AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/1/1-2)
Unit 1 · Topic 1.2
1.2 Terrestrial Biomes
A biome is a large area with a typical climate and the plants and animals adapted to it. Average temperature and precipitation mostly decide which biome you find, which also controls soil nutrients and what can grow there.
Key terms
- biome
- climate
- tropical rainforest
- desert
- taiga
- tundra
Climate decides the biome
A biome is a large region, often spanning several continents, defined by its climate and its dominant plant life. Climate means the long-term pattern of weather in a place, especially average temperature and average yearly precipitation (rain and snow). Weather is what's happening today; climate is the average over decades.
Temperature and precipitation decide which plants can grow, and the plants decide which animals can live there. That's why the same biome can appear in very different parts of the world: deserts exist in Africa, Australia and North America because those places share a similar climate.
Scientists often show climate with a climatograph, a graph with months on the horizontal axis, average temperature as a line and average precipitation as bars. Reading a climatograph is a common way to identify a biome.
The same factors explain why resources like fresh water and timber are spread so unevenly. Rainfall, temperature, latitude, elevation, landforms, soil and nutrients together decide whether a region grows thick forests, holds plenty of water or has very little of either.
The major land biomes
These ranges are rough guides. Real biomes blend into each other, so on the exam use the overall pattern: wet or dry, hot or cold, and whether the seasons vary.
| Biome | Climate (rough guide) | Key features |
|---|---|---|
| Tropical rainforest | Warm all year; very wet, often over 200 cm of rain a year | Highest biodiversity on land; thin, nutrient-poor soil |
| Temperate deciduous forest | Four seasons; moderate rain spread through the year | Trees drop leaves in fall; fairly fertile soil |
| Temperate rainforest | Mild, cool and very wet (coastal) | Huge conifers, like those in the Pacific Northwest |
| Taiga (boreal forest) | Long, cold winters; short summers; moderate precipitation | Conifers like spruce and fir; acidic, nutrient-poor soil |
| Tundra | Very cold; very little precipitation | No trees; permafrost (permanently frozen ground); short growing season |
| Temperate grassland | Hot summers, cold winters; not enough rain for many trees | Very deep, fertile soil; fires and grazing keep trees out |
| Savanna | Warm all year; distinct wet and dry seasons | Grasses with scattered trees; large grazing herds |
| Desert | Very dry, often under 25 cm of precipitation a year; can be hot or cold | Plants store water or have small leaves; sparse life |
| Shrubland (chaparral) | Hot, dry summers; mild, rainy winters | Fire-adapted shrubs |
Nutrients and soil
Biomes differ in how many nutrients are in the soil. In a tropical rainforest, heat and moisture make dead material decompose very quickly, and plants absorb the nutrients right away. Heavy rain also washes nutrients out of the soil. So most nutrients are stored in living plants, not in the soil, which is why rainforest land cleared for farming loses its fertility within a few years.
In the taiga, cold temperatures slow decomposition, and fallen conifer needles make the soil acidic, so soils are thin and poor. Temperate grasslands have the most fertile soils: grass roots die back and add organic matter every year, and there's not enough rain to wash nutrients away. This is why grasslands like the U.S. prairie became major farming regions.
Biomes can shift
Because biomes follow climate, they move when the climate changes. As average temperatures rise, biome boundaries tend to move toward the poles and up mountainsides. For example, shrubs and trees are spreading north into parts of the Arctic tundra. Species that can't move or adapt fast enough may decline, especially those already near mountaintops or the poles.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Reading a climatograph
A climatograph shows monthly average temperatures below 0°C for about seven months of the year, rising to about 15°C in midsummer. Total yearly precipitation is about 50 cm, most of it in summer. The vegetation is dominated by evergreen conifers. Identify the biome and explain one feature of its soil.
Show the solutionHide the solution
- Step 1: Look at temperature first: long, cold winters below freezing and short, cool summers point to a high-latitude biome, either taiga or tundra.
- Step 2: Use the other clues to choose. Tundra has no trees and usually less precipitation. Conifer forest with about 50 cm of precipitation fits the taiga.
- Step 3: Explain the soil: cold temperatures slow decomposition, and the needles that fall from conifers are acidic, so taiga soil is thin, acidic and low in nutrients.
Answer: Taiga (boreal forest); its soil is thin, acidic and nutrient-poor because decomposition is slow and conifer needles acidify it.
- Example 2
The rainforest soil trap
A student says, ‘Tropical rainforests have the most plant growth, so they must have the most fertile soil, and cleared rainforest land should make excellent farmland.’ Evaluate the claim.
Show the solutionHide the solution
- Step 1: Check the first part: tropical rainforests do have very high plant growth, thanks to constant warmth, sunlight and rain.
- Step 2: Check the second part: most nutrients in a rainforest are held in living plants. Fast decomposition and heavy rain that washes nutrients out keep the soil itself thin and nutrient-poor.
- Step 3: Apply it: when the forest is cut and burned, the ash adds nutrients briefly, but they're used up or washed away within a few years, so yields drop.
Answer: The claim is wrong. Rainforest soils are nutrient-poor because nutrients are stored in plants, so cleared land usually becomes poor farmland within a few years.
Common mistakes
- Assuming lush plant growth means fertile soil. Tropical rainforest soils are poor; temperate grassland soils are the richest.
- Defining a desert by heat. Deserts are defined by low precipitation, and some, like the Gobi, are cold.
- Mixing up taiga and tundra. Taiga has conifer forests; tundra is too cold for trees and has permafrost.
- Reading only the temperature line on a climatograph. Always check the precipitation bars too, since many biomes have similar temperatures.
On the exam
- Practice identifying biomes from described climatographs or climate data. Use both temperature and precipitation in your reasoning.
- If asked how climate change affects biomes, say that biome boundaries shift toward the poles and to higher elevations as temperatures rise.
Connected topics
Videos
Check yourself
4 questions on 1.2 Terrestrial Biomes. Pick an answer to see if you got it, and why.
| Site | Mean annual temperature (°C) | Annual precipitation (cm) |
|---|---|---|
| W | 26 | 260 |
| X | −12 | 20 |
| Y | 0 | 50 |
| Z | 22 | 15 |
Hypothetical climate data
Which biome is most likely found at Site X?
Site W supports the highest plant growth of the four sites, yet its soil is thin and low in nutrients. Which statement best explains this?
Suppose the mean annual temperature at Site Y rose by 4 °C over the next century while precipitation stayed about the same. Which change is most likely?
The taiga (boreal forest) has dense stands of evergreen trees, yet its soils are thin, acidic and low in nutrients. Which statement best explains this?
0 of 4 answered