Life on Land: Terrestrial Biomes, Climate, and Soil (Chapter 2, Part 1)

Life on Land: Terrestrial Biomes, Climate, and Soil

Introduction to Terrestrial Biomes

Terrestrial biomes are major divisions of the Earth's land environments, primarily distinguished by their predominant plant types. Understanding biomes is essential for studying how climate and other factors shape the distribution of life on land.

• Biome: A large ecological area on the Earth's surface, with flora and fauna adapting to their environment. Examples include deserts, tropical forests, grasslands, and tundra.

• Natural history: The study of how organisms in a particular area are influenced by factors such as climate, soils, predators, and competition.

• Key factors: Climate, soil type, and interactions among organisms determine the structure and function of biomes.

Large-Scale Patterns of Climatic Variation

Climate is the most important factor in determining the distribution of terrestrial biomes. Large-scale climate variation is primarily due to the uneven heating of the Earth's surface by the sun, which is influenced by the planet's shape and tilt.

• Temperature and water availability: These two factors influence the dominant plant types in a biome.

• Global air circulation: The equator receives the most heat energy, while the poles receive the least. Air flows from areas of high temperature (equator) to low temperature (poles).

• Warm air vs. cold air: Warm air holds more water vapor than cold air, affecting precipitation patterns.

• Solar-driven air circulation: Warm, moist air rises at the equator, cools, condenses, and falls as rain, creating rainforests. Cooler, dry air descends at around 30° N/S latitude, creating deserts.

Example: Rainforests are typically found near the equator, while major deserts are found near 30° N/S latitude.

Global Distribution of Biomes

The distribution of biomes across the globe is closely linked to climate patterns. The following table summarizes the major terrestrial biomes and their global locations:

Biome	Location	Climate Characteristics
Arctic Tundra	High latitudes (near Arctic Circle)	Cold, low precipitation
Boreal Forest	Subarctic regions	Cold, moderate precipitation
Temperate Grassland	Mid-latitudes	Moderate temperature, low precipitation
Tropical Rain Forest	Near equator	Warm, high precipitation
Desert	30° N/S latitude	Hot or cold, very low precipitation
Temperate Forest	Mid-latitudes	Moderate temperature, moderate precipitation
Additional info: Other biomes include Mediterranean shrubland, tropical dry forest, and mountains.

Climate Diagrams

Climate diagrams are useful tools for visualizing the relationship between climate and terrestrial vegetation. They plot temperature and precipitation over time, helping to identify periods of adequate moisture for plant growth.

• Horizontal axis: Time (months of the year)

• Vertical axes: Temperature (°C) and precipitation (mm)

• Interpretation: Plant growth is typically possible when the precipitation line is above the temperature line.

• Conversion: 1 degree Celsius is equivalent to 20 mm precipitation for diagram scaling.

Other Factors That Shape Terrestrial Biomes

While latitude and climate are major determinants of biome distribution, microclimate and soil type also play significant roles.

• Microclimate: Local climate conditions, such as those created by mountains, can influence temperature and precipitation. Higher elevations are colder, and mountains can create rain shadow effects.

• Rain shadow effect: Moisture is higher on the windward side of mountains, while the leeward side is drier.

• Soil type: Soil properties affect water availability and nutrient supply for plants.

Example: The western United States exhibits diverse biomes due to variations in elevation, climate, and soil.

Soil: Structure and Properties

Soil is a mixture of organic and inorganic material, formed by interactions among living organisms, climate, weathering, and parent material. The structure and composition of soil influence plant growth and ecosystem function.

• Soil horizons:

  • O horizon: Freshly fallen organic matter; most superficial layer, becomes more fragmented and decomposed with depth.

  • A horizon: Mixture of mineral materials (clay, silt, sand) and organic matter from the O horizon; also called topsoil.

  • B horizon: Clay, humus, and other materials leached from the A horizon; compositional horizon.

  • C horizon: Weathered parent material; only horizon without plant roots.

• Organic component: Living organisms and dead plant/animal matter form complex food webs.

• Inorganic component: Parent material (mineral/rock) undergoes weathering, breaking down into smaller fragments.

Soil Water Holding Capacity

The ability of soil to retain water depends on the size of its particles and the spaces between them. This property is crucial for plant growth and ecosystem health.

• Large pores: Less water is bound to soil particles, resulting in drier soil.

• Small pores: Water is tightly bound, making it difficult for plants to extract.

• Medium pores: Optimal for water retention and plant uptake.

• Clay soils: Although they have high water capacity, pure clay soils make relatively little water available to plants.

Three-Phase Soil System

Soil consists of three phases: solid (organic and inorganic), water, and air. Its structure and properties result from long-term interactions among parent material, climate, vegetation, topography, and time.

• Solid phase: Mineral and organic matter

• Liquid phase: Water

• Gas phase: Air

Example: Soil organisms form complex food webs, but much remains unknown about their diversity and ecological roles.

Additional info: The study notes have expanded on brief points to provide definitions, examples, and context for General Biology students.