The vegetation of the forest-alpine transition in the Grunningsdalen area, Telemark, S. Norway

This study is divided into two parts. The general part contains a review of theories of the nature of vegetation. It is concluded that present evidence points in the direction of species individuality and vegetational continuum as responses to continuous environmental gradients, on a regional, and mostly also on a local scale. Accordingly, a gradient approach to classification of the forest and alpine vegetation of the investigated area was designed as an alternative to traditional phytosociological classification. The importance of the concept of corresponding vegetation types in different regions is stressed. It is argued that four environmental gradients explain the major variation in Southern Norwegian forest and alpine vegetation. In the special part, the vegetation of the Grunningsdalen area is treated. Two gradients, the vertical gradient and the complex-gradient topographic moisture snow cover, are considered sufficient to explain the major variation in forest and alpine vegetation in the area. The vegetation is classified into 16 site-types by division of the gradients into four zones (according to altitude) and four series (according to moisture - snow cover) by means of floristic criteria known to reflect particular ecological conditions. For each of the site-types a description of the vegetation and an extensive comparison with corresponding Fennoscandian vegetation is given. On the basis of results from the present study area regional variation in Southern Norwegian poor vegetation corresponding to the xeric, subxeric, and submesic series, and phytosociological classification in the forestalpine transition are discussed. Various numerical classification and ordination methods are used in an analysis of the floristic composition of the site-types and the autecology of the species. The main phytosociological gradient in the investigated vegetation runs from dry and high altitude to wet and low altitude, most closely approaching the moisture gradient. Diversity relations are discussed. It is strongly emphasized that a hierarchic system is unable to give a consistent classification of a vegetation that must be regarded as a multidimensional network of variation along environmental gradients. Viewed in the light of the results of this study, a gradient approach to classification seems most suitable for a variety of Fennoscandian ecosystems.

Insect Herbivory Strongly Modifies Mountain Birch Volatile Emissions

Insect herbivory is known to augment emissions of biogenic volatile organic compounds (BVOCs). Yet few studies have quantified BVOC responses to insect herbivory in natural populations in pan-Arctic regions. Here, we assess how quantitative and qualitative BVOC emissions change with increasing herbivore feeding intensity in the Subarctic mountain birch (Betula pubescens var pumila (L.)) forest. We conducted three field experiments in which we manipulated the larval density of geometrid moths (Operophtera brumata and Epirrita autumnata), on branches of mountain birch and measured BVOC emissions using the branch enclosure method and gas chromatography-mass spectrometry. Our study showed that herbivory significantly increased BVOC emissions from the branches damaged by larvae. BVOC emissions increased due to insect herbivory at relatively low larvae densities, causing up to 10% of leaf area loss. Insect herbivory also changed the blend composition of BVOCs, with damaged plants producing less intercorrelated BVOC blends than undamaged ones. Our results provide a quantitative understanding of the relationship between the severity of insect herbivore damage and emissions of BVOCs at larvae densities corresponding to background herbivory levels in the Subarctic mountain birch. The results have important and practical implications for modeling induced and constitutive BVOC emissions and their feedbacks to atmospheric chemistry.

Ecosystem change and stability over multiple decades in the Swedish subarctic: complex processes and multiple drivers

The subarctic environment of northernmost Sweden has changed over the past century, particularly elements of climate and cryosphere. This paper presents a unique geo-referenced record of environmental and ecosystem observations from the area since 1913. Abiotic changes have been substantial. Vegetation changes include not only increases in growth and range extension but also counterintuitive decreases, and stability: all three possible responses. Changes in species composition within the major plant communities have ranged between almost no changes to almost a 50 per cent increase in the number of species. Changes in plant species abundance also vary with particularly large increases in trees and shrubs (up to 600%). There has been an increase in abundance of aspen and large changes in other plant communities responding to wetland area increases resulting from permafrost thaw. Populations of herbivores have responded to varying management practices and climate regimes, particularly changing snow conditions. While it is difficult to generalize and scale-up the site-specific changes in ecosystems, this very site-specificity, combined with projections of change, is of immediate relevance to local stakeholders who need to adapt to new opportunities and to respond to challenges. Furthermore, the relatively small area and its unique datasets are a microcosm of the complexity of Arctic landscapes in transition that remains to be documented.

A C-repeat binding factor transcriptional activator (CBF/DREB1) from European bilberry (Vaccinium myrtillus) induces freezing tolerance when expressed in Arabidopsis thaliana

Freezing stress affects all plants from temperate zones to the poles. Global climate change means such freezing events are becoming less predictable. This in turn reduces the ability of plants to predict the approaching low temperatures and cold acclimate. This has consequences for crop yields and distribution of wild plant species. C-repeat binding factors (CBFs) are transcription factors previously shown to play a vital role in the acclimation process of Arabidopsis thaliana, controlling the expression of hundreds of genes whose products are necessary for freezing tolerance. Work in other plant species cements CBFs as key determinants in the trait of freezing tolerance in higher plants. To test the function of CBFs from highly freezing tolerant plants species we cloned and sequenced CBF transcription factors from three Vaccinium species (Vaccinium myrtillus, Vaccinium uliginosum and Vaccinium vitis-idaea) which we collected in the Arctic. We tested the activity of CBF transcription factors from the three Vaccinium species by producing transgenic Arabidopsis lines overexpressing them. Only the Vaccinium myrtillus CBF was able to substantially activate COR (CBF-target) gene expression in the absence of cold. Correspondingly, only the lines expressing the Vaccinium myrtillus CBF were constitutively freezing tolerant. The basis for the differences in potency of the three Vaccinium CBFs was tested by observing cellular localisation and protein levels. All three CBFs were correctly targeted to the nucleus, but Vaccinium uliginosum CBF appeared to be relatively unstable. The reasons for lack of potency for Vaccinium vitis-idaea CBF were not due to stability or targeting, and we speculate that this was due to altered transcription factor function.

Enhanced UV-B and elevated CO(2) impacts sub-arctic shrub berry abundance, quality and seed germination

This study investigated the effects of long-term-enhanced UV-B, and combined UV-B with elevated CO(2) on dwarf shrub berry characteristics in a sub-arctic heath community. Germination of Vaccinium myrtillus was enhanced in seeds produced at elevated UV-B, but seed numbers and berry size were unaffected. Elevated UV-B and CO(2) stimulated the abundance of V. myrtillus berries, whilst UV-B alone stimulated the berry abundance of V. vitis-idaea and Empetrum hermaphroditum. Enhanced UV-B reduced concentrations of several polyphenolics in V. myrtillus berries, whilst elevated CO(2) increased quercetin glycosides in V. myrtillus, and syringetin glycosides and anthocyanins in E. hermaphroditum berries. UV-B × CO(2) interactions were found for total anthocyanins, delphinidin-3-hexoside and peonidin-3-pentosidein in V. myrtillus berries but not E. hermaphroditum. Results suggest positive impacts of UV-B on the germination of V. myrtillus and species-specific impacts of UV-B × elevated CO(2) on berry abundance and quality. The findings have relevance and implications for human and animal consumers plus seed dispersal and seedling establishment.

Ectomycorrhizal roots select distinctive bacterial and ascomycete communities in Swedish subarctic forests

Ectomycorrhizal (ECM) roots represent important niches for interactions with bacteria and ascomycete fungi, since they have a large surface area and receive a direct supply of plant assimilates from their tree hosts. We tested the hypothesis that the roots colonized by specific ECM fungi harbour distinct bacteria/ascomycete communities. Roots were collected from two different locations in a subarctic shrub forest dominated by Betula pubescens. Bacterial and ascomycete communities were analysed by PCR-DGGE and sequencing, in roots colonized by five frequently observed ECM fungi, Leccinum variicolor, Piloderma fallax, Tomentellopsis submollis, Lactarius torminosus and Pseudotomentella tristis. The bacterial communities associated with P. fallax- or P. tristis-colonized roots were distinct from those associated with roots colonized by three other ECM fungi at both sampling locations. Bacterial communities associated with T. submollis-, L. torminosus- and L. variicolor-colonized roots were more similar to each other. Lactarius- and Pseudotomentella-colonized roots hosted distinct ascomycete communities at one site while only the community associated with Lactarius was distinct at the second location. The results thus suggest that while the community structure of bacteria colonizing ECM roots can be influenced by the local soil environment, there can also be a strong selective effect of particular fungal symbionts.

Contrasting strategies for UV-B screening in sub-Arctic dwarf shrubs

The content and distribution of UV-absorbing phenolic compounds was investigated in leaves of three species of Vaccinium co-existing at a site in north Sweden. Vaccinium myrtillus L., Vaccinium vitis-idaea L., and Vaccinium uliginosum L. exhibit markedly different strategies, in terms of localization and content of leaf phenolics and in their responses to UV-B enhancement. Plants were exposed to either ambient radiation or to enhancement of UV-B corresponding to 15% (clear sky) depletion of stratospheric ozone for approximately 10 years prior to commencement of this study. Vaccinium myrtillus contained the highest concentration of methanol-extractable UV-B-absorbing compounds, which was elevated in plants exposed to enhanced UV-B. Fluorescence and confocal laser scanning microscopy showed that these compounds were distributed throughout the leaf, and were particularly concentrated in chlorophyll-containing cells. In V. vitis-idaea, most phenolic compounds were cell wall-bound and concentrated in the walls of the epidermis; this pool increased in response to UV-B enhancement. It is suggested that these two plants represent extreme forms of two divergent strategies for UV-B screening, the different responses possibly being related to leaf longevity in the two species. The response of V. uliginosum was intermediate between the other two, with high concentrations of cell wall-bound phenolics in the epidermis but with this pool decreasing, and the methanol-soluble pool tending to increase, after exposure to enhanced UV-B. One explanation for this response is that this plant is deciduous, like V. myrtillus, but has leaves that are structurally similar to those of V. vitis-idaea.

Arctic microorganisms respond more to elevated UV-B radiation than CO2

Surface ultraviolet-B radiation and atmospheric CO2 concentrations have increased as a result of ozone depletion and burning of fossil fuels. The effects are likely to be most apparent in polar regions where ozone holes have developed and ecosystems are particularly sensitive to disturbance. Polar plant communities are dependent on nutrient cycling by soil microorganisms, which represent a significant and highly labile portion of soil carbon (C) and nitrogen (N). It was thought that the soil microbial biomass was unlikely to be affected by exposure of their associated plant communities to increased UV-B. In contrast, increasing atmospheric CO2 concentrations were thought to have a strong effect as a result of greater below-ground C allocation. In addition, there is a growing belief that ozone depletion is of only minor environmental concern because the impacts of UV-B radiation on plant communities are often very subtle. Here we show that 5 years of exposure of a subarctic heath to enhanced UV-B radiation both alone and in combination with elevated CO2 resulted in significant changes in the C:N ratio and in the bacterial community structure of the soil microbial biomass.

Shoot biomass, δ13C, nitrogen and chlorophyll responses of two arctic dwarf shrubs to in situ shading, nutrient application and warming simulating climatic change

As climatic change might induce ecophysiological changes in plants which affect their long-term performance, we investigated responses in above-ground biomass, δ¹³C, nitrogen and chlorophyll of two evergreen arctic dwarf shrubs, Cassiope tetragona and Empetrum hermaphroditum, to 5 (biomass, N) or 6 years of shading, nutrient application and air/soil warming at a dwarf shrub dominated tree-line heath (450 m a.s.l) and a high altitude fellfield (1100 m a.s.l.) in Swedish Lapland. Warming enhanced the green biomass (equivalent to the last 3-4 years of leaf production) and the ratio of green to brown biomass of C. tetragona at the fellfield, and diluted the shoot N concentration. Fertilizer application led to higher shoot N concentration and larger green-to-brown biomass ratio at both sites, and fertilizer application and warming generally had an additive effect on the green biomass. We conclude that both warming and increased soil nutrient availability stimulated the growth of C. tetragona at the fellfield whereas at the heath there was a clear increase in production only if enhanced temperature was combined with nutrient application. Across treatments C. tetragona at the fellfield had 0.6‰ higher δ¹³C and 1.4 mg g^-1 more leaf N, and the soil organic matter δ¹³C was 1.0‰ higher at the fellfield than at the heath. However, an increase in shoot N concentration with altitude does not necessarily lead to higher δ¹³C as no differences in δ¹³C were observed when leaf N of the two dwarf shrubs was increased by fertilizer application c. tetragona in non-warmed plots had higher δ¹³C values than those from warmed plots at the same altitude, which provides the first in situ experimental validation of the theory that temperature partly is responsible for altitudinal trends in plant carbon isotope discrimination. Increased biomass and chlorophyll concentration of C. tetragona in warmed plots points to increased assimilation, at least at the fellfield. As the δ¹³C-based and, therefore, time-integrated estimate of the ratio of CO₂ concentration in the leaf intercellular spaces to that in the atmosphere (C _i/C _a) also increased, warming probably enhanced the stomatal conductance relatively more than the C assimilation, which may be harmful if climatic change leads to reduced soil moisture content and increased plant competition for water. At both sites C. tetragona and E. hermaphroditum responded to shade by increasing the concentration of shoot N and photosynthetic pigments whereas biomass production (and therefore also net photosynthesis) did not decline. Shade was accompanied by a 0.6-1.3‰ (E. hermaphroditum) or 1.2-2.2‰ (C. tetragona) decrease in δ¹³C. This could be due to enhanced stomatal conductance with shading, and perhaps to shade reducing the ericoid mycorrhizal uptake of soil organic C, a factor which has been overlooked as an influence on plant δ¹³C.

Growth responses of Polytrichum commune and Hylocomium splendens to simulated environmental change in the sub-arctic

Total moss cover and the growth responses of the two dominant bryophytes within a sub-arctic heath ecosystem, Polytrichum commune Hedw, and Hylocomium splendens (Hedw.) Br. Eur., were investigated in response to simulated environmental change. Temperature, water supply and nutrients (NPK fertilizer) were increased in a factorial field study in Swedish Lapland. After 3 yr of treatment, total moss caver on fertilized plots was found to be less than 50% of that on unfertilized plots. H. splendens made a smaller contribution to total cover, and also had fewer shoots on the fertilizer treated plots. The relative contribution of P. commune to the bryophyte community was greater on the fertilized plots, although total shoot numbers of P. commune were not greater. There was, however, a larger number of dead shoots of P. commune on the fertilized plots. Total moss cover, relative contribution to moss cover by P. commune and H. splendens, and the numbers of shoots of these two species, were unresponsive to both the temperature and water treatments. The growth of the current year's segment of H. splendens was retarded by both the fertilizer and temperature treatments. Furthermore, the dry weight of the physiologically active shoot was found to be significantly altered by all three treatments. The current year's growth of P. commune showed increases in d. wt and morphological parameters in response to the fertilizer treatment. There were also interactive effects of both the fertilizer and temperature treatments with the water treatment on the current year's stem length increment.

Root mechanical properties related to disturbed and stressed habitats in the Arctic

Various mechanical and architectural properties of roots were measured on plants characteristic of different levels of soil disturbance associated with frost-heave cycles on sorted polygons in Swedish Lapland: one of these measures, resilience, has not, apparently, been recorded previously in literature. Some roots were sampled from species which occurred on both disturbed and stable soils. Root length, root angle, numbers of roots and root diameter varied greatly between species but the degree of branching of roots was generally low. Among all species, and particularly forbs and graminoids which are the most common life forms on polygons, there was a clear trend of decreasing root diameter with increasing soil disturbance, while the species growing on both disturbed and stable ground showed no clear trend associated with soil disturbance. The small root diameter of species growing on the polygons could result from the different species and life forms found there together with a younger age class distribution of plants, younger plants having smaller roots. Only one third of the species growing on the most disturbed soils had rhizomes, whereas twice as many species possessed rhizomes in areas of lower disturbance. The tensile strength, breaking strain (a measure of how much a root is deformed by stretching), breaking stress (a measure of root strength corrected for the cross-sectional area) and the resilience (a measure of the elastic-recovery of stretched roots) of the roots varied highly significantly among the species. The tensile strength varied 6.5-fold, the breaking strain varied by up to almost 8-fold and the breaking stress 13-fold. The modulus of resilience varied least, by a factor of only two. There was no clear trend in the way mechanical properties of the roots of the three species occurring on both stable and frost-heaved soils varied between the two regimes. However, among all species, those from the frost-heaved soils tended to have the weakest roots, because of their small diameter, but the strongest roots per cross-sectional area (i.e. they had the greatest breaking stress). Among life-forms, graminoids toler̀Gated a significantly greater strain than forbs and dwarf shrubs and much higher stress before they broke. Dwarf shrubs and forbs had significantly higher resilience than graminoids and the highest tensile strengths. Forbs showed the lowest values for all of the measured variables except for resilience, which fell between the values for graminoids and dwarf shrubs. There were no significant differences in any of the mechanical measures of root resistance among species of different frost-heaving regimes, indicating that there are few architectural or mechanical properties which enable roots to survive in the most disturbed soil. Indeed, the absence of a complete plant canopy and an increase in abundance of young plants on the polygon area compared with the stable areas suggest that if there are any adaptations to strong disturbance, they are less than adequate and the hypothesis that arctic plants avoid, rather than adapt, to severe environment is supported.