Major Biomes

Why This Matters

Biomes aren't just categories to memorize—they're the result of climate patterns, energy flow, and biogeochemical cycles working together across Earth's surface. When you understand why a tropical rainforest develops near the equator while tundra dominates polar regions, you're demonstrating mastery of how solar radiation, atmospheric circulation, and precipitation patterns shape life on Earth. These connections between climate and ecosystems are exactly what Earth Systems Science exams test.

You're being tested on your ability to explain feedback loops, carbon storage mechanisms, and human-environment interactions within each biome. Don't just memorize that deserts get less than 250 mm of rain annually—know why they form where they do and how they fit into global climate regulation. Each biome below illustrates specific principles about energy transfer, nutrient cycling, and ecosystem resilience that you'll need for both multiple choice and FRQ success.

---

High-Productivity Biomes: Maximum Biomass and Carbon Storage

These biomes receive abundant solar energy and precipitation, driving high rates of photosynthesis and creating complex food webs. Net primary productivity (NPP) is highest where water and sunlight are both plentiful.

Tropical Rainforest

• Highest biodiversity on Earth—warm temperatures (25-28°C year-round) and rainfall exceeding 2000 mm annually create ideal conditions for speciation
• Massive carbon reservoir stored primarily in living biomass rather than soil, making deforestation a major source of $CO_2$ emissions
• Rapid nutrient cycling—decomposition happens so quickly that nutrients stay in plants, not soil, explaining why cleared rainforest land loses fertility fast

Temperate Deciduous Forest

• Four distinct seasons drive the deciduous adaptation—trees shed leaves to conserve water during cold winters when soil water is frozen
• Nutrient-rich soils develop from annual leaf litter decomposition, creating deep humus layers that support agriculture
• Moderate NPP (lower than rainforests) but significant carbon storage in both biomass and soil organic matter

---

Cold-Climate Biomes: Temperature as the Limiting Factor

In these biomes, low temperatures restrict growing seasons and decomposition rates. Cold slows enzymatic activity, meaning organic matter accumulates rather than cycling quickly.

Coniferous Forest (Taiga)

• Largest terrestrial biome by area—stretches across Canada, Scandinavia, and Russia in a circumpolar band
• Conical tree shape and needle leaves are adaptations that shed snow and reduce water loss during frozen winters
• Major carbon sink—slow decomposition in cold soils means carbon accumulates in thick organic layers and permafrost zones

Tundra

• Permafrost defines this biome—permanently frozen ground (within 1 meter of surface) prevents deep root growth and tree establishment
• Extremely low NPP due to short growing season (50-60 days) and limited nutrients locked in frozen soil
• Climate change hotspot—thawing permafrost releases stored methane ($CH_4$) and $CO_2$, creating a dangerous positive feedback loop

---

Water-Limited Biomes: Precipitation as the Controlling Variable

These biomes exist where evaporation exceeds precipitation for much of the year. Aridity shapes everything—from plant spacing to animal behavior to fire regimes.

Desert

• Less than 250 mm annual precipitation defines deserts, but temperature varies widely (hot deserts vs. cold deserts)
• Extreme adaptations include CAM photosynthesis in cacti, nocturnal activity in animals, and deep taproots or shallow spreading roots
• High albedo (reflectivity) means deserts reflect significant solar radiation, influencing regional and global energy budgets

Grassland

• Precipitation (250-750 mm) falls between desert and forest thresholds—enough for grasses but not enough to support dense tree cover
• Deep root systems store carbon in soil organic matter, making grassland soils among the most carbon-rich per unit area
• Fire and grazing maintain grassland structure by preventing woody plant encroachment—remove these disturbances and forests may take over

Savanna

• Distinct wet and dry seasons create a mosaic of grasses with scattered drought-resistant trees
• Fire-adapted ecosystems—many savanna plants have thick bark, underground storage organs, or rapid regrowth strategies
• Supports megafauna like elephants and wildebeest whose grazing and migration patterns shape vegetation structure and nutrient distribution

---

Aquatic Biomes: Water as the Medium

Aquatic biomes cover over 70% of Earth's surface and operate by different rules than terrestrial systems. Light penetration, nutrient availability, and water chemistry replace temperature and precipitation as primary limiting factors.

Freshwater Systems

• Rivers, lakes, and wetlands support only 0.01% of Earth's water but contain disproportionately high biodiversity
• Wetlands function as "Earth's kidneys"—filtering pollutants, storing floodwater, and providing critical habitat for breeding species
• Eutrophication vulnerability—excess nitrogen and phosphorus from agriculture triggers algal blooms and oxygen depletion

Marine Systems

• Oceans absorb ~30% of anthropogenic $CO_2$—making them crucial carbon sinks but causing ocean acidification as carbonic acid forms
• Phytoplankton produce ~50% of Earth's oxygen through photosynthesis in the sunlit euphotic zone (top 200 meters)
• Thermohaline circulation distributes heat globally, connecting tropical and polar regions through deep water currents

---

Quick Reference Table

---

Self-Check Questions

1. Which two biomes store the most carbon in soil rather than living biomass, and why does decomposition rate explain this difference?

2. Compare the limiting factors in tundra versus desert biomes—both have low NPP, but for fundamentally different reasons. What are they?

3. If permafrost thaws across the tundra, explain the feedback loop that could accelerate global warming. Which greenhouse gas is most concerning and why?

4. A region receives 600 mm of annual precipitation. Could it be a grassland, savanna, or desert? What additional information would you need to classify it correctly?

5. An FRQ asks you to explain how deforestation in the Amazon affects global carbon cycling differently than clearing temperate deciduous forest. What's the key distinction in where carbon is stored, and how does this affect $CO_2$ release?