Robert G. Björk

The production and turnover of extramatrical mycelium of ectomycorrhizal fungi in forest soils: role in carbon cycling

There is growing evidence of the importance of extramatrical mycelium (EMM) of mycorrhizal fungi in carbon (C) cycling in ecosystems. However, our understanding has until recently been mainly based on laboratory experiments, and knowledge of such basic parameters as variations in mycelial production, standing biomass and turnover as well as the regulatory mechanisms behind such variations in forest soils is limited. Presently, the production of EMM by ectomycorrhizal (EM) fungi has been estimated at ~140 different forest sites to be up to several hundreds of kg per ha per year, but the published data are biased towards Picea abies in Scandinavia. Little is known about the standing biomass and turnover of EMM in other systems, and its influence on the C stored or lost from soils. Here, focussing on ectomycorrhizas, we discuss the factors that regulate the production and turnover of EMM and its role in soil C dynamics, identifying important gaps in this knowledge. C availability seems to be the key factor determining EMM production and possibly its standing biomass in forests but direct effects of mineral nutrient availability on the EMM can be important. There is great uncertainty about the rate of turnover of EMM. There is increasing evidence that residues of EM fungi play a major role in the formation of stable N and C in SOM, which highlights the need to include mycorrhizal effects in models of global soil C stores.

Nurse plant effect of the cushion plant Silene acaulis (L.) Jacq. in an alpine environment in the subarctic Scandes, Sweden

Background: Facilitation plays important roles in the structuring of plant communities and several studies have found that it tends to increase with environmental severity in alpine plant communities. In addition, cushion plants have been shown to act as nurse plants, moderating extreme environmental conditions, and providing resources for other species, with substantial effects on local plant diversity. Aims: This study addresses the nurse plant effects of Silene acaulis – a common, circumpolar alpine plant species with a compact cushion-forming growth form – along an altitude transect in the mid- to high-alpine zones in northern Sweden. Methods: The numbers of species in paired S. acaulis cushions and identical-sized control plots along an altitude transect between 1150 m and 1450 m above sea level were compared, and differences in species composition were analysed. Results: At altitudes above c. 1280 m, but not at lower altitudes, more species were found inside the cushions than in their paired control plots. Species composition was similar inside cushions and in control plots. Conclusions: Our results suggest that S. acaulis acts as a nurse plant at altitudes higher than a certain threshold (c. 1280 m at the investigated site). It appears to play an important role in creating focal points for local vascular plant diversity in high-alpine environments, where vegetation is open and occurs in small patches.

Expansion of deciduous tall shrubs but not evergreen dwarf shrubs inhibited by reindeer in Scandes mountain range

Summary One of the most palpable effects of warming in Arctic ecosystems is shrub expansion above the tree line. However, previous studies have found that reindeer can influence plant community responses to warming and inhibit shrubification of the tundra. We revisited grazed (ambient) and ungrazed study plots (exclosures), at the southern as well as the northern limits of the Swedish alpine region, to study long‐term grazing effects and vegetation changes in response to increasing temperatures between 1995 and 2011, in two vegetation types (shrub heath and mountain birch forest). In the field layer at the shrub heath sites, evergreen dwarf shrubs had increased in cover from 26% to 49% but were unaffected by grazing. Deciduous dwarf and tall shrubs also showed significant, though smaller, increases over time. At the birch forest sites, the increase was similar for evergreen dwarf shrubs (20–48%) but deciduous tall shrubs did not show the same consistent increase over time as in the shrub heath. The cover and height of the shrub layer were significantly greater in exclosures at the shrub heath sites, but no significant treatment effects were found on species richness or diversity. July soil temperatures and growing season thawing degree days (TDD) were higher in exclosures at all but one site, and there was a significant negative correlation between mean shrub layer height and soil TDD at the shrub heath sites. Synthesis. This study shows that shrub expansion is occurring rapidly in the Scandes mountain range, both above and below the tree line. Tall, deciduous shrubs had benefitted significantly from grazing exclosure, both in terms of cover and height, which in turn lowered summer soil temperatures. However, the overriding vegetation shift across our sites was the striking increase in evergreen dwarf shrubs, which were not influenced by grazing. As the effects of an increase in evergreen dwarf shrubs and more recalcitrant plant litter may to some degree counteract some of the effects of an increase in deciduous tall shrubs, herbivore influence on shrub interactions is potentially of great importance for shaping arctic shrub expansion and its associated ecosystem effects.

Can distribution of trees explain variation in nitrous oxide fluxes

Abstract The impact of distance to tree stems on nitrous oxide (N2O) fluxes was examined to determine whether it is possible to improve the accuracy of flux estimates from boreal forest soils. Dark static chambers were placed along transects between pairs of trees within a Norway spruce stand and fluxes of N2O and carbon dioxide (CO2) were measured during the period 1999–2003. The groundwater table was measured on every sampling occasion along the transects. In addition, radiation transmission, potential diffusion rate and biomass of forest floor vegetation were measured once at each chamber site along one of the transects and soil samples were collected at three depths, from which pH, denitrification enzyme activity, soil moisture, organic matter, and carbon and nitrogen content were determined. There was a high level of variation in the N2O fluxes, both spatially and temporally. However, the spatial variation in the N2O fluxes within the transect could not be explained by differences in any of the measured variables. Sometimes, mainly when no major peaks occurred, N2O fluxes were significantly correlated with CO2 release. It is concluded that distance to stems cannot be used to improve the design of sampling schemes or for extrapolating flux levels to larger scales.

Patchy field sampling biases understanding of climate change impacts across the Arctic

Effective societal responses to rapid climate change in the Arctic rely on an accurate representation of region-specific ecosystem properties and processes. However, this is limited by the scarcity and patchy distribution of field measurements. Here, we use a comprehensive, geo-referenced database of primary field measurements in 1,840 published studies across the Arctic to identify statistically significant spatial biases in field sampling and study citation across this globally important region. We find that 31% of all study citations are derived from sites located within 50 km of just two research sites: Toolik Lake in the USA and Abisko in Sweden. Furthermore, relatively colder, more rapidly warming and sparsely vegetated sites are under-sampled and under-recognized in terms of citations, particularly among microbiology-related studies. The poorly sampled and cited areas, mainly in the Canadian high-Arctic archipelago and the Arctic coastline of Russia, constitute a large fraction of the Arctic ice-free land area. Our results suggest that the current pattern of sampling and citation may bias the scientific consensuses that underpin attempts to accurately predict and effectively mitigate climate change in the region. Further work is required to increase both the quality and quantity of sampling, and incorporate existing literature from poorly cited areas to generate a more representative picture of Arctic climate change and its environmental impacts.Analysing a database of >1,800 field studies in the terrestrial Arctic, the authors identify large spatial biases in sampling, with nearly one-third of all citations derived from sites located within 50 km of two research stations.

Complex effects of mammalian grazing on extramatrical mycelial biomass in the Scandes forest-tundra ecotone

Abstract Mycorrhizal associations are widespread in high‐latitude ecosystems and are potentially of great importance for global carbon dynamics. Although large herbivores play a key part in shaping subarctic plant communities, their impact on mycorrhizal dynamics is largely unknown. We measured extramatrical mycelial (EMM) biomass during one growing season in 16‐year‐old herbivore exclosures and unenclosed control plots (ambient), at three mountain birch forests and two shrub heath sites, in the Scandes forest‐tundra ecotone. We also used high‐throughput amplicon sequencing for taxonomic identification to investigate differences in fungal species composition. At the birch forest sites, EMM biomass was significantly higher in exclosures (1.36 ± 0.43 g C/m2) than in ambient conditions (0.66 ± 0.17 g C/m2) and was positively influenced by soil thawing degree‐days. At the shrub heath sites, there was no significant effect on EMM biomass (exclosures: 0.72 ± 0.09 g C/m2; ambient plots: 1.43 ± 0.94). However, EMM biomass was negatively related to Betula nana abundance, which was greater in exclosures, suggesting that grazing affected EMM biomass positively. We found no significant treatment effects on fungal diversity but the most abundant ectomycorrhizal lineage/cortinarius, showed a near‐significant positive effect of herbivore exclusion (p = .08), indicating that herbivory also affects fungal community composition. These results suggest that herbivory can influence fungal biomass in highly context‐dependent ways in subarctic ecosystems. Considering the importance of root‐associated fungi for ecosystem carbon balance, these findings could have far‐reaching implications.

The impact of shrub browsing by mountain hare and reindeer in subarctic Sweden

Background: Climate warming has been causing an increase in tall shrub cover around the Arctic, however, mammalian herbivory has been shown to inhibit shrub expansion. Though the effect of reindeer (Rangifer tarandus) and many other mammals has been widely studied in this context, the role of the mountain hare (Lepus timidus) in subarctic Scandinavia remains unknown. Aims: To quantify browsing from mountain hare and reindeer on tall shrubs in different vegetation types and to investigate differences in shrub preference between the two. Methods: In the summers of 2013 and 2014, we counted signs of browsing by hare and reindeer on tall shrub species in 31 study plots at three alpine locations in the Scandes range, Sweden. Results: Hare browsing was significantly more frequent than that by reindeer in two (dry-mesic heath and dry meadow) out of seven vegetation types studied. Reindeer browsing was significantly higher in the low herb meadow and Långfjället shrub heath. Two shrub species, Betula nana and Salix hastata, were significantly more browsed by hare, while reindeer browsing was significantly higher on S. phylicifolia and S. lapponum. Conclusions: Our results show that mountain hares can cause extensive damage to tall shrubs in the subarctic and may have a stronger impact on shrub communities than previously recognised.