6.3 Biomes and vegetation distribution

Biomes and Climate Zones

Biomes are large-scale ecosystems defined by their dominant plant communities and the climate conditions that support them. Temperature and precipitation are the two biggest drivers of where biomes occur, which is why biome maps and climate zone maps look so similar. Understanding this climate-vegetation link is central to climatology.

Defining Biomes and Climate Relationships

A biome is a major ecological community type that extends over a large geographic area and is shaped by long-term climate averages. The two most important climate variables are temperature and precipitation, which together determine what kinds of plants can thrive in a region, and those plants in turn define the biome.

• Biomes divide into terrestrial (land-based) and aquatic (water-based) categories. This guide focuses on terrestrial biomes.
• Ecotones are transitional zones between adjacent biomes. They contain species from both sides and often shift position as climate conditions change.
• The Holdridge life zone system classifies biomes quantitatively using biotemperature, annual precipitation, and the potential evapotranspiration ratio. It's one of the most widely used frameworks for linking climate data directly to expected vegetation.
• Climate change is actively altering biome boundaries. As temperatures rise, biomes shift poleward and upslope, meaning the map of biomes is not static.

Biome Classification Systems

The Köppen climate classification is the system you'll encounter most often in climatology, and it correlates tightly with biome distribution. Each Köppen climate type tends to support a predictable set of vegetation communities.

Several factors beyond latitude shape where biomes fall:

• Altitude creates vertical biome zonation in mountainous regions. Climbing a tropical mountain, you can pass through rainforest, cloud forest, and alpine tundra in a matter of kilometers.
• Ocean currents transport heat and moisture, influencing coastal biomes. Cold currents (like the Humboldt off South America) can produce coastal deserts even at tropical latitudes.
• Microclimates create localized variation within a biome. A sheltered valley floor and an exposed ridgeline in the same region may support quite different plant communities.
• Seasonal variation in temperature and precipitation determines whether a biome supports evergreen or deciduous vegetation, continuous growth or seasonal dormancy.

Terrestrial Biomes and Vegetation

Forest Biomes

Tropical rainforests are the most biodiverse terrestrial biome. They feature tall trees forming a multi-layered canopy (emergent layer, canopy, understory, and forest floor). Found near the equator in places like the Amazon Basin and Congo Basin, they receive roughly 2,000+ mm of rainfall annually with consistently warm temperatures (around 25–27°C year-round).

Temperate deciduous forests occur in mid-latitude regions like the eastern United States, Central Europe, and parts of East Asia. Trees shed their leaves in autumn as an adaptation to cold winters, and the forest supports a diverse understory during the growing season. These regions experience distinct seasons with moderate precipitation spread throughout the year.

Coniferous forests (taiga/boreal forest) form the largest terrestrial biome by area, stretching across subarctic regions of northern Canada, Scandinavia, and Siberia. Dominated by evergreen conifers like spruce and fir, these forests endure long, harsh winters (temperatures dropping below $-30°C$) and short growing seasons of just a few months.

Grassland and Desert Biomes

Grasslands are dominated by grasses rather than trees, typically because precipitation is too low or too seasonal to support dense forest. They include:

• Tropical savannas (like the East African Savanna), which have warm temperatures year-round with a distinct wet and dry season. Scattered trees coexist with grasses.
• Temperate grasslands (like the North American Great Plains and Central Asian steppes), which experience hot summers, cold winters, and periodic drought. Fire and grazing help maintain grass dominance over trees.

Deserts receive less than 250 mm of precipitation per year. They occur in subtropical zones where descending air in Hadley cells suppresses rainfall (Sahara, Arabian Desert) and in rain shadow regions (Atacama, Gobi). Vegetation is sparse: cacti, drought-resistant shrubs, and succulents dominate. Not all deserts are hot; polar deserts exist in Antarctica and the Arctic.

Mediterranean biomes feature hot, dry summers and mild, wet winters. Found in coastal regions between roughly 30° and 40° latitude (California, the Mediterranean Basin, parts of Chile and Australia), they support drought-adapted shrubland known as chaparral or maquis. Fire is a natural part of these ecosystems, and many plant species have evolved to resprout after burning.

Tundra and Alpine Biomes

Arctic tundra occurs in high-latitude regions like northern Alaska, northern Canada, and the Siberian coast. Permafrost (permanently frozen ground) underlies the surface, preventing deep root growth. Vegetation consists of mosses, lichens, sedges, and low shrubs. Growing seasons last only 6–10 weeks, and mean annual temperatures stay well below freezing.

Alpine biomes occur at high elevations on mountains worldwide, even in the tropics. They share many characteristics with arctic tundra (cold temperatures, high winds, short growing seasons) but receive more intense UV radiation. Plants adapt through compact growth forms like cushion plants and dwarf shrubs that stay close to the ground to avoid wind damage.

Climate's Influence on Biome Distribution

Temperature and Precipitation Factors

Temperature and precipitation are the primary controls on biome distribution, and both vary predictably with latitude and altitude.

Latitude effects:

• Equatorial regions receive the most direct solar radiation, producing consistently warm temperatures that support tropical biomes.
• Polar regions receive oblique sunlight, resulting in cold temperatures and tundra or ice cap biomes.
• Mid-latitudes fall between these extremes, supporting temperate forests and grasslands depending on moisture availability.

Altitude effects:

• Temperature decreases with elevation at an average environmental lapse rate of about $6.5°C$ per 1,000 m. This is why mountain peaks can be snow-covered even in the tropics.
• Precipitation often increases on windward slopes due to orographic lifting (air forced upward by terrain cools and releases moisture). Leeward slopes receive less precipitation, creating rain shadows.

Climate Classification and Biome Correlation

The Köppen system groups climates into five main types, each associated with characteristic biomes:

Seasonal and Microclimate Influences

Seasonal climate patterns shape how vegetation behaves within a biome:

• Deciduous forests respond to seasonal temperature swings by dropping leaves in autumn, reducing water loss during winter when soil moisture is locked in ice or snow.
• Savannas and grasslands cycle between growth during wet seasons and dormancy during dry seasons. Fire during the dry season clears dead material and recycles nutrients.

Microclimates add another layer of complexity within biomes:

• Slope aspect matters significantly. In the Northern Hemisphere, south-facing slopes receive more direct sunlight and tend to be warmer and drier than north-facing slopes, often supporting different vegetation on the same mountain.
• Canopy effects in forests create understory microclimates that are cooler, more humid, and receive less light than the canopy above. This allows shade-tolerant species to thrive beneath the dominant trees.

Plant and Animal Adaptations in Biomes

Plant Adaptations

Plants have evolved structural and physiological traits that match the climate conditions of their biome. Three categories of adaptation stand out:

Leaf modifications:

• Broad, flat leaves in tropical rainforests maximize light capture in the dim understory. Many also have "drip tips" that channel heavy rainfall off the leaf surface.
• Needle-like leaves in coniferous forests have a small surface area and waxy coating that reduce water loss and prevent snow from accumulating on branches.

Root systems:

• Desert plants like mesquite develop deep taproots (sometimes exceeding 30 m) to reach underground water sources. Others, like cacti, have shallow but widespread root networks to capture brief rainfall events.
• Tundra plants have shallow, spreading roots because permafrost prevents downward growth. These roots exploit the thin, nutrient-rich active layer that thaws each summer.

Growth strategies:

• Deciduous trees shed leaves to conserve water and energy during cold or dry seasons when photosynthesis would be inefficient.
• Succulents like cacti and agaves store water in thick, fleshy stems or leaves, allowing them to survive months without rain.

Animal Adaptations

Animals in each biome face distinct survival challenges related to temperature, food availability, and predation.

Physiological adaptations:

• Cold-biome animals like polar bears and seals rely on thick fur, blubber, or countercurrent heat exchange in their limbs to retain body heat.
• Hot-biome animals dissipate excess heat through specialized structures. Fennec foxes have oversized ears with extensive blood vessel networks that radiate heat, and jackrabbits use a similar strategy.

Behavioral adaptations:

• Migration allows animals to track favorable conditions seasonally. Arctic terns travel pole to pole annually, while wildebeest follow rainfall across the Serengeti.
• Hibernation lets animals like ground squirrels and bears survive food-scarce winters by drastically lowering their metabolic rate and living off stored fat.

Camouflage:

• Arctic foxes and snowshoe hares change fur color seasonally, shifting from brown in summer to white in winter to match their surroundings.
• Grassland animals like zebras use disruptive coloration (bold stripe patterns) that may confuse predators, especially when the herd is moving.