Terrestrial Biomes

Core Concepts

Climate is the master control

Two variables explain most of the variation among terrestrial biomes: temperature and precipitation. Both are driven ultimately by solar energy and latitude: the equator receives sunlight more directly than the poles, so average temperature falls with latitude. Precipitation follows global air circulation — warm air rising at the equator cools and releases rain, producing wet tropical climates, while dry descending air near roughly 30° north and south latitude creates bands of desert and grassland. Mountains interrupt this pattern: air forced upward sheds moisture on the windward side, leaving a rain shadow on the leeward side. Temperature also falls with altitude (textbooks commonly cite roughly 6.5 °C per 1,000 m), which is why climbing a tropical mountain passes through forest, then grassland, then tundra-like zones.

The tropical biomes

Tropical rainforests are warm year-round with heavy, fairly constant rainfall (commonly cited as 200 cm or more per year). They have the highest biodiversity of any terrestrial biome, yet their soils are often poor: rapid decomposition and heavy rain cycle nutrients through living plants rather than storing them in soil. Tropical dry forests have pronounced wet and dry seasons, with trees dropping leaves in the dry season. Tropical savannas are grasslands with scattered trees, seasonal rainfall, and frequent fires; fire and grazing keep woody plants from taking over.

Deserts

Deserts are defined by dryness, not heat: textbooks commonly cite less than about 25 cm of annual precipitation. They can be hot (Sahara) or cold (Gobi), with wide day-night temperature swings. Adaptations to water scarcity include succulence (cacti storing water in fleshy stems), CAM photosynthesis (opening stomata at night to avoid daytime water loss), and nocturnal or burrowing animals.

Chaparral and temperate grasslands

Chaparral has a Mediterranean climate — hot, dry summers and mild, wet winters — dominated by drought-tolerant shrubs; fire is a recurring force, and many chaparral species resprout from roots or germinate after burning. Temperate grasslands have hot summers and cold winters with moderate rainfall (commonly cited as 25–75 cm per year). Their deep root systems build some of the most fertile soils on Earth, and fire and grazing keep trees out.

Temperate and boreal forests

Temperate forests have distinct seasons with moderate precipitation (commonly cited as 75–150 cm per year). Deciduous trees such as oaks and maples shed their leaves in autumn, an adaptation to the cold, dry winter when water is hard to pull from frozen ground; soils are more fertile than in the tropics. Boreal forests (taiga) ring the high northern latitudes with long, cold winters; conifers dominate — needle-like, waxy leaves reduce water loss, and conical shape sheds snow — and permafrost (soil frozen year-round) limits root depth.

Arctic tundra

The tundra is very cold with a short growing season and low precipitation. Permafrost underlies the landscape, preventing trees and leaving mosses, lichens, low shrubs, and grasses. Productivity is low, many animals migrate in and out seasonally, and the tundra stores enormous amounts of carbon in frozen organic matter.

Latitude, altitude, and ecotones

Because altitude mimics latitude, a single mountain can hold several biome-like zones within a few kilometers of elevation gain. Boundaries between biomes are usually gradual: ecotones are transition zones where species from two adjacent biomes mix. Fire suppression, grazing, deforestation, and climate change can all push these boundaries, so the biome map is a moving target.

Common Confusions