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Many arctic species originated outside the Arctic and some of their physiological responses are similar to those in temperate latitudes. Unique adaptations to the Arctic have rarely been found. The recent influx of other species has, however, broken down reproductive barriers and gene flow has been stimulated. In extreme arctic environments, selection forces driving evolution are mainly of the physical environment and plant interactions are positive. Elsewhere, biotic factors, such as herbivory, are important and plant interactions become negative through competition. Physical selective forces operate in winter and summer. Low winter temperatures rarely affect arctic plants, but snow depth and duration influence species distributions. Deep and persistent snow deforms plants and limits the period of resource acquisition. Cryptogams are common in such snow beds. Little or no snow cover exposes plants to abrasion by wind-blown particles and desiccation. In such fell-field sites, deciduous species and xerophytes, such as evergreen cushion plants, are common. Arctic summers are short and developmental processes are extended beyond one growing season, with perennials predominating. Cushion plants efficiently increase their temperatures above ambient, while evergreen and deciduous ericaceous dwarf shrubs coexist and have complementary strategies for intercepting radiation in a low canopy. "hndra soils are generally infertile and may be disturbed by freezehhaw cycles. Nutrients are conserved by recycling within shoots and between ramets within clones. Vegetative proliferation enhances the survival of young ramets, while physiological integration between ramets enables young ramets to forage across patchy environments. Negative plant-animal relationships are particularly important in the Subarctic. Periodic infestations of moth caterpillars defoliate large areas of mountain birch and stimulate increases in populations of their predators. Periodic population peaks of small rodents graze or kill much vegetation and they may moderate the dynamic structure of plant communities, as the plant species have different abilities to regenerate.

U. 2002. Growth of two peat-forming mosses in subarctic mires: species interactions and effects of simulated climate change. -Oikos 99: 151-160.In patches of co-occurring species in natural plant communities, there is a finely poised balance between species in the ways in which they respond to prevailing moisture and temperature regimes. However, environmental change scenarios, in which temperature, moisture and ultraviolet-B radiation are suggested to increase, may favour one of the species. The imbalance is likely to occur at the levels of interactions between patches of the different species and at the shoot level when neighbouring shoots belong to different species. We increased temperature and UV-B in a two-way factorial experiment and increased water supply independently in two subarctic mire communities dominated by the mosses Sphagnum fuscum and Dicranum elongatum. The effects of simulated increase in UV-B were studied using two separate radiation systems, i.e. a ''square wave'' system and a ''modulated'' system. When precipitation was enhanced, both species showed an increase in growth but this was not sustained beyond 5 mm per day. S. fuscum showed a 50% greater response to enhanced precipitation than did D. elongatum, as would be expected from their habitat preferences. Under ambient temperature, S. fuscum grew 67% faster than D. elongatum and this relative difference in response was maintained after one year under a temperature enhancement. The response by species over the winter period was moderated by their neighbours. S. fuscum growth was enhanced when it grew next to D. elongatum whereas D. elongatum grew better with neighbours of its own species. Increased temperature and UV-B radiation did not affect the interaction between the species. Although a balance was maintained between the two species over the short duration of the experiment, potential was shown for an imbalance to occur over longer periods and particularly if winter warming and precipitation are greater than those in summer. During the peak growing season 20% increased UV-B over ambient had a negative effect on S. fuscum under increased temperature but there were no overall seasonal effects on either species, irrespective of method of UV supplementation.

The widespread bryophyte Hylocomium splendens was sampled in a hierarchical fashion from populations representing four Scandinavian vegetation zones. Allozyme electrophoresis revealed variation at 11 out of 13 screened loci, allowing accurate identification of genotypes. From a total sample of 298 shoots 79 genotypes could be detected, giving the proportion of distinguishable genotypes (PD) of 0.265. The total allelic diversity (HT) based on polymorphic loci was 0.274. The relative differentiation among populations was low (GsT = 0.073), indicating a high level of gene flow. Differences in population structuring occurred between a subarcticalpine site vs. three lowland sites. The subarctic-alpine population had one widespread clone, which appeared to be propagated by dispersal of vegetative fragments. That population also comprised many rare genotypes, often occurring together within 10 x 10 cm patches. The lowland populations had genotypes that were locally common and often dominant within the patches. When identical genotypes were observed in multiple patches within these populations, it was usually statistically highly probable that they had arisen by independent sexual recombinations.

In order to document the natural CO environment of the moss Hylocomium splendens, and ascertain whether or not the moss was adapted to this, and its interactions with other microenvironmental factors, two studies were carried out. Firstly, the seasonal variations of CO concentration, photosynthetically active radiation (PAR), tissue water content and temperature were measured in the natural microenvironment of H. splendens in a subarctic forest during the summer period (July-September). Secondly, the photosynthetic responses of the species to controlled CO concentrations, PAR, temperature, and hydration were measured in the laboratory. CO concentrations around the upper parts of the plant, when PAR was above the compensation point (30 μmol m s), were mostly between 400 and 450 ppm. They occasionally increased up to 1143 ppm for short periods. PAR flux densities below saturating light levels for photosynthesis (100 μmol m s), occurred during 65% (July), 76% (August) and 96% (September) of the hours of the summer period. The temperature optimum of photosynthesis was 20° C: this temperature coincided with PAR above the compensation point during 5%, 6% and 0% of the time in July, August and September, respectively. Optimal hydration of tissues was infrequent. Hence PAR, temperature and water limit CO uptake for most of the growing season. Our data suggest that the higher than normal ambient CO concentration in the immediate environment of the plant counteracts some of the limitations in PAR supply that it experiences in its habitat. This species already experiences concentrations of atmospheric CO predicted to occur over the next 50 years.

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Summary 1.The correlation between climatic variables and past (up to 20 years) growth was studied in seven circumarctic populations of the moss Hylocomium splendens, using retrospective analyses of growth. We hypothesized that relationships between growth and climate would be simpler in an ectohydric moss than in higher plants and that the moss could provide high signal-to-noise ratios of responses to climatic variation. 2. Growth parameters of the moss were strongly correlated with early summer temperatures and with the length of the growing season. Annual segment mass, growth rates and degeneration rates were highest at the mildest subarctic sites and lowest at the high arctic site. In contrast, 'longevity' (age of the oldest segment) increased at the climatically harsher sites. 3. Between-year growth variations at two contrasting sites were significantly correlated with June and July temperatures and, to a lesser extent, with early-season precipitation at one of the sites. 4. The moss currently tolerates a wide range of climates and large interannual variations in temperature and is likely to be at risk from climatic change only at the southern edge of its range. 5. The climate-change component most likely to affect the growth of H. splendens in the Arctic and Subarctic will be a lengthening of the growing season and in increase in early summer temperatures provided that moisture is not limiting. 6. Hylocomium splendens is a suitable species for monitoring climatic change at a circumarctic scale.