1. Chapter 26
Biomes
Different regions of the earth are known by the dominant vegetation occupying each region. The dominant vegetation is maintained by the combined effect of biotic and abiotic factors. Each such region is called a biome. Similar biomes have similar characteristics whether they are located in North America, South America, Asia, Europe, Africa or Australia.

3. 1. Tundra:
- Occupies 20% of the globe above the Arctic Circle (Arctic Tundra) and also below the Arctic Circle above the treelines on tops of mountains (Alpine Tundra).
- Very cold, drying winds, freezing temperatures throughout most of the year, a short growing season (2-3 months) with long periods of sunlight, precipitation is less than 10" (25 cm) per year.
- Soil is shallow ( cm deep), nutrient-poor largely organic that is waterlogged during growing season. A permafrost layer, cm up to 1 meter deep, prevents water draining. No recycling of nutrients due to the cold and anaerobic conditions of the soil.
- Vegetation: Grasses, sedges, mosses, lichens, and dwarf evergreen species. Plants are usually perennials and often reproduce by bulbs, rhizomes, tubers or root systems that remain below ground. The tundra ecosystem is easily disrupted and does not restore itself quickly when damaged.

7. 2. Taiga (Northern Coniferous or Boreal Forests):
- Covers the globe just below the Arctic Tundra.
- Long and cold winters (-50oC in mid winter) with very short days. Summer (20o -30oC) with very long days (18 h photoperiod).
- Permafrost occurs in over 65% of the taiga. Fires are frequent. Fires melt the permafrost and stimulate plant growth for a few seasons. Soils are acidic and nutrient-poor.
- Precipitation mostly in the summer cm.
- Vegetation: Is dominated by conifer forests which include spruce and pines. To adapt to the taiga, conifers have thick-walled epidermis, sunken stomata and a thick cuticle. Deciduous trees such as birch, poplar willow and alder occur in some wetter areas.
To adapt to poor taiga soils, the alder roots carry nitrogen-fixing actinomycetes

10. 3. Temperate Deciduous Forests:
- Cover large areas in the Northern Hemisphere. In North America, they occur from the Great Lakes south to the Gulf of Mexico and extend from the Mississippi River to the eastern seaboard. These forests also exist in Western Europe and Asia.
- Temperature varies greatly. It falls below 4oC in midwinter and rises above 20oC in summer. Annual precipitation of cm occurs mostly during the summer.
- Deciduous trees are broad-leaved species e.g. maples, oaks and hickories. Conifers exist in areas where soil conditions are favorable. Wild flowers bloom in the spring before the canopy excludes the light to the forest floor.

13. 4. Grassland:
- Located in the heart of the continent. On their margins they are surrounded by forest and desert biomes.
- Annual precipitation varies greatly 25 cm in some parts up to 100 cm in others. Temperatures range from -45oC to 45oC.
- Vegetation is grasses (for grazing cattle) and cereal crops (corn and wheat). A lot of wild flowering plants.

16. 5. Deserts:
- In the Southwest (Sonoran desert).
- Very little annual precipitation less than 12.5 cm annually or in some parts the soil is too porous to retain water.
- Due to low humidity fluctuation in day/night temperatures is very high. Very high light intensity and no or very little water vapor to filter the sun's rays. Plant adaptations;
(i) Crassulacean acid metabolism (CAM) as means of photosynthesis. In CAM plants, organic acids are formed in their chloroplasts at night. In day, carbon dioxide (CO2) is released from the organic acids and used by plants to photosynthesize. This occurs because stomata are closed during the day to reduce transpiration and thus no CO2 enters the plants through stomata.
(ii) Leaves reduced to spines (stems perform photosynthesis) or covered with thick cuticle e.g. Cacti.
(iii) Cacti have widespread shallow roots.
(iv) Special internal structures in stems to store water.

18. 6. Mountain Forest:
- Extends from the Pacific Northwest to the Rocky mountains, the Sierra Nevada and California coast ranges south. Such forests are originally deciduous forests that extended west but due to climate change (reduced rainfall in summer) the mountain forests were formed. Very large Coniferous replaced most of the deciduous trees, however, some maples, birches and oaks still exist at the bottom of the mountains. Annual precipitation in some parts exceeds 250 cm (explains the big size Douglas firs). The large redwood trees of California depend on fog to reduce transpiration and maintain their size. Mountain forests show a clear altitudal zonation of species.
Due to the dry summers and frequent forest fires some trees are adapted to fires. Example: (i) The thick bark of Douglas fir can be burnt but does not transmit much heat to the internal tissues.
(ii) The giant redwoods of the Sierra Nevada can survive after their bark is burnt in a fire. (iii) The cones of some pines do not open to disperse their seeds until they are exposed to fire. (iv) The seeds of some species of pine germinate at a higher rate after they have been exposed to fire.

20. 7. Tropical Rain Forest:
- Around the equator. Such forests occupy about 5% of the earth and half the forests.
- Annual rainfall 200 to 400 cm. There is no dry season.
- Temperatures 25oC to 35oC and the humidity is no less than 80%.
- The number of flora exceeds all the other biomes combined. The vegetation is broadleaf evergreen trees (160 ft high), epiphytes such as orchids (their roots don't touch the ground so they grow on rainwater (containing a few minerals) accumulating in their leaf bases.