THE NATURAL ENVIRONMENT
Geography 101 Online
The environment and climate of the midlatitudes is much more variable than the tropics. As the Earth orbits the Sun, the seasons alternate back and forth between the northern and southern hemisphere and plants must cope with sometimes extreme seasonal changes in temperature and moisture. In general, temperate (midlatitude) biomes have less species diversity than their tropical counterparts.
While the climate of the tropics is dominated by the ITCZ and subtropical High pressure centers, the midlatitude zones are dominated by the movements of air masses, as described in Chapter 6 -> Air Masses. Also, ocean currents moving in opposite directions on opposite sides of continents and mountain ranges can produce distinctly different climate and vegetation zones between east and west coasts. Look at North America in the map below, for example. The vegetation zones clearly vary more in an east-west direction than in the north-south direction, as was the case in the tropics.
Temperate deciduous forest dominates large tracts of the Eurasian and North American continents. Deciduous means that the trees lose their leaves during the cold, or sometimes dry, season. In colder climates, this helps prevent frost damage as trees "harden" their tissues for the coming winter. Broadleaf trees, like oak, elm, hemlock, maple, and beech, dominate these forests. They cover most of the eastern portions of North America and Asia, as well as most of Europe south of Scandinavia.
The wet western coasts of North and South America and most of New Zealand and Tasmania support temperate evergreen forest. These are dominated by coniferous species, such as redwood and fir, but also containing many broadleaf species. The climate of these areas is fairly stable year-round as onshore winds bring plenty of rain to the windward slopes of coastal mountain ranges and keep temperatures cool in summer and relatively warm in winter. It remains something of a mystery in biogeography why broadleaf forests dominate the eastern part of North America, while at the same latitudes, conifers dominate the west coast.
A transitional biome between grassland and forest, Mediterranean shrub, occurs on the fringes of the Mediterranean Sea and in small patches on the west coasts of North and South America, Western Australia, and the southern tip of Africa. These areas have unusual climates with wet winters and long, dry summers (that's why Europeans head to the Med for their summer vacations). Other places in the world receive most of their rainfall in summer. Mediterranean shrub looks superficially like savanna, being dominated by grasses with a few scattered, gnarled, short trees. Mediterranean shrub has adapted to frequent fires and many species require it to initiate seed germination. (NOTE: Hawai'i is one of the few places on Earth to receive most of its rainfall in the winter months).
Toward the interior of continents in midlatitudes, rainfall becomes less dependable and climate becomes more continental (see Chapter 3 -> Heat). The climate becomes too dry to support continuous forest and grasses dominate. Unlike savannas, few trees grow to interrupt the horizon, giving enormous vistas of sky and grass. Huge swaths of grassland cover the interiors of the Americas and Asia. These areas are called by various names around the world, such as steppe in central Asia, veldt in South Africa, and prairie in North America.
The wetter grasslands produce deep, black soils and include some of the richest agricultural land in the world, called tall grass prairie in North America and pampa in South America. Deeply rooted perennial grasses, such as Big Bluestem, originally covered these areas, growing up to two meters in height. The sod of these grasses was so tough that early American pioneers chopped it into bricks to build temporary cabins, called soddies. Today, most of the tall grass prairie has been plowed under and replaced with crops like corn and soybeans. Only a few small patches have been protected.
In drier areas, short grass prairie dominates. The low growing grasses, such as Buffalo grass, can form a continuous layer, or in drier areas, form tufts separated by patches of bare soil. With relatively low fertility and limited rainfall, the short grass prairies are primarily used for grazing cattle and sheep. Most of the Asian steppe is short grass prairie.
Temperate deserts are the poleward extension of tropical deserts into the interiors of continents. The tropical deserts of North Africa and the Middle East, for example, extend north and east into the interior of Asia. These locations are dry, either because of mid-continental location, such as central Asia, or because they lie in the rainshadow of mountain ranges, such as California's interior deserts.
A cold winter season characterizes temperate deserts. Otherwise, they are similar to the tropical deserts in receiving less than 25 cm (10 inches) of rainfall per year and requiring the same plant strategies for survival, with the additional requirement that vegetation may have to endure freezing temperatures.
Kapiolani Community College Geography