Nearctic Rocky Tundra - Code: Ne10B

The feel of the tundra varies markedly through the seasons. It is covered with snow or wind driven snow for most of fall, winter and early spring. In late May through to late June, the landscape is bright green with a low covering of forbes, mosses and grasses. However, in midsummer it is a blend of earthy oranges, olive greens, and browns of the mosses and lichens, along with dying annuals from earlier in the season.  Rocky Tundra has a very low-lying, three-layered ground cover with few bare patches over rock. There are bare patches where only lichens and mosses grow, but over most of the area, where soil has been able to develop, there are shrubs and a few taller grasses and flowers. There are fewer plant species here than in more temperate regions, but because of the large range of microhabitats caused by dramatic landscapes with uneven snow distribution, many microhabitats exist allowing for more biodiversity than otherwise expected.
 
It has sparse vegetation cover, with 20%-50% cover in gullies and snow beds, and less on headlands, ridgelines and steeper scree slopes where it merges with Cryptic Tundra. In the north of these tundras, permafrost thawing is very limited and the vegetation is dominated by lichens such as reindeer lichen (Cladonia rangiferina), mosses such as wooly feather moss (Tomentypnum nitens) and sedges such as Curly Sedge (Carex rupestris), Short-leaved Sedge (Carex misandra) and Spike Sedge (Carex nardina). In other slightly better drained snowbeds, the vegetation is  dominated by Arctic Willow (Salix arctica) and dwarf birch (Betula nana) groves with a canopy between 2cm and 5cm, mixed with plants like White Dryas (Dryas octopetala) Purple Saxifrage (Saxifraga oppositifolia) and on Greenland, Arctic dandelion (Taraxacum arcticum).
 
In the southern regions, a deeper thawing of the permafrost to 50cm occurs, peat formation is possible and vascular plants become more common. The vegetation comprises mainly perennial plants often no higher than 12 in. (30cm) aboveground, especially dwarf shrubs such as Arctic White Heather (Cassiope tetragona) along with Lingonberry (Vaccinium vitis-idaea), Northern Labrador Tea (Rhododendron tomentosum) and Crowberry (Empetrum sp), as well as mosses, lichens and Bigelow’s Sedge (Carex bigelowii). The Rocky is being encroached upon by the shrubby tundra from the south, with many of the herbs and forbs being replaced by shrubs
 
 Though there is a general rule that tundra lay north of the 50f (10 C)  summer isotherm, much of the Rocky Tundra is restricted to areas of permafrost. The growing environment of this habitat is extremely harsh, with winter average highs of 8 to -4 °F(-13C  to -20C), mean annual temperatures of 44 to 30 °F (7 to -1 °C)  and three months of night, three of day, and much of the year in twighlight, growth is limited to around half the year.  These high latitudes  have a cool growth periods, with  average summer high temperatures of between 36 and 43 °F (2 and 6°C). The rainfall between 12in and 24in ( 300mm and 600mm), is very arid, but with such low temperatures, the effective evapotranspiration rates allow for a (seemingly) humid environment where protected from desiccating winds with abundant surface water in early summer. Some vegetation in snow banks only starts growing at the end of July after the snow bank has melted off, and may be snow free for only a month of the year.
 
The weathering in these harsh environments is much more mechanical than chemical, resulting in lithosols with limited humic layers; this high importance of soil parent material in these immature soils means that changes in underlying rock chemistry from acid to alkaline can play important roles in determining the individual plants which grow there, but not as important as in Cryptic Tundras because in these Rocky Tundras, many more nutrients are derived from defecation of birds and mammals which are generally missing from the Cryptic Tundra.
 
During the Pleistocene from 2.5 million years to 11, 800 years ago cold, Ice-Age conditions ranges over most of North America, but it was not the tundra that exists today. These “Mammoth Steppes” were much colder and drier than today's Rocky and Boggy Tundras, with mainly grasses over well-drained soils, and more akin to an extreme version of a Shortgrass Prairie or the Desert Steppe in Asia. During these exceptionally cold and dry periods, the Rocky and Boggy Tundras were more associated with warmer refugia, such as Beringia and estuarine environments.