S7) or in their “greenness” (NDVI; Fig. S9). In general, the dominant vascular plant species within OTCs and control plots was Salix herbacea (10% cover), and the dominant moss species was Racomitrium ericoides (21% cover). Other common but low abundant (ca. 1%) vascular plant species were Festuca richardsonii, Bistorta vivipara, and Agrostis stolonifera. We also identified some lichen species, such as Stereocaulon sp. and Cladonia sp., but they were in low abundance and therefore were not included in our surface cover analysis.

3.3. Litter mass loss

Overall, mass loss of green and rooibos tea was larger in OTCs than in control plots (Fig. 3 and Table S8). At 2 cm depth, mass loss of green and rooibos tea was 68.2 ± 1.8% and 28.1 ± 0.7% in the control plots, and 71.7 ± 1.4% and 29.6 ± 1.6% in the OTCs, respectively (Fig. 3). Surprisingly, values were similar at 8 cm depth (Fig. 3). Mass loss of green and rooibos tea was greater in 2018–2019 (71.1 ± 1.3% and 30.2 ± 0.8%, respectively) than in 2021–2022 (65 ± 1.7% and 27.1 ± 0.4%, respectively).

We used mass loss data to calculate S and k (Fig. S9) as in Keuskamp et al. (2013). S decreased by 25% with the OTC treatment (0.14 ± 0.03 in comparison to 0.18 ± 0.03 in the control plots) (Fig. S8 and Table S9). In 2018–2019, S was marginally (P < 0.1) higher below the biocrust (0.13 ± 0.02 at 2 cm depth) than in the underlying soil (0.10 ± 0.01 at 8 cm depth; Fig. S8 and Table S9). In 2021–2022, when S was ca. 2 times lower than in 2018–2019, depth did not have a significant effect on S (Table S9). On the other hand, k varied between years and depths, but not with the OTC treatment (Fig. S8 and Table S10). k was ca. 10 times lower in 2021–2022 than in 2018–2019. Also, in 2018–2019 depth had a negative effect on k, meaning that decomposition rates at 8 cm depth were lower than in the topsoil layer (2 cm depth), right below the biocrust (Table S11).

3.4. Biocrust respiration (R_B)

The best statistical model (-72 BIC) identified with glmulti does not include an effect of the OTC treatment on R_B, only differences at the temporal scale (Fig. 4a and Table S13). Since we used different instruments to measure R_B, at least part of the observed differences in R_B between years could be linked to the performance of the gas analyzers. Because of this, and because of the purpose of our research, we included the OTC treatment in the best statistical model (Table S14) and focus our analysis on the overall differences in paired measurements of R_B between control and OTC plots (Fig. 4b). The modified model (Table S14), which had the same performance (-72 BIC) as the original (Table S13), indicated a marginal (P < 0.1) but positive effect of OTCs on R_B (Fig. 4b and Table S14).

On the dry day of June 13th, 2019 (Fig. S4), R_B was − 0.141 ± 0.066 µmol CO₂ m^− 2 s^− 1 lower in the OTCs than in the control plots. The rest of the measuring times, between 2019 and 2022, R_B was larger in the OTCs than in the control plots. The greatest observed difference was on June 30th, 2021, when CO₂ emissions from Anthelia jurkatzkana biocrust patches within OTCs were 0.146 ± 0.116 µmol CO₂ m^− 2 s^− 1 greater than at ambient conditions.

As expected, our warming treatment increased average surface temperature by ca. 1°C, a warming similar to the 1.1°C increase of global surface temperature since 1850 (IPCC, 2023) and to the 0.8°C increase in air temperature in the last decade near our study site (Fig. S10). Additionally, it created summer, heatwave-like peaks in which surface temperatures inside OTCs were > 10°C higher than in controls, a common observation in ITEX sites (Bokhorst et al., 2013; Hollister et al. 2023). Our initial hypothesis of a warming-induced acceleration of ecological succession was based on the expected increase in average surface temperatures. However, our findings suggest that ecosystem responses at the CRUST experiment were driven to a large extent by the simulated summer heat pulses.

Heatwaves, which are becoming more frequent in the high north (Dobricic et al., 2020), can be a major source of stress for tundra plants like Salix polaris (Jónsdóttir et al., 2023) and close relatives (Marchand et al., 2005), possibly including S. herbacea, the dominant vascular plant in our study site. Contrary to our hypothesis of vascular plants growing on biocrust, we observed net decreases of vascular plant cover and increases of biocrust cover. It is important to notice that for the period 2018–2021, when we estimated cover using the quadrat method (Fig. S2), an increase of biocrust cover means an increase of the visible biocrust. It is possible that during this period the observed increase of biocrust cover was because of reduction in mosses and vascular plants, thus exposing more of the existing biocrust. In a transplant experiment in the Colorado Plateau (Antoninka et al., 2022), biocrusts from different sources had higher cover and community stability at the highest and coldest elevation. In this sense, the observed increase of biocrust cover in our experiment between 2018 and 2021 could be due to the resistance and/or resilience of Anthelia jurkatzkana biocrust to environmental change. On the other hand, the point intercept that we applied in 2022 accounts for biocrust covered by leaves. Therefore, our observation in 2022 of higher biocrust cover in OTCs (60.4 ± 2.7%) than in control plots (52.6 ± 3.6%) indicates a net increase in biocrust cover. It is not rare for biocrust be more resistant to environmental stress than vascular plants (e.g., Condon et al., 2024). In our case, this observation could be at least partially explained by the capacity of biocrust organisms to rapidly adjust their metabolic rates to sudden changes in the environment (Chongfeng et al., 2017).

Surface cover responses at the CRUST experiment may still be transient. In a similar experiment in northern Sweden, Alatalo et al., (2018) found that the effects of simulated warming on the relative abundance of bryophytes and lichens (two common components of biocrust) varied between years five and seven. In their experiment, warming reduced the relative abundance of bryophytes, with most of the change in the first five years. Warming also decreased the relative abundance of lichens, but in that case most of the reduction happened in the last two years of the experiment. In our case, too, the effects of OTC on surface cover differed between years and the most conspicuous differences between control and manipulated plots were at the end of the experiment, particularly for the cover of vascular plants. Future analyses at the CRUST experiment will show whether the effects of OTCs on surface cover will stabilize (as in the case of the bryophytes in Alatalo et al., 2018) or will intensify (as in the case of lichen in Alatalo et al., 2018, and as with the cover of vascular plants in our study). Also, in a laboratory experiment we found that as long as water is not limiting warming (between 0 and 25°C) can increase N fixation rates in Anthelia biocrust (Salazar et al., 2022). This suggests that as global temperatures continue rising, biocrusts from the highlands of Iceland and similar cold environments may fix more N₂ into terrestrial ecosystems, which could, in the long-term, create favorable nutritional conditions for the establishment of vascular plants.

In addition to simulating warming, OTCs exclude grazers. Sheep grazing has transformed the landscape of North-Atlantic ecosystems for centuries (Ross et al., 2016). In the communal highland grazing areas of Iceland, where our experiment is located, sheep grazing is one of the main drivers of ecosystem degradation (Arnalds and Barkarson, 2003; Barrio et al., 2018; Marteinsdóttir et al., 2017). At the CRUST site, we have seen higher abundances of palatable species such as Bistorta vivipara in OTCs than in control plots (Gunnlaugsdóttir, 2022), which might be an effect of excluding selective grazing rather than of warming. However, instead of an increase in total vascular plant cover within OTCs that could have been explained in part by herbivory exclusion, in the last two years of measurements we observed substantial decreases of vascular plant cover. Because of this, we believe that the differences between OTC and control plots in our experiment were caused, to a larger extent, by the environmental manipulation (both warming and drying) rather than by the unintentional grazing exclusion.

Warming may also have changed the relative abundances of the organisms forming the biocrust. A 13 yrs manipulation experiment at the Colorado Plateau showed that an in situ warming of + 2–4°C with infrared lamps decreased the cover of mosses by 15% (which is in line with our findings) and differentially affected dark (i.e. late successional) and light (i.e. early successional) biocrusts (Phillips et al., 2022). Overall, in that experiment warming resulted in a stagnation of the system in an early successional state dominated by light-pigmented cyanobacteria such as Microcoleus vaginatus. We know that on a short-term time scale (hours to days), warming can alter the microbial community and function of the Anthelia jurkatzkana biocrust of our study site (Salazar et al., 2022). Similar analyses but at the multiyear scale remain to be done.

Regarding our analysis of biocrust function, we first need to acknowledge one caveat in our k measurements. For logistic reasons, we incubated the tea bags in the field for ca. 1 year, instead of 90 days as recommended by Keuskamp et al. (2013). This means that when we collected the tea bags the hydrolysable or labile fraction (i.e. the sum of non-polar extraction, water soluble and acid soluble fractions) could have been exhausted or almost exhausted. Since k is calculated based on a direct relationship with the hydrolysable fraction of the rooibos tea, it is possible that the observed lack of effect of the OTCs on k in our experiment was an artifact caused by an underestimation of k values. Also, k decreases with incubation time. Therefore, the differences in k values between years could reflect differences in incubation periods. In this sense, k values from 2018–2019, calculated based on a shorter incubation period than in 2021–2022, and which suggest an expected negative relationship between depth and k (similar to Miralles et al., 2020), are more reliable. In warm drylands, experimental warming can increase decomposition rates by degrading biocrust and therefore increasing soil-litter mixing (Chuckran et al., 2020). We did not see any evidence of biocrust degradation, which could at least partially explain why warming did not have any effect on k rates in our experiment.

In the case of S rates, which are calculated based on a constant hydrolysable fraction of green tea, we have high confidence in our estimates for both measuring periods. Moreover, all tea bags, in control and OTC plots, were under the same conditions for the same period of time, and therefore the observed differences in S between treatments are comparable. Although biocrust effects are often restricted to the top layer of the soil and decrease with depth (Miralles et al., 2020), in our study warming decreased litter mass loss and S both at 2 and 8 cm depth, suggesting a possible “cascading effect” of the biological activity in the biocrust down to deeper layers of the soil. Overall, our study suggests that warming within limits projected for the high north in the coming decades could increase mass loss in and under the biocrust from our site and similar ecosystems, but reduce the capacity of such biocrust to stabilize labile carbon into recalcitrant forms.

Our findings of higher respiration rates, as well as higher litter cover and mass losses, within the OTCs than in control plots, may indicate a warming-induced increase in turnover rates of carbon. Warming generally decreases biocrust respiration in warm drylands (García-Palacios et al., 2018). This is because of higher mortality of biocrust-forming organisms (e.g., lichens) and/or because of microbial thermal acclimation, as indicated by reductions in the activity of extracellular enzymes such as β-glucosidase (García-Palacios et al., 2018). However, in wetter conditions, such as wet years in semi-arid grasslands (Escolar et al., 2015) or in biocrust-dominated ecosystems regularly covered with snow (Darrouzet-Nardi et al., 2015; this study), warming can increase biocrust respiration rates and ultimately increase carbon losses. A previous study in our site suggests that surface temperature and respiration are unaffected by OTCs for most part of the snow-free season (Gunnlaugsdóttir, 2022). However, our interannual comparison of daytime (ca. 11:00–13:00 UTC) biocrust respiration in the peak of the summer (June-August) suggests a marginal, positive effect of the OTC-warming. Overall, this is consistent with a recent synthesis of 56 OTC in-situ experiments located at Arctic and alpine tundra sites that reports an average 30% increase in ecosystem respiration with warming (Maes et al., 2024).

At least part of our observations, such as the negative ΔR_B recorded in the dry June of 2019, could have been caused by drying rather than by warming. Mean annual precipitation is projected to increase in Arctic and sub-Arctic regions in the coming decades (IPCC, 2023). However, biocrust-dominated ecosystems such as the one from our experiment will continue experiencing periods of dryness, especially at sites similar to ours where soils have limited water holding capacity and capillary water transfer from sub-surface horizons during droughts is hindered by near-surface coarse tephra layers (i.e. ash; Arnalds, 2015). More data is needed to better understand the ways in which changes in moisture and temperature could affect carbon dynamics in the vast areas of the high north covered with biocrust. Our findings point towards warming increasing carbon emissions from biocrust in the highlands of Iceland, which combined with our observations of higher litter mass loss and lower S values in OTCs than controls, could mean future depletion of soil carbon and possibly other soil nutrients (Ferrenberg et al., 2022).

Warming-induced biocrust development has been reported in many locations of the Arctic and sub-Arctic, often on bare ground after a glacier retreat (e.g., Breen and Levesque, 2006; Yoshitake et al., 2018). In our study, however, the bare ground cover was negligible. What we observed, was an orchestrated response of the different plant-soil components of an understudied type of biocrust habitat. An observation that fills an empty space in the puzzle that is understanding how tundra ecosystems could be further affected by climate change.

Biocrust: Biological soil crust

CRUST: Climate Research Unit at Subarctic Temperatures

IMO: Icelandic Meteorological Office.

ITEX: International Tundra Experiment

k: decomposition rates

NDVI: Normalised Difference Vegetation Index

OTC: Open Top Chamber

TBI: Tea Bag Index

R_B: Biocrust respiration

RCPs: Representative Concentration Pathways

S: Stabilization factor

Acknowledgments

We thank the Icelandic Research Fund for partially funding this work with the grant no. 163336. We thank the staff of the Nordic Master Program in Environmental Changes at Higher Latitudes (EnCHiL), for supporting Eyrún Gyða in her research, including measurements at the CRUST experiment that are included in this article. We thank the Icelandic Meteorological Office and Emmanuel P. Pagneux for their help with meteorological data. Finally, we thank Gideon Asamoah, a fellow of the 2022 cohort of the GRÓ-Land Restoration Training program, for his help with the collection of tea bags in the summer of 2022.

Funding

Much of this work was financed with the 2016 Icelandic Research Fund grant no. 163336.

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.

Author Contributions

Alejandro Salazar, Ingibjörg S. Jónsdóttir and Ólafur Andrésson contributed to the study conception and design. All the authors contributed to material preparation, data collection and/or analysis. The first draft of the manuscript was written by Alejandro Salazar and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Data Availability

Our data has been submitted to the data repository Dryad and is available for review at https://datadryad.org/stash/share/kzARpDSC8MrHjXBspYoqf8pTnmvjbNy2gSul7K4GYNk. (doi:10.5061/dryad.tht76hf6r). This is a temporary link to share during the during the peer-review process. If the manuscript is accepted for publication, the dataset will be available to the general public after the publication of the article.

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Increased biocrust cover and activity in the highlands of Iceland after five growing seasons of experimental warming

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Methods

2.1. Experimental design and abiotic measurements

2.2. Aboveground cover and plant community analysis

2.3. Litter decomposition and stabilization factor

2.4. Biocrust respiration

2.5. Statistical analysis

3. Results

3.1. Simulated warming

3.2. Surface cover

3.3. Litter mass loss

3.4. Biocrust respiration (R_B)

4. Discussion

Abbreviations

Declarations

References

Supplementary Files

Status:

Journal Publication

Version 1

Increased biocrust cover and activity in the highlands of Iceland after five growing seasons of experimental warming

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Methods

2.1. Experimental design and abiotic measurements

2.2. Aboveground cover and plant community analysis

2.3. Litter decomposition and stabilization factor

2.4. Biocrust respiration

2.5. Statistical analysis

3. Results

3.1. Simulated warming

3.2. Surface cover

3.3. Litter mass loss

3.4. Biocrust respiration (RB)

4. Discussion

Abbreviations

Declarations

References

Supplementary Files

Status:

Journal Publication

Version 1

3.4. Biocrust respiration (R_B)