Summary• The aim was to elucidate the effects of elevated winter temperatures on the dehardening process of mountain birch ( Betula pubescens ssp. czerepanovii ) ecotypes and to evaluate their susceptibility to frost damage under warming climate conditions.• Ecotypes from 60 to 71 ° N latitudes and 20 -750 m altitudes were grown in northern Norway (70 ° N) and subjected to simulation of the photoperiod in southern Norway (60 ° N) by artificial illumination from September onwards. In November, the seedlings were transported to the south (60 ° N) to overwinter at ambient or 4 ° C above ambient temperatures. Frost hardiness and lipid peroxidation were determined during January-April.• The higher winter temperature accelerated dehardening, and there were significant differences between the ecotypes. Among tree individuals of southern origin, the alpine ecotype exhibited the most rapid rate of dehardening, whereas the oceanic type showed the slowest rate. Lipid peroxidation supported the above findings.• Since temperature elevation was unequal for the ecotypes with respect to climatic change, the frost hardiness results were normalized to obtain an equal +4 ° C temperature rise. The risk of frost injury seemed to be lowest in the northernmost ecotypes under a temperature elevation of +4 ° C, obviously due to their adaptation to a wider temperature range.

Data series for bud burst, beginning of flowering and petal fall for 20 species of deciduous trees and conifers at four sites in different regions of southern Norway have been analysed and related to temperature series. On average, the spring phenophases occurred 7 days earlier during the period 1971-2005. The most significant linear trends were observed for the earliest phases. The trends in this period were compared with trends in other periods, the longest one starting in 1927. Those starting in cold decades and ending in 2005 were in most instances statistically significant, whereas hardly any significant trend appeared for series starting in warm decades. This fact showed that the results of trend studies are very sensitive to the choice of starting year. There were significant decadal variations in 40% of the series. The dates of occurrence of the phenophases, varying from the first days of May to the first days of June, correlated with seasonal temperature series, in most cases strongest to mean temperatures for the seasons March-May and April-May. The North Atlantic Oscillation Index (NAOI) for January and February appeared to have some predictive power for the date of occurrence of the recorded phases. The basis for this may be that the oscillations described by the index are of importance for the fulfilment of physiological chilling requirements needed to break bud dormancy. The same genotypes of the trees were grown in region West Norway and in Central Norwegian region; during the period 1965-2005 the trends towards earlier bud burst were more pronounced and steeper at the western site.

In a mature black spruce (Piceamariana (Mill.) B.S.P.) forest near the University of Alaska, Fairbanks, AK, samples of four common moss species, Polytrichumcommune Hedw., Hylocomiumsplendens (Hedw.) B.S.G., Pleuroziumschreberi (Brid.) Mitt., and Sphagnumsubsecundum Nees. were collected at intervals during the 1976 season to determine the diurnal variation in leaf water content and daily water loss as functions of temperature, moisture, radiation, and wind speed. The field measurements were followed by laboratory experiments on intact cores in an open system and on excised shoots in closed cuvettes. In these experiments, water loss rates varied by species and were affected by vapor pressure deficits and wind speed; where vapor pressure deficit is more constant than these other factors, variations in light intensities had less effect on water loss rates. Polytrichumcommune, which translocates water from the soil, avoided moisture stress to a greater extent than the other moss species which were more dependent on water absorption through the leaves. Hylocomiumsplendens was below the water content for compensation for almost 50% of the July measurement period. Observed patterns of the rates of water loss and of the moisture required to reach field capacity are correlated with the moisture status of the mosses in the field. Field measurements of photosynthetically active radiation (PAR) at the moss surface and above the forest canopy showed that the transmission of diffuse radiation through the forest canopy increased with increasing cloudiness. PAR at the moss surface was above the compensation level for photosynthesis with sunny and cloudy conditions. Sunflecks (short periods of direct sunlight), which had an intensity of about 76% of the radiation incident on the forest canopy, occurred on up to 35% of the ground surface and provided a major source of the radiation received. Species varied in their pattern of attenuation of light through the moss canopy. A mixed stand of P. commune and P. schreberi and stands of H. splendens transmit light deeply. Sphagnumsubsecundum, on the other hand, shows rapid attenuation of light high in the canopy. The observed pattern of light attenuation helps explain the vertical position of the green–brown interface and the death of green material in the moss canopy.

Carbon dioxide exchange rates in excised 2-year-old shoot sections of five common moss species were measured by infrared gas analysis in mosses collected from different stands of mature vegetation near Fairbanks, Alaska. The maximum rates of net photosynthesis ranged from 2.65 mg CO gh in Polytrichum commune Hedw. to 0.25 in Spagnum nemoreum Scop. Intermediate values were found in Sphagnum subsecundum Nees., Hylocomium splendens (Hedw.) B.S.G., and Pleurozium schreberi (Brid.) Mitt. Dark respiration rates at 15°C ranged from 0.24 mg CO gh in S. subsecundum to 0.57 mg CO gh in H. splendens. The dark respiration rates were found to increase in periods of growth or restoration of tissue (i.e., after desiccation). There was a strong decrease in the rates of net photosynthesis during the winter and after long periods of desiccation.Due to increasing amounts of young, photosynthetically active tissue there was a gradual increase in the rates of net photosynthesis during the season to maximum values in late August. As an apparent result of constant respiration rates and increasing gross photosynthetic rates, the optimum temperature for photosynthesis at light saturation and field capacity increased during the season in all species except Polytrichum, with a corresponding drop in the compensation light intensities. Sphagnum subsecundum seemed to be the most light-dependent species.Leaf water content was found to be an important limiting factor for photosynthesis in the field. A comparison between sites showed that the maximum rates of net photosynthesis increased with increasing nutrient content in the soil but at the permafrostfree sites photosynthesis was inhibited by frequent moisture stress.

Abstract. The biomass and chemical composition in six dominant field and bottom layer species was followed during four years after a fire in a Pinus sylvestris forest in western Norway. Three fire intensity levels were distinguished: low, medium and high. The overall biomass in the different species was estimated from the biomass per shoot, the shoot densities in pure stands of the investigated species and the percentage cover at medium burned sites. Corresponding measurements were made at control plots outside the burned area. A strong post‐fire increase in the overall biomass of Calluna vulgaris, Polytrichum spp., Deschampsia flexuosa and Pteridium aquilinum was found, against a much slower regrowth of Vaccinium myrtillus and V. vitis‐idaea. The Vaccinium species and Calluna were the dominant species at the control plot. Accumulation of carbohydrates seemed to take place in green and non‐green tissue of Pteridium and Deschampsia. A corresponding accumulation of nitrogen was found in green and non‐green Calluna and in non‐green Vaccinium myrtillus tissue, and of phosphorus in green and non‐green tissue of both Vaccinium species and Calluna as a result of fire. The regrowth of Calluna was mainly from seeds that appeared to be present in the soil before the fire. The regeneration of Pteridium and the two Vaccinium species took place almost exclusively vegetatively from below‐ground rhizomes that had survived the fire. Three years after the fire the overall biomass at the burned site was higher than at the control site, due to improved light and nutrient conditions. However, the biomass is expected to decrease in the future and the species composition will probably change as nutrients are leached out of the soil and Pinus and deciduous trees (Betula pubescens and Salix caprea) regenerate from seeds and roots, leading to increased competition in the field and shrub layer.