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Unit 27

THE GLOBAL DISTRIBUTION OF PLANTS

Unit Overview

This unit examines the distribution of plants across the Earth's continents by focusing on biomes. The main sections are as follows:

  • Biomes
  • Principal terrestrial biomes

A biome is the broadest justifiable subdivision of the plant and animal worlds. It is an ecological unit that is present at the sub-continental scale. The location of a biome is controlled by air masses, solar radiation, topography, and the distribution of landmasses and oceans.

The principal terrestrial biomes are tropical rainforest, tropical savanna, desert, temperate grassland, temperate forest, Mediterranean Scrub, northern coniferous forest, and tundra. As a result of its many variations in air masses, solar radiation, topography, a continent such as North America contains many of these biomes.

Unit Objectives

  • To briefly survey the principal terrestrial biomes


Glossary of Key Terms

Biome The broadest justifiable subdivision of the plant and animal world, an assemblage and association of plants and animals that forms a regional ecological unit of subcontinental dimensions.
Desert biome Characterized by sparse, xerophytic vegetation or even the complete absence of plant life.
Mediterranean scrub biome Consists of widely spaced evergreen or deciduous trees and often dense, hard-leaf evergreen scrub; thick waxy leaves are well adapted to the long dry summers, Sometimes referred to as chaparral or maquis.
Northern coniferous forest biome The upper-midlatitude boreal forest (known in Russia as the snowforest or taiga); dominated by dense stands of slender, conebearing, needleleaf trees.
Savanna biome The transitional vegetation of the environment between the tropical rainforest and the subtropical desert; consists of tropical grasslands with widely spaced trees.
Temperate deciduous biome Dominated by broadleaf trees; herbaceous plants are also abundant, especially in spring before the trees grow new leaves.
Temperate evergreen forest biome Dominated by needleleaf trees; especially common along western midlatitude coasts where precipitation is abundant.
Temperate grassland biome Occurs over large midlatitude areas of continental interiors; perennial and sod-forming grasses are dominant.
Tropical rainforest biome Vegetation is dominated by tall, closely spaced evergreen trees; a teeming arena of life that is home to a greater number and diversity of plant and animal species than any other biome.
Tundra biome Microtherm plant assemblage of the coldest environments; dominated by perennial mosses, lichens, and sedges.


Unit Outline

  • Biomes
    • The challenges of mapping plant and animal regions on the global scale
    • Biomes represent the broadest possible spatial units of plant association
    • Terrestrial vs. marine biomes
    • Factors that determine the distribution of biome regions
      • climate
      • topography (see vertical sequencing in Fig. 27.3)
  • Principle terrestrial biomes
    • Tropical rainforest biome
      • contains the greatest number of species
      • dense canopies of tall trees admit minimal light to the forest floor
      • monsoon rainforest subtype
    • Tropical savanna biome
      • transitional environment between tropical rainforest and desert
      • tropical grassland with widely-spaced trees
      • associated with pronounced wet and dry seasons of the Aw climate
      • large herds of grazing animals common, and grass bumings are frequent
    • Desert biome
      • Earth's driest environments (BW climate prevails)
      • sparse xerophytic vegetation where plants exist at all
        • perennials (e.g. cactuses) store water and are mostly dormant
        • ephemerals grow quickly after short seasonal rains and soon die off
      • animal life limited and must also be adapted to extreme aridity
    • Temperate grassland biome
      • common in continental interiors, especially in Eurasia and North America.
      • range from short-grass (steppe) to tall-grass prairies
      • highly susceptible to human influence-and environmental degradation
    • Temperate forest biome
      • major subtypes
        • temperate deciduous forest
        • temperate evergreen forest
      • human influences widespread, especially clearing for agriculture
    • Mediterranean scrub
      • Csa and Csb climates produce hot dry summers and cool dry winters
      • widely-spaced evergreen or deciduous trees
      • trees interspersed with often dense, hard-leaf evergreen scrub with waxy leaves adapted to survive long arid summers
      • chaparral ( coastal California)
      • maquis/macchia (Mediterranean Europe)
    • densely populated, intensively farmed, and environments greatly modified by long-term human activity
  • Northern coniferous forest biome
    • upper-midlatitude boreal forest (taiga)
    • dominated by needleleaf trees well adapted to harsh winters and drought
  • Tundra
    • most continuous biome, occurring throughout the poleward margins of the Northern Hemisphere continents
    • only cold-tolerant plants survive, such as mosses, lichens, and sedges
    • poor drainage due to widespread permafrost in subsoil
    • surprisingly varied fauna, with huge bird and insect populations during the short summer
    • extremely fragile environment from human standpoint


Review Questions

  1. Discuss the challenges for mapping plant and animal assemblages at the global scale, and compare them to those faced by physical geographers who map climates and soils at this level of generalization.
  2. Prepare a brief profile of each terrestrial biome, highlighting its major vegetational features, climatic constraints, and human impacts.