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The response of plants to temperature has gained renewed interest as researchers speculate on the biotic response to climate change. It is of particular interest in the Arctic, due to recent warming trends and anticipated continued warming for the region. This long‐term, multispecies study confirms that changes in temperature affect the functioning of plants in their natural environment. It also demonstrates that the influence of temperature should be considered in the context of natural variability within a given location. The study examined natural temperature gradients, interannual climate variation, and experimental warming at sites near Barrow (71°18′ N, 156°40′ W) and Atqasuk (70°29′ N, 157°25′ W) in northern Alaska, USA. At each of the four sites, 24 plots were experimentally warmed for 5–7 years with small, open‐top chambers, and plant growth and phenology were monitored; an equal number of unmanipulated control plots were monitored. The response of seven traits from 32 plant species occurring in at least one site is reported when there were at least three years of recordings. Plants responded to temperature in 49% of the measured traits of a species in a site. The most common response to warming was earlier phenological development and increased growth and reproductive effort. However, the total response of a species, for all traits examined, was individualistic and varied among sites. In 14% of the documented responses, the plant trait was correlated with thawing degree‐day totals from snowmelt (TDDsm), and temperature was considered the dominant factor. In 35% of the documented responses, the plant trait responded to warming, but the interannual variation in the trait was not correlated with TDDsm and temperature was considered subordinate to other factors. The abundance of temperature responses that were considered subordinate to other factors suggests that prediction of plant response to temperature that does not account for natural variability may overestimate the importance of temperature and lead to unrealistic projections of the rate of vegetation change due to climate warming.

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Aim In the alpine life zone, plant diversity is strongly determined by local topography and microclimate. We assessed the extent to which aspect and its relatedness to temperature affect plant species diversity, and the colonization and disappearance of species on alpine summits on a pan‐European scale. Location Mountain summits in Europe's alpine life zone. Methods Vascular plant species and their percentage cover were recorded in permanent plots in each cardinal direction on 123 summits in 32 regions across Europe. For a subset from 17 regions, resurvey data and 6‐year soil temperature series were available. Differences in temperature sum and Shannon index as well as species richness, colonization and disappearance of species among cardinal directions were analysed using linear mixed‐effects and generalised mixed‐effects models, respectively. Results Temperature sums were higher in east‐ and south‐facing aspects than in the north‐facing ones, while the west‐facing ones were intermediate; differences were smallest in northern Europe. The patterns of temperature sums among aspects were consistent among years. In temperate regions, thermal differences were reflected by plant diversity, whereas this relationship was weaker or absent on Mediterranean and boreal mountains. Colonization of species was positively related to temperature on Mediterranean and temperate mountains, whereas disappearance of species was not related to temperature. Main conclusions Thermal differences caused by solar radiation determine plant species diversity on temperate mountains. Advantages for plants on eastern slopes may result from the combined effects of a longer diurnal period of radiation due to convection cloud effects in the afternoon and the sheltered position against the prevailing westerly winds. In northern Europe, long summer days and low sun angles can even out differences among aspects. On Mediterranean summits, summer drought may limit species numbers on the warmer slopes. Warmer aspects support a higher number of colonization events. Hence, aspect can be a principal determinant of the pace of climate‐induced migration processes.

Warming-induced expansion of trees and shrubs into tundra vegetation will strongly impact Arctic ecosystems. Today, a small subset of the boreal woody flora found during certain Plio-Pleistocene warm periods inhabits Greenland. Whether the twenty-first century warming will induce a re-colonization of a rich woody flora depends on the roles of climate and migration limitations in shaping species ranges. Using potential treeline and climatic niche modelling, we project shifts in areas climatically suitable for tree growth and 56 Greenlandic, North American and European tree and shrub species from the Last Glacial Maximum through the present and into the future. In combination with observed tree plantings, our modelling highlights that a majority of the non-native species find climatically suitable conditions in certain parts of Greenland today, even in areas harbouring no native trees. Analyses of analogous climates indicate that these conditions are widespread outside Greenland, thus increasing the likelihood of woody invasions. Nonetheless, we find a substantial migration lag for Greenland's current and future woody flora. In conclusion, the projected climatic scope for future expansions is strongly limited by dispersal, soil development and other disequilibrium dynamics, with plantings and unintentional seed dispersal by humans having potentially large impacts on spread rates.

Three species of dwarf, prostrate willow (Salix arctica, S. rotundifolia and S. herbacea) were subjected to experimental summer warming in high arctic Canada, arctic Alaska, and subarctic Sweden, respectively, as part of the International Tundra Experiment. Phenological and growth responses of these species were compared for the second season of the experiment. Stigmas became receptive and pollen dispersal occurred significantly earlier for S. rotundifolia and S. herbacea in the ITEX open‐top chambers, but not for S. arctica. Warming had no effect on the timing of seed dispersal, leaf yellowing, or leaf senescence. The length and dry weight of the largest leaves were greater for warmed plants, and was significant for S. rotundifolia. The number of catkins/plot did not differ among species or treatments, but the fruit : flower ratio was reduced in the experimental plots.

Willow (Salix arctica) and sedges (Carex stans and Eriophorum triste) were the dominant plants available as forage for herbivores in the high Arctic of Greenland. Willow leaves were of high quality as forage in early stages, of phenology, but crude protein and digestibility declined markedly by late stages whereas sedges, remained high in forage quality throughout the growing season. Densities of fecal pellets indicated that muskoxen (Ovibos moschatus) made heaviest use of sedge-dominated vegetation types in both winter and summer, although increased use of willow communities was observed in early summer. Hares (Lepus arcticus) favored willow-dominated communities in both winter and summer. Evidence of collared lemming (Dicrostonyx groenlandicus) winter use was mainly in willow-dominated communities where snow had accumulated, whereas in summer they were present in drier habitas dominated by willows, but with greater plant diversity. Analyses of plant tissues in feces indicated that graminoids composed over 60% of the diet of muskoxen in winter and over 40% in summer. Willows were of nearly equal importance in the muskox diet in summer, and forbs, Dryas integrifolia, and moss collectively composed over 20% of the diet in both summer and winter. Grass accounted for nearly 50% of the diet of hares in both summer and winter, with willows, forbs, and moss accounting for most of the remainder. Willows and graminoids dominated the diet of lemmings, with willows being somewhat more important in summer and graminoids in winter. Moss was a noteworthy dietary component of lemmings. Differences in body and digestive-tract morphology among the three mammalian herbivores account for differences in locomotive efficiency, predator avoidance, and foraging efficiency which interact with vegetation quality, density, and patchiness. The resulting patterns of use of the landscape result in minimal overlap in use of forage resources and help to explain the distribution and co-existence of high Arctic herbivores.