Global Distribution and Characteristics of Ecosystems

Tropical Rainforests: Climate, Vegetation, and Soil

Tropical rainforests cover just 6% of the Earth's land surface but are home to over 50% of all known species, making their biodiversity the highest on Earth.

• Climate: Found at $0^\circ$ latitude, intense solar heating causes air to rise, cool, and condense (equatorial low pressure). This creates daily convectional rainfall (over $2{,}000\text{mm}$/year) and consistently hot temperatures (averaging $27^\circ\text{C}$).
• Vegetation: Plants compete for light through stratification (layering). The five layers are: Emergents ($40$–$50\text{m}$), Main Canopy, Under Canopy, Shrub Layer, and Ground Layer.
• Adaptations: Trees have buttress roots for stability in shallow soils, and drip tips to shed heavy rain. Lianas (vines) climb trees, while epiphytes grow directly on branches to access sunlight.
• Soil & Nutrients: Rainforests sit on latosols—deep, red, iron-rich soils. They are surprisingly nutrient-poor because heavy rains cause intense leaching (minerals washed away). Nutrients are rapidly cycled and stored almost entirely in the living biomass, with only a very thin layer of humus on the forest floor.

Hot Deserts: Extreme Aridity

You might picture deserts as constantly baking hot, but without a blanket of clouds to trap the day's heat, nighttime temperatures can plummet below freezing.

• Climate: Located $15^\circ$–$30^\circ$ North and South of the equator. Here, air from the Hadley cell sinks, creating high pressure that prevents cloud formation. This results in extreme aridity (less than $250\text{mm}$ rainfall) and a massive diurnal range (daytime $50^\circ\text{C}$, nighttime $<0^\circ\text{C}$).
• Vegetation: Xerophytes (succulents like cacti) store water in fleshy stems and have spines instead of leaves to reduce transpiration. Ephemerals have dormant seeds that only sprout after rare rainfall.
• Soil: Desert soils are sandy, stony, thin, and grey with very low organic matter. High evaporation draws salts to the surface, causing salinisation and making the soil highly infertile.

Polar and Tundra Environments

Imagine trying to dig a garden where the ground has been literally frozen solid for thousands of years.

• Polar Biomes ($66.5^\circ$–$90^\circ$): Frigid, dense air sinks here to create high-pressure "cold deserts" with $<100\text{mm}$ precipitation. Crucially, polar regions do not have exposed soil; they are covered by permanent ice. Animals like polar bears adapt with thick blubber and hollow fur.
• Tundra Biomes (Arctic regions): These areas feature permafrost (permanently frozen ground). During the brief summer, the top active layer thaws for 50–60 days, creating waterlogged, acidic soils.
• Tundra Vegetation: Plants like bearberry are very low-growing to avoid freezing winds and have extremely shallow roots because they cannot penetrate the solid permafrost below.

Temperate and Coniferous Forests

Understanding seasonal leaf loss explains exactly how trees in the UK survive the freezing, dark days of winter.

• Temperate Deciduous Forests ($40^\circ$–$60^\circ$): Subpolar low pressure brings steady rainfall. Trees here are deciduous—they shed their leaves in winter to stop water loss via transpiration. Their dropped leaves create deep, incredibly fertile Brown Earth soils.
• Coniferous Forests / Taiga ($60^\circ$): Found in colder, drier latitudes, these trees have needle-like leaves to survive frost. The soils, known as podsols, are acidic and ash-grey due to the slow breakdown of pine needles.

Grasslands and Mediterranean Biomes

Every time you watch a wildlife documentary featuring vast herds of zebra migrating across open plains, you are looking at a tropical grassland biome.

• Tropical Grasslands / Savanna ($15^\circ$–$30^\circ$): Governed by the shifting ITCZ, these biomes have distinct wet and dry seasons. The red, porous laterites (soils) suffer from heavy seasonal leaching. Trees like the Baobab have fire-resistant bark to survive dry-season wildfires.
• Temperate Grasslands ($30^\circ$–$40^\circ$): Located far inland, continentality causes extreme temperature swings. Grasses die back to their deep roots in winter to survive freezing. They create Chernozems (Black Earth), which are the most fertile soils globally.
• Mediterranean ($40^\circ$–$45^\circ$): Characterised by hot, dry summers and mild, wet winters. Plants have sclerophyllous leaves (small, tough, waxy) to prevent wilting. The reddish, clay-rich soil is called Terra Rossa; it is highly permeable and well-drained but lacks moisture in summer.

Worked Example: Calculating Temperature Range

In exam questions, you may need to interpret climate graphs to prove a biome's identity (e.g., distinguishing a desert's high diurnal range).

• Formula: $\text{Highest Temperature} - \text{Lowest Temperature} = \text{Temperature Range}$

• Step 1: Identify the peak daytime temp ($50^\circ\text{C}$) and the lowest nighttime temp ($-2^\circ\text{C}$).

• Step 2: Subtract the lowest from the highest: $50 - (-2) = 52$.

• Answer: The diurnal temperature range is $52^\circ\text{C}$.