Botanists define the Arctic as being above the polar treeline. Along this line, the mean July temperature is around 10°C: somewhat higher on continents and lower in oceanic areas. The treeline criterion is applicable for northern Fennoscandia and Iceland, even if the forests there are only remnants of what was present before the Viking settlement. It is not applicable to the Faroe Islands that have not had any known Holocene forests. While the treeline follows a fairly uniform line across Eurasia, in North America it varies from above 68° N in Alaska to approximately 54° N in southern Hudson Bay, Canada. The position is influenced by ocean currents and prevailing winds. Where the Gulf Stream reaches Scandinavia above 70° N, mountain tops have an alpine tundra that has some
plant species that occur predominantly further north, but many species are reaching their northernmost limit. This area is not considered Arctic tundra (see Arctic: Definitions and Boundaries).
Within the Arctic, the flora has been divided into five zonal units that reflect a July mean temperature range of about 2°C. The coldest zone has a desertlike appearance with less than 5% ground cover composed of herbs like Saxifraga, Draba, Cerastium, and Papaver. This zone is absent from a broad area around Bering Strait. The next zone, coming south, has a typical component of prostrate shrubs, like Dryas and Salix. The plant cover is discontinuous, but not desertlike. Peat accumulation occurs, particularly in mires with Carex and Eriophorum. The third zone is dominated by the dwarf-shrub heather, Cassiope tetragona, which occurs widely around the pole, but does not grow in Novaya Zemlya, on alkaline, fine-textured soils, and in small oceanic areas. The vegetation cover is closed and minerotrophic fens often cover large areas. Fireweed, Epilobium latifolium, communities are present along most rivers. In the next zone, dwarf-shrubs still dominate, but heather is replaced by species of birch (Betula), Empetrum, Salix, and Vaccinium, some of which only grow on acidic substrates. In the amphi-Beingian area, tussock tundra dominated by the cottongrass, Eriophorum vaginatum, dominates. The zone closest to the boreal forest features shrubs (taller than 0.5 m) in the genera Salix, Alnus, and Betula and a podzol soil profile is often developed. Tall herbs dominate in snow-bed vegetation and the moss Sphagnum produces oligotrophic bogs.
Proximity to the Arctic ocean also influences vegetation because the land warms up as the longest day approaches, but the ocean often remains ice-covered until early August when much of the summer plant growth has occurred. While the Canadian Arctic Archipelago reaches from 62° to 82° N, the average maximum and minimum temperature through that range of latitudes is uniform for most of the area. Many places below the treeline in continental North America record colder temperatures than does the far north Eureka weather station at 82° N on Ellesmere Island.
The distribution of plants is also influenced by topography, precipitation, soil type, and soil pH. Areas of the Arctic have less than 250 mm annual precipitation and this contributes to the desertlike appearance of the coldest zone. In these areas, groundwater seeping up from melting permafrost is available for plant growth in the summer, so that the area is not a desert in the sense that water is limiting growth. One site on eastern Ellesmere Island at 80° N has sufficient moisture and soil to have more than 140 species, a number similar to that recorded on Baffin Island at 62° N. Many of the species at the two sites are the same. The differences in species composition are thought to reflect differences in the plants that have been able to reach the areas since the last glaciation. The Russian Wrangel Island has many unique and relict species, and this is thought to have resulted in part from its isolation from the mainland in recent times and its position as a refugium during marine transgressions in the Pleistocene.
Woody species in the High Arctic are shrubs that form prostrate mats. These are Mountain avens, Dryas integrifolia, other Dryas species common in Eurasia, and the Arctic willow. The western Canadian Arctic Islands along the vast expanse of Arctic Ocean are among the coldest areas in the Arctic, with a July mean temperature below 4°C at 75° N, and in this area there is a mini-treeline where the woody shrubs cease to grow.
The circumpolar Arctic land vegetation has 1600-2000 species of flowering, or vascular, plants in 95 families. There are no Arctic palms, and gym-nosperms, if present, occur in a taiga zone at the tree-line. The numbers of mosses, liverworts, and lichens relative to the numbers of vascular plants are high. There are ferns (19+) and their relatives, horsetails (7), clubmosses (10), and quillworts (2). Among the flowering, about one-quarter are monocotyledons, mainly grasses (approximately 200 species in many species aggregates), sedges (160), rushes (15-17), and reeds (15-17), with approximately 75 species in 17 smaller monocotyledon families, including around 20 orchids. Among the dicotyledons, the major families are those containing carnations (Caryophyllaceae), daisies
(Asteraceae), heaths (Ericaceae), legumes (Fabaceae), mustards (Brassicaceae), roses (Rosaceae), saxifragas (Saxifragaceae), and snapdragons (Scrophulariaceae). There are many Arctic willows (Salicaceae) that occur near the treeline, but only Salix arctica occurs at high latitudes. Many smaller families have fewer than five species.
Some of the uncertainty in the numbers of species to be recognized in the tundra flora reflects differences among botanists in different Arctic countries in the concept of what is a plant species. Other uncertainties result from many species that are in the process of spe-ciating, that is, the process of developing as distinct species, and these are sometimes better regarded as species aggregates. Speciation may result from founder effects where a limited number of individual plants of a species reach an island and, cut off from the main source of genetic material, slowly evolve in isolation.
Another complication results from the sex life of many Arctic plants that may self-fertilize, either as a fallback to attempt to set seed if crosspollination does not occur normally or from choice (cleiostogamy or agamospermy). This can result in large populations of almost identical plants that are not interbreeding to set seed, but sometimes have a unique characteristic that cause them to be regarded as distinct species by some botanists. By contrast, other species successfully hybridize and result in viable hybrids that are unique new species. Recent DNA evidence suggests that successful hybridization may have occurred long ago, but may also be occurring currently. Many plants have high chromosome numbers, some of which are a result of the simple doubling of lower numbers, while others represent the coming together of two or more species to produce a new species, either in the distant past or in the more recent present.
The floristic integrity of the Arctic is very high, even at the species level. The circumpolar species account for 30-80 of local Arctic floras and less than 10% are endemic, or naturally occurring only in the Arctic. Grasses are successful because many of them are pioneering species that are able to colonize disturbed sites, near shorelines of oceans or lakes and rivers, or on slumping, frost-heaved soils. Grasses respond vigorously to additional nitrogen at sites around present or past communities, animal burrows, and bird cliffs. Some species are aquatic and grow in standing water. While grasses are able to colonize many different habitats, they form less of the total plant biomass than do the sedges. Several sedge species grow well in wet or soggy ground, forming meadows that are a major source of food for muskox-en and caribou or reindeer. The heaths and rose families have woody members that grow close to the ground in dry and barren tundra.
Since 1997, major efforts have been made to increase understanding among botanists and resolve differences in botanic tradition toward publishing a Checklist of the Panarctic Flora to reflect a consensus of scientific names to be used and document species where differences of opinion exist.
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