Background: Arctic lichens and mosses are covered by snow for more than half the year and are generally considered as being dormant for most of this period. However, enhanced frequency of winter warming events due to climate change can cause increased disturbance of their protective subnivean environment. Aim: To further understand cryptogamic responses to midwinter warming we compared the ecophysiological performance of one lichen and one moss species during a simulated warming event. Methods: We measured photosynthesis and dark respiration in samples of the moss Hylocomium splendens and the lichen Peltigera aphthosa removed from under snow, and on natural refreezing after the warming event, which was simulated by using infrared heaters suspended above the ground. Results: The moss exposed to light at +5 °C immediately after removal from their subnivean environment and from warmed plots showed positive net gas exchange within 332 s; the lichen required 1238 s. Photosynthesis and nitrogen fixation rates were equal to that, or higher than, during the preceding growing season. Upon refreezing after the event, moss photosynthesis declined considerably. Conclusions: The moss, and to a lesser extent the lichen, may contribute to subnivean midwinter ecosystem respiration, and both are opportunistic, and can take advantage of warmer winter phases for photosynthesis and growth. This ought to be taken into account in vegetation change projections of cryptogam-rich ecosystems.

Background: Tussock tundra is a main component of the Low Arctic vegetation cover. As it is adapted to underlying permafrost, its marginal sites at its southern distribution limit could be excellent indicators of climate change. There are still some scattered outpost stands of tussock tundra in the subarctic-alpine area of northern Fennoscandia, now showing signs of decline. Aims: The objective was to document changes in community structure of the tussock tundra over a 12-year period in experimentally warmed plots and in non-manipulated controls. In addition, the study included a survey of the present cover of tussock tundra in northern Swedish Lapland and a long-term monitoring of annual flowering intensity in the dominant species, the arctic hare’s-tail cottongrass, Eriophorum vaginatum. Methods: The extent of tussock tundra in the region was assessed in a helicopter survey in 2005, followed by ground truthing in 2006. Climate and permafrost have been monitored at Latnjajaure, northern Swedish Lapland, since 1992. An experimental warming study employing open-top chambers (OTCs) was initiated at Latnjajaure for a number of habitats in 1993€-1995, and all tussock tundra plots were surveyed in 1995 and 2006. Non-manipulated, permanently marked E. vaginatum tussocks were monitored for flowering frequency annually in 1992-2008. Results: The helicopter survey indicated that tussock tundra covers only few km2 in northernmost Sweden. A stand at the lower end of its altitude range was already in an advanced stage of transition into shrub tundra. In the OTCs at Latnjajaure, evergreen boreal dwarf-shrubs (particularly Vaccinium vitis-idaea) increased in above-ground biomass about eightfold between 1995 and 2006, and almost fourfold in the controls. There has been a significant warming trend in the study area of 0.12 °C per year, likely explaining why the control plots have also changed. The annual monitoring of E. vaginatum flowering indicated a tendency for relaxed synchrony of masting episodes in recent years, probably caused by longer growing seasons. Conclusions: Tussock tundra stands at the southern margin of the ecosystem’s range are undergoing rapid changes at present. Increased air temperature and permafrost degradation are likely to be the main drivers of the observed change.

Background: Considerable changes in vegetation structure and distribution are predicted in high latitude ecosystems as a result of amplified climate change. However, some documented plant community changes do not follow model predictions. Aim: We compared the growth of and the responses to climate variation by the thermophilic aspen (Populus tremula) and its sub-Arctic competitor mountain birch (Betula pubescens ssp. czerepanovii) over the past 100 years. Methods: Repeat photography, high-resolution vegetation transects, dendro-ecological analysis, and local climate record archives were used to study changes in vitality and distributional range of the two tree species in response to climate variability. Results: Aspen grew 45% faster and had seven times higher recruitment numbers than birch. However, no aspen stand expansion was observed, most likely because of browsing by moose (Alces alces). Birch, on the other hand, suffered from cyclic outbreaks of the autumnal moth (Epirrita autumnata). One-hundred-year-old birch trees experienced on average 9.0 years of reduced growth due to moth herbivory compared to 1.4 years for aspen. Moreover, these moth outbreaks on birch stimulated recruitment of aspen in birch stands. Conclusions: As the sub-Arctic continues to become warmer, the dynamics between aspen and birch in forest ecosystems will likely depend on the number of vertebrate browsers relative to the number of aspen recruits, while major moth outbreaks on birch may facilitate the spread of aspen by reducing competition. Our results suggest that alternating episodes of apparent species range stabilities (homeostasis) and abrupt non-linear shifts may characterise species migration patterns in this ecosystem.

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.