Semidesert (also called half desert) is a xerophytic shrub vegetation with a poorly developed herbaceous lower layer. Trees are generally absent. Semidesert shrub vegetation is well developed in subtropical and middle-latitude dry climates having a small annual total rainfall and high summer temperatures. An examples is the sagebrush (Artemisia tridentata) vegetation of the middle and southern Rocky Mountain region and Colorado Plateau. Semidesert shrub vegetation seems recently to have expanded widely into areas of the western United States that were formerly steppe grasslands as a result of overgrazing and trampling by livestock.
On the world map of vegetation, semidesert is included with dry-desert vegetation. The xerophytic shrub vegetation is to be expected along the less arid margins of the deserts and in favorable highland locations within the desert. Pedogenic processes are calcification producing brown soils, and gray desert soils. Salinization is found in poorly drained sites.
Heath is a low, dense layer of shrubs commonly not more than 10 inch (25 cm) in height which occurs in regions of highly equable but cool climate in middle and high latitudes. The shrubs are usually dominated by members of the Heath family (Ericaceae); mosses are also important. A common plant of heath vegetation is heather (Calluna vulgaris). In very cold climates heath includes small shrubby birches and willows.
A cool, marine west coast climate is favorable to development of heath. Well-distributed precipitation and a temperature regime with small annual range are required. The pedogenic process is podzolization and soils are acidic. Examples of heath vegetation are seen in Ireland and other exposed west coasts of the British Isles (moors) and in western and north central Europe.
Cold woodland, the last on our list of formation classes in the savanna biochore, is a form of vegetation limited to very cold sub arctic and tundra climates. Trees are low in height and well spaced apart; a shrub layer may be well developed. The ground cover of lichens and mosses is distinctive. Cold woodland is essentially equivalent to what is widely referred to as taiga, and occurs along the northern fringes of the high latitude, needleleaf forests. Cold woodland is thus transitional into the treeless tundra and arctic heath.
In North America representative trees of the cold woodland are black spruce (Picea mariana) and tamarack (Larix laricina). In northern Scandinavia a scrubby birch (Betula odorata) forms a woodland with a low layer of lichens, such as reindeer mosses (Cladonia rangiferina) (in Siberia larch (Larix ) is the woodland tree. Birch-lichen woodland is also found in the sub arctic regions of northwestern Canada and Alaska.
Cold woodland is dominated by the cold continental climate with its severe winters in which all soil moisture is frozen for many months of the year. A shallow depth of thaw accompanies the short summer when insolation continues throughout much of the day. The pedogenic process is toward gleization in the poorly drained sites; towards podzolization on upland sites.
Cold woodland can be found in the tundra areas along the boundary with needleleaf forest.
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