At the Root of the Wood Wide Web: Self Recognition and Non-Self Incompatibility in Mycorrhizal Networks

Arbuscular mycorrhizal (AM) fungi are mutualistic symbionts living in the roots of 80% of land plant species, and developing extensive, below-ground extraradical hyphae fundamental for the uptake of soil nutrients and their transfer to host plants. Since AM fungi have a wide host range, they are able to colonize and interconnect contiguous plants by means of hyphae extending from one root system to another. Such hyphae may fuse due to the widespread occurrence of anastomoses, whose formation depends on a highly regulated mechanism of self recognition. Here, we examine evidences of self recognition and non-self incompatibility in hyphal networks formed by AM fungi and discuss recent results showing that the root systems of plants belonging to different species, genera and families may be connected by means of anastomosis formation between extraradical mycorrhizal networks, which can create indefinitely large numbers of belowground fungal linkages within plant communities.Key Words: arbuscular mycorrhizal symbiosis, extraradical mycelium, anastomosis, plant interconnectedness, self recognition, non-self incompatibility, mycorrhizal networks     

Common mycorrhizal networks provide a potential pathway for the transfer of hydraulically lifted water between plants

Plant roots may be linked by shared or common mycorrhizal networks (CMNs) that constitute pathways for the transfer of resources among plants. The potential for water transfer by such networks was examined by manipulating CMNs independently of plant roots in order to isolate the role(s) of ectomycorrhizal (EM) and arbuscular mycorrhizal fungal (AMF) networks in the plant water balance during drought (soil water potential -5.9 MPa). Fluorescent tracer dyes and deuterium-enriched water were used to follow the pathways of water transfer from coastal live oak seedlings (Quercus agrifolia Nee; colonized by EM and AMF) conducting hydraulic lift (HL) into the roots of water-stressed seedlings connected only by EM (Q. agrifolia) or AMF networks (Q. agrifolia, Eriogonum fasciculatum Benth., Salvia mellifera Greene, Keckiella antirrhinoides Benth). When connected to donor plants by hyphal linkages, deuterium was detected in the transpiration flux of receiver oak plants, and dye-labelled extraradical hyphae, rhizomorphs, mantles, and Hartig nets were observed in receiver EM oak roots, and in AMF hyphae of Salvia. Hyphal labelling was scarce in Eriogonum and Keckiella since these species are less dependent on AMF. The observed patterns of dye distribution also indicated that only a small percentage of mycorrhizal roots and extraradical hyphae were involved with water transfer among plants. Our results suggest that the movement of water by CMNs is potentially important to plant survival during drought, and that the functional ecophysiological traits of individual mycorrhizal fungi may be a component of this mechanism.     

Use of 15N stable isotope to quantify nitrogen transfer between mycorrhizal plants

Aims Mycorrhizas (fungal roots) play vital roles in plant nutrient acquisition, performance and productivity in terrestrial ecosystems. Arbuscular mycorrhizas (AM) and ectomycorrhizas (EM) are mostly important since soil nutrients, including NH₄⁺, NO₃^? and phosphorus, are translocated from mycorrhizal fungi to plants. Individual species, genera and even families of plants could be interconnected by mycorrhizal mycelia to form common mycorrhizal networks (CMNs). The function of CMNs is to provide pathways for movement or transfer of nutrients from one plant to another. In the past four decades, both ¹⁵N external labeling or enrichment (usually expressed as atom%) and ¹⁵N naturally occurring abundance (δ¹⁵N, ‰) techniques have been employed to trace the direction and magnitude of N transfer between plants, with their own advantages and limitations.     

Increased ectomycorrhizal fungal abundance after long-term fertilization and warming of two arctic tundra ecosystems

Shrub abundance is expected to increase with enhanced temperature and nutrient availability in the Arctic, and associated changes in abundance of ectomycorrhizal (EM) fungi could be a key link between plant responses and longer-term changes in soil organic matter storage. This study quantifies the response in EM fungal abundance to long-term warming and fertilization in two arctic ecosystems with contrasting responses of the EM shrub Betula nana. Ergosterol was used as a biomarker for living fungal biomass in roots and organic soil and ingrowth bags were used to estimate EM mycelial production. We measured 15N and 13C natural abundance to identify the EM-saprotrophic divide in fungal sporocarps and to validate the EM origin of mycelia in the ingrowth bags. Fungal biomass in soil and EM mycelial production increased with fertilization at both tundra sites, and with warming at one site. This was caused partly by increased dominance of EM plants and partly by stimulation of EM mycelial growth. We conclude that cycling of carbon and nitrogen through EM fungi will increase when strongly nutrient-limited arctic ecosystems are exposed to a warmer and more nutrient-rich environment. This has potential consequences for below-ground litter quality and quantity, and for accumulation of organic matter in arctic soils.     

Above‐ and below‐ground plant biomass response to experimental warming in northern Alaska

Question: Does experimental warming, designed to simulate future warming of the Arctic, change the biomass allocation and mycorrhizal infection of tundra plants? Location: High Arctic tundra near Barrow, Alaska, USA (71°18′N 156°40′W). Methods: Above and below ground plant biomass of all species was harvested following 3–4 yr of 1‐2°C of experimental warming. Biomass allocation and arbuscular mycorrhizal infection were also examined in the two dominant species, Salix rotundifolia and Carex aquatilis. Results: Above‐ground biomass of graminoids increased in response to warming but there was no difference in total plant biomass or the ratio of above‐ground to below‐ground biomass for the community as a whole. Carex aquatilis increased above‐ground biomass and proportionally allocated more biomass above ground in response to warming. Salix rotundifolia increased the amount of above‐ and below‐ground biomass allocated per leaf in response to warming. Mycorrhizal infection rates showed no direct response to warming, but total abundance was estimated to have likely increased in response to warming owing to increased root biomass of S. rotundifolia. Conclusions: The community as a whole was resistant to short‐term warming and showed no significant changes in above‐ or below‐ground biomass despite significant increases in above‐ground biomass of graminoids. However, the patterns of biomass allocation for C. aquatilis and S. rotundifolia did change with warming. This suggests that long‐term warming may result in changes in the above‐ground to below‐ground biomass ratio of the community.     

Long‐term experimental manipulation of climate alters the ectomycorrhizal community of Betula nana in Arctic tundra

Climate warming is leading to shrub expansion in Arctic tundra. Shrubs form ectomycorrhizal (ECM) associations with soil fungi that are central to ecosystem carbon balance as determinants of plant community structure and as decomposers of soil organic matter. To assess potential climate change impacts on ECM communities, we analysed fungal internal transcribed spacer sequences from ECM root tips of the dominant tundra shrub Betula nana growing in treatments plots that had received long‐term warming by greenhouses and/or fertilization as part of the Arctic Long‐Term Ecological Research experiment at Toolik Lake Alaska, USA. We demonstrate opposing effects of long‐term warming and fertilization treatments on ECM fungal diversity; with warming increasing and fertilization reducing the diversity of ECM communities. We show that warming leads to a significant increase in high biomass fungi with proteolytic capacity, especially Cortinarius spp., and a reduction of fungi with high affinities for labile N, especially Russula spp. In contrast, fertilization treatments led to relatively small changes in the composition of the ECM community, but increased the abundance of saprotrophs. Our data suggest that warming profoundly alters nutrient cycling in tundra, and may facilitate the expansion of B. nana through the formation of mycorrhizal networks of larger size.     

Isotopic evidence of partial mycoheterotrophy in the Gentianaceae: Bartonia virginica and Obolaria virginica as case studies

? Premise of the study: An estimated 10% of plant species have evolved to steal C from their symbiotic fungal partners (mycoheterotrophy), and while physiological evidence for full and partial mycoheterotrophy is well developed in the Orchidaceae and Ericaceae, it is lacking for the majority of other mycoheterotrophic taxa. The family Gentianaceae not only contains several lineages of achlorophyllous mycoheterotrophs, but also contains species that are putative partially mycoheterotrophic. The North American genera Bartonia and Obolaria (Gentianaceae) are green but have leaves reduced to scales or foliose bracts and so have ambiguous mycoheterotrophic status. ? Methods: We investigated the natural abundance (13)C and (15)N profiles of both genera along with total N and chlorophyll content and investigated mycorrhizal infection using light microscopy. ? Key results: The shoots of B. virginica were significantly more enriched in (15)N than the surrounding vegetation but not in (13)C. In contrast, the shoots of O. virginica are not enriched in (15)N compared to the surrounding vegetation but were significantly enriched in (13)C. Total N concentrations were significantly higher than the surrounding vegetation in B. virginica, while the collaroid roots of both species were infected by arbuscular mycorrhizal fungi. ? Conclusions: This microscopic evidence coupled with the natural abundance stable isotope profiles strongly suggests that both species are partially mycoheterotrophic. However, differences in the root-shoot stable isotopic patterns relative to surrounding vegetation between B. virginica and O. virginica are suggestive of the utilization of different physiological pathways or extent of commitment to mycoheterotrophic C gain.     

The nature of spatial transitions in the Arctic

Aim Describe the spatial and temporal properties of transitions in the Arctic and develop a conceptual understanding of the nature of these spatial transitions in the face of directional environmental change. Location Arctic tundra ecosystems of the North Slope of Alaska and the tundra‐forest region of the Seward Peninsula, Alaska Methods We synthesize information from numerous studies on tundra and treeline ecosystems in an effort to document the spatial changes that occur across four arctic transitions. These transitions are: (i) the transition between High‐Arctic and Low‐Arctic systems, (ii) the transition between moist non‐acidic tundra (MNT) and moist acidic tundra (MAT, also referred to as tussock tundra), (iii) the transition between tussock tundra and shrub tundra, (iv) the transition between tundra and forested systems. By documenting the nature of these spatial transitions, in terms of their environmental controls and vegetation patterns, we develop a conceptual model of temporal dynamics of arctic ecotones in response to environmental change. Results Our observations suggest that each transition is sensitive to a unique combination of controlling factors. The transition between High and Low Arctic is sensitive primarily to climate, whereas the MNT/MAT transition is also controlled by soil parent material, permafrost and hydrology. The tussock/shrub tundra transition appears to be responsive to several factors, including climate, topography and hydrology. Finally, the tundra/forest boundary responds primarily to climate and to climatically associated changes in permafrost. There were also important differences in the demography and distribution of the dominant plant species across the four vegetation transitions. The shrubs that characterize the tussock/shrub transition can achieve dominance potentially within a decade, whereas spruce trees often require several decades to centuries to achieve dominance within tundra, and Sphagnum moss colonization of non‐acidic sites at the MNT/MAT boundary may require centuries to millennia of soil development. Main conclusions We suggest that vegetation will respond most rapidly to climatic change when (i) the vegetation transition correlates more strongly with climate than with other environmental variables, (ii) dominant species exhibit gradual changes in abundance across spatial transitions, and/or (iii) the dominant species have demographic properties that allow rapid increases in abundance following climatic shifts. All three of these properties characterize the transition between tussock tundra and low shrub tundra. It is therefore not surprising that of the four transitions studied this is the one that appears to be responding most rapidly to climatic warming.     

Experimental manipulations of snow-depth: effects on nutrient content of caribou forage

We investigated the potential effects of global climate change on arctic tundra vegetation used as caribou forage. A total of 96 experimental plots was established at six sites on the coastal plain of the Arctic National Wildlife Refuge, Alaska, in 1993 and 1994. We erected snow-fences to increase the amount of snow deposition, and therefore delay the date of the snowmelt on 48 plots (referred to as increased snow/late melting plots). We used black mesh netting on the surface of the snow to increase the rate of melting on 24 plots; the remaining 24 plots served as controls. In July 1994, we collected green leaves from Eriophorum vaginatum, Salix planifolia, and Betula nana and analysed these samples for total carbon and total nitrogen content. Ratios of carbon to nitrogen differed among treatments for all three species. Generally, C:N ratios for B. nana and E. vaginatum on increased snow/late melting plots were lower than on control plots. C:N ratios for S. planifolia on increased snow/late melting plots did not differ from controls, but were lower than on plots which started to melt early. These results may be due to the timing of nitrogen translocation from leaf and stem tissue into storage organs, or due to an increase in available nitrogen input to the system. Further sampling is needed to adequately determine the mechanism responsible for increased nitrogen content of caribou forage in areas with increased amount of snow and delayed snowmelt.     

Rapid, landscape scale responses in riparian tundra vegetation to exclusion of small and large mammalian herbivores

Productive tundra plant communities composed of a variety of fast growing herbaceous and woody plants are likely to attract mammalian herbivores. Such vegetation is likely to respond to different-sized herbivores more rapidly than currently acknowledged from the tundra. Accentuated by currently changing populations of arctic mammals there is a need to understand impacts of different-sized herbivores on the dynamics of productive tundra plant communities. Here we assess the differential effects of ungulate (reindeer) and small rodent herbivores (voles and lemmings) on high productive tundra vegetation. A spatially extensive exclosure experiment was run for three years on river sediment plains along two river catchments in low-arctic Norway. The river catchments were similar in species pools but differed in species abundance composition of both plants and vertebrate herbivores. Biomass of forbs, deciduous shrubs and silica-poor grasses increased by 40–50% in response to release from herbivory, whereas biomass of silica-rich grasses decreased by 50–75%. Hence both additive and compensatory effects of small rodents and reindeer exclusion caused these significant changes in abundance composition of the plant communities. Changes were also rapid, evident after only one growing season, and are among the fastest and strongest ever documented in Arctic vegetation. The rate of changes indicates a tight link between the dynamics of productive tundra vegetation and both small and large herbivores. Responses were however not spatially consistent, being highly different between the catchments. We conclude that despite similar species pools, variation in plant species abundance and herbivore species dynamics give different prerequisites for change.     

大兴安岭典型森林沼泽植物叶片和细根碳氮磷化学计量特征

探究大兴安岭典型森林沼泽不同植物叶片和细根生态化学计量特征,能够为进一步认识高纬度气候敏感生态系统养分利用策略和物质循环过程提供依据。对大兴安岭地区兴安落叶松-苔草、兴安落叶松-笃斯越桔-藓类和兴安落叶松-杜香-泥炭藓3种典型森林沼泽19种优势和亚优势维管植物叶片和细根碳氮磷计量特征(C∶N∶P)进行比较,分析不同森林沼泽类型、植物生长型和菌根类型叶片和细根C∶N∶P差异,通过标准化主轴回归分析叶片与细根C∶N∶P的关系。结果表明: 叶片C∶N∶P在种间水平具有最大的变异(42.5%～84.6%),且叶片和细根种间变异大小均为N∶P>C∶N>C∶P。土壤养分和水分含量较高的兴安落叶松-苔草沼泽叶片与细根C∶N和C∶P值较低,且3种森林沼泽植物叶片和细根N∶P均小于10,受N限制。草本植物叶片C∶P和细根C∶N、C∶P显著低于木本植物。外生菌根和杜鹃花类菌根植物叶片和细根C∶N和C∶P高于丛枝菌根和无菌根植物,且杜鹃花类菌根植物叶片和细根C∶P显著高于外生菌根植物。不同森林沼泽、生长型、菌根类型植物叶片和细根C∶N和C∶P差异明显,而N∶P相对稳定。森林沼泽植物叶片与细根C∶N、C∶P和N∶P呈线性正相关,植物地上与地下部分在生态化学计量特征上存在协同。     

Ericaceous plant–fungus network in a harsh alpine–subalpine environment

In terrestrial ecosystems, plant species and diverse root‐associated fungi form complex networks of host–symbiont associations. Recent studies have revealed that structures of those below‐ground plant–fungus networks differ between arbuscular mycorrhizal and ectomycorrhizal symbioses. Nonetheless, we still remain ignorant of how ericaceous plant species, which dominate arctic and alpine tundra, constitute networks with their root‐associated fungi. Based on a high‐throughput DNA sequencing data set, we characterized the statistical properties of a network involving 16 ericaceous plant species and more than 500 fungal taxa in the alpine–subalpine region of Mt. Tateyama, central Japan. While all the 16 ericaceous species were associated mainly with fungi in the order Helotiales, they varied remarkably in association with fungi in other orders such as Sebacinales, Atheliales, Agaricales, Russulales and Thelephorales. The ericaceous plant–fungus network was characterized by high symbiont/host preferences. Moreover, the network had a characteristic structure called ‘anti‐nestedness’, which has been previously reported in ectomycorrhizal plant–fungus networks. The results lead to the hypothesis that ericaceous plants in harsh environments can host unexpectedly diverse root‐associated fungal taxa, constituting networks whose structures are similar to those of previously reported ectomycorrhizal networks but not to those of arbuscular mycorrhizal ones.     

Changes in the structure and function of northern Alaskan ecosystems when considering variable leaf‐out times across groupings of species in a dynamic vegetation model

The phenology of arctic ecosystems is driven primarily by abiotic forces, with temperature acting as the main determinant of growing season onset and leaf budburst in the spring. However, while the plant species in arctic ecosystems require differing amounts of accumulated heat for leaf‐out, dynamic vegetation models simulated over regional to global scales typically assume some average leaf‐out for all of the species within an ecosystem. Here, we make use of air temperature records and observations of spring leaf phenology collected across dominant groupings of species (dwarf birch shrubs, willow shrubs, other deciduous shrubs, grasses, sedges, and forbs) in arctic and boreal ecosystems in Alaska. We then parameterize a dynamic vegetation model based on these data for four types of tundra ecosystems (heath tundra, shrub tundra, wet sedge tundra, and tussock tundra), as well as ecotonal boreal white spruce forest, and perform model simulations for the years 1970–2100. Over the course of the model simulations, we found changes in ecosystem composition under this new phenology algorithm compared with simulations with the previous phenology algorithm. These changes were the result of the differential timing of leaf‐out, as well as the ability for the groupings of species to compete for nitrogen and light availability. Regionally, there were differences in the trends of the carbon pools and fluxes between the new phenology algorithm and the previous phenology algorithm, although these differences depended on the future climate scenario. These findings indicate the importance of leaf phenology data collection by species and across the various ecosystem types within the highly heterogeneous Arctic landscape, and that dynamic vegetation models should consider variation in leaf‐out by groupings of species within these ecosystems to make more accurate projections of future plant distributions and carbon cycling in Arctic regions.     

Tree seedling performance and below-ground properties in stands of invasive and native tree species

The establishment and subsequent impacts of invasive plant species often involve interactions or feedbacks with the below-ground subsystem. We compared the performance of planted tree seedlings and soil communities in three ectomycorrhizal tree species at Craigieburn, Canterbury, New Zealand – two invasive species (Pseudotsuga menziesii, Douglas-fir; Pinus contorta, lodgepole pine) and one native (Nothofagus solandri var. cliffortioides, mountain beech) – in monodominant stands. We studied mechanisms likely to affect growth and survival, i.e. nutrient competition, facilitation of carbon and nutrient transfer through mycorrhizal networks, and modification of light and soil conditions by canopy trees. Seedlings were planted in plastic tubes filled with local soil, and placed in monospecific stands. Effects of root competition from trees and mycorrhizal connections on seedling performance were tested by root trenching and use of tubes with or without a fine mesh (20 μm), allowing mycorrhizal hyphae (but not roots) to pass through. Survival and growth were highest in stands of Nothofagus and lowest under Pseudotsuga. Surprisingly, root trenching and mesh treatments had no effect on seedling performance, indicating canopy tree species affected seedling performance through reduced light availability and altered soil conditions rather than below-ground suppression from root competition or mycorrhizal facilitation. Seedlings in Pseudotsuga stands had lower mycorrhizal colonisation, likely as a result of the lower light levels. Soil organic matter levels, microbial biomass, and abundance and diversity of microbe-consuming nematodes were all highest under Nothofagus, and nematode community assemblages differed strongly between native and non-native stand types. The negative effects of non-native trees on nematodes relative to Nothofagus are likely due to the lower availability of soil organic matter and microbial biomass in these stands, and therefore lower availability of resources for nematodes. This study shows that established stands of non-native invasive tree species may adversely affect tree seedlings and soil communities through modifications of the microenvironment both above and below ground. As such, invasion and domination of new landscapes by these species is likely to result in fundamental shifts in community- and ecosystem-level properties relative to those under native forest cover.     

Fast assimilate turnover revealed by in situ 13CO2 pulse-labelling in Subarctic tundra

Climatic changes in Arctic regions are likely to have significant impacts on vegetation composition and physiological responses of different plant types, with implications for the regional carbon (C) cycle. Here, we explore differences in allocation and turnover of assimilated C in two Subarctic tundra communities. We used an in situ ¹³C pulse at mid-summer in Swedish Lapland to investigate C allocation and turnover in four contrasting tundra plant communities. We found a high rate of turnover of assimilated C in leaf tissues of Betula nana and graminoid vegetation at the height of the growing season, with a mean residence time of pulse-derived ¹³C of 1.1 and 0.7?days, respectively. One week after the pulse, c. 20 and 15%, respectively, of assimilated label-C remained in leaf biomass, representing most likely allocation to structural biomass. For the perennial leaf tissue of the graminoid communities, a remainder of approximately 5% of the pulse-derived C was still traceable after 1?year, whereas none was detectable in Betula foliage. The results indicate a relatively fast C turnover and small belowground allocation during the active growing season of recent assimilates in graminoid communities, with comparatively slower turnover and greater investment in belowground allocation by B. nana vegetation.     

丛枝菌根网络介导的植物间信号交流研究进展及展望

丛枝菌根(arbuscularmycorrhizal,AM)真菌是一类能够与绝大多数陆地植物形成共生关系的土壤真菌,其根外菌丝可以侵染不同植物根系且可以进行菌丝融合,从而形成丛枝菌根网络(arbuscular mycorrhizal networks, AMNs)。AMNs可以在植物之间转运水分及营养元素如碳(C)、氮(N)、磷(P)等,最近研究表明AMNs还可以在植物遭受环境胁迫时向邻近植物传递防御信号,对周围植物起到“预警”作用。目前,关于环境胁迫条件下AMNs介导的信号物质传递研究仍处于起步阶段,许多问题亟待回答。该文首先回顾了目前有关AMNs介导的信号物质传递研究进展,继而梳理了这一研究领域值得进一步探究的科学问题,包括AMNs在植物间传递防御信号的可能途径及相关机制, AMNs介导的信号传递对菌根共生体系的可能影响,以及AMNs研究中常用的技术及其发展,最后讨论了AMNs介导的信号物质传递在作物保护等方面的可能应用。     

Pathways of tundra encroachment by trees and tall shrubs in the western Brooks Range of Alaska

Climate change is expected to increase woody vegetation abundance in the Arctic, yet the magnitude, spatial pattern and pathways of change remain uncertain. We compared historical orthophotos photos (1952 and 1979) with high-resolution satellite imagery (2015) to examine six decades of change in abundance of white spruce Picea glauca and tall shrubs (Salix spp., Alnus spp.) near the Agashashok River in northwest Alaska. We established ~3000 random points within our ~5500 ha study area for classification into nine cover types. To examine physiographic controls on tree abundance, we fit multinomial log-linear models with predictors derived from a digital elevation model and with arctic tundra, alpine tundra and ‘tree’ as levels of a categorical response variable. Between 1952 and 2015, points classified as arctic and alpine tundra decreased by 31% and 15%, respectively. Meanwhile, tall shrubs increased by 86%, trees mixed with tall shrubs increased by 385% and forest increased by 84%. Tundra with tall shrubs rarely transitioned to forest. The best multinomial model explained 71% of variation in cover and included elevation, slope and an interaction between slope and ‘northness’. Treeline was defined as the elevation where the probability of tree presence equaled that of tundra. Mean treeline elevation in 2015 was 202 m, corresponding with a June–August mean air temperature > 11°C, which is > 4°C warmer than the 6–7°C isotherm associated with global treeline elevations. Our results show dramatic increases in the abundance of trees and tall shrubs, question the universality of air temperature as a predictor of treeline elevation and suggest two mutually exclusive pathways of vegetation change, because tundra that gained tall shrubs rarely transitioned to forest. Conversion of tundra to tall shrubs and forest has important and potentially contrasting implications for carbon cycling, surface energy exchange and wildlife habitat in the Arctic.     

Natural Abundance of <Superscript>15</Superscript>N in Nitrogen-Limited Forests and Tundra Can Estimate Nitrogen Cycling Through Mycorrhizal Fungi: A Review

The hyphae of ectomycorrhizal and ericoid mycorrhizal fungi proliferate in nitrogen (N)-limited forests and tundra where the availability of inorganic N is low; under these conditions the most common fungal species are those capable of protein degradation that can supply their host plants with organic N. Although it is widely understood that these symbiotic fungi supply N to their host plants, the transfer is difficult to quantify in the field. A novel approach uses the natural ¹⁵N:¹⁴N ratios (expressed as δ¹⁵N values) in plants, soils, and mycorrhizal fungi to estimate the fraction of N in symbiotic trees and shrubs that enters through mycorrhizal fungi. This calculation is possible because mycorrhizal fungi discriminate against ¹⁵N when they create compounds for transfer to plants; host plants are depleted in ¹⁵N, whereas mycorrhizal fungi are enriched in ¹⁵N. The amount of carbon (C) supplied to these fungi can be stoichiometrically calculated from the fraction of plant N derived from the symbiosis, the N demand of the plants, the fungal C:N ratio, and the fraction of N retained in the fungi. Up to a third of C allocated belowground, or 20% of net primary production, is used to support ectomycorrhizal fungi. As anthropogenic N inputs increase, the C allocation to fungi decreases and plant δ¹⁵N increases. Careful analyses of δ¹⁵N patterns in systems dominated by ectomycorrhizal and ericoid mycorrhizal symbioses may reveal the ecosystem-scale effects of alterations in the plant–mycorrhizal symbioses caused by shifts in climate and N deposition. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Butterflies (Lepidoptera, Rhopalocera) in the Arctic fauna

The Arctic fauna includes 106 species of diurnal butterflies: Papilionidae (6 species), Pieridae (20), Lycaenidae (18), Nymphalidae (30), Satyridae (27), and Hesperiidae (5). Among them, representatives of the family Nymphalidae predominate as to the features characterizing the biological progress in the Arctic, as well as to the number of the most strongly pronounced arctic forms. The family Satyridae shares the first place with Nymphalidae by the number of species, but differs from the latter in the uneven or local distribution. The family Pieridae demonstrates a wide distribution of polyzonal and boreal species in the tundra zone. The distribution patterns of Lycaenidae are different in the Eurasian and Beringian-American sectors. Species of Papilionidae and Hesperiidae occur only in the southern part of the tundra zone. Each family is characterized by specific distribution in the Arctic subzones and landscapes and by latitudinal trends in its specific ratio in the faunas. There are 30 to 40 arctic species, including arctic proper (euarctic and hemiarctic) and hypoarctic, arctoalpine, arctomontane, and arctoboreal species. The species developing successfully under high-latitude conditions are Boloria chariclea, B. polaris, B. improba, Colias nastes, C. hecla, and Erebia fasciata; the first two species can be considered true euarctic forms. Specific features of the latitudinal and longitudinal distribution of the butterfly species in different parts of the Arctic are discussed.     

Plant species-specific changes in root-inhabiting fungi in a California annual grassland: responses to elevated CO2 and nutrients

Five co-occurring plant species from an annual mediterranean grassland were grown in monoculture for 4 months in pots inside open-top chambers at the Jasper Ridge Biological Preserve (San Mateo County, California). The plants were exposed to elevated atmospheric CO₂ and soil nutrient enrichment in a complete factorial experiment. The response of root-inhabiting non-mycorrhizal and arbuscular mycorrhizal fungi to the altered resource base depended strongly on the plant species. Elevated CO₂ and fertilization altered the ratio of non-mycorrhizal to mycorrhizal fungal colonization for some plant species, but not for others. Percent root infection by non-mycorrhizal fungi increased by over 500% for Linanthus parviflorus in elevated CO₂, but decreased by over 80% for Bromus hordeaceus. By contrast, the mean percent infection by mycorrhizal fungi increased in response to elevated CO₂ for all species, but significantly only for Avena barbata and B. hordeaceus. Percent infection by mycorrhizal fungi increased, decreased, or remained unchanged for different plant hosts in response to fertilization. There was evidence of a strong interaction between the two treatments for some plant species and non-mycorrhizal and mycorrhizal fungi. This study demonstrated plant species- and soil fertility-dependent shifts in below-ground plant resource allocation to different morpho-groups of fungal symbionts. This may have consequences for plant community responses to elevated CO₂ in this California grassland ecosystem. Received: 2 June 1997 / Accepted: 22 August 1997