AP Environmental Science

## Learning Outcomes

- Understand how **temperature** and **precipitation** determine major terrestrial biomes
- Identify the main characteristics of the nine major APES terrestrial biomes
- Explain how latitude, altitude, and global air circulation affect biome distribution
- Distinguish between commonly confused biomes using climate and soil evidence
- Interpret a **climograph** to identify a likely biome on the AP Environmental Science exam
- Connect biome patterns to **net primary productivity (NPP)** and biosphere limits

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand major terrestrial biomes, with a focus on the following syllabus points:

- Know that terrestrial biomes are defined mainly by long-term patterns of **temperature** and **precipitation**
- Identify the major APES biome categories: tropical rainforest, tropical seasonal forest/savanna, subtropical desert, temperate rainforest, temperate seasonal forest, woodland/shrubland, temperate grassland/cold desert, boreal forest (taiga), and tundra
- Explain how **climographs** are used to identify biomes from monthly temperature and precipitation data
- Understand how soil characteristics differ across biomes, including leached tropical soils, fertile grassland soils, acidic boreal soils, and permafrost-limited tundra soils
- Explain why many deserts occur near **30° north and 30° south** because of global air circulation
- Understand how **altitude** can create biome patterns similar to changes in latitude
- Connect climate conditions to **net primary productivity**, with warm and wet biomes generally showing higher productivity than cold or dry biomes

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What two abiotic factors are most important in determining terrestrial biomes?
2. How can you tell a tropical rainforest from a tropical seasonal forest or savanna on a climograph?
3. Why are many deserts found around 30° north and 30° south latitude?
4. What is the difference between boreal forest (taiga) and tundra?
5. Which biome is likely to have deep, fertile topsoil, and which is likely to have nutrient-poor, heavily leached soil?

## Introduction

Terrestrial biomes are one of the highest-yield topics in AP Environmental Science Unit 1 because they connect climate, soils, productivity, and species distributions. On the exam, you are rarely rewarded for memorising long species lists. You are rewarded for recognizing patterns: warm versus cold, wet versus dry, seasonal versus year-round, forest versus grassland, and high versus low productivity.

A strong APES answer on biomes usually begins with climate. Temperature and precipitation shape plant growth, plant growth shapes animal communities, and both together affect nutrient cycling and productivity. That is why biome questions often use a **climograph**, not a photograph. The exam wants to know whether you can infer the biome from data.

> **Key Term: terrestrial biome**
> A large land-based ecological region defined mainly by climate, especially temperature and precipitation, and characterized by typical vegetation and soils.

## Introduction to climate controls and biome patterns

### Why climate is the main predictor of biomes

If two places on different continents have similar temperature and rainfall patterns, they often develop similar vegetation forms even if they are far apart. This is because climate sets the limits for plant survival. Plants need enough water, suitable temperatures for enzyme activity, and a growing season long enough to reproduce.

This also explains why APES treats vegetation as an outcome, not the starting cause. Students often reverse the chain. The correct sequence is:

- climate influences water availability and temperature
- those conditions favor certain plant adaptations
- dominant vegetation shapes habitat and food sources for animals

For example, grasses dominate where rainfall is too low or too seasonal to support dense forest. Conifers dominate where winters are long and cold, because needle leaves reduce water loss and tolerate freezing conditions.

> **Key Term: climograph**
> A graph showing a location’s average monthly temperature and precipitation, used to identify likely biome type.

A climograph matters because the same annual rainfall total can still produce different biomes if the rainfall pattern differs. A place with heavy rain in every month is different from a place with a long dry season and a short wet season.

> **Test Tip:** On biome questions, identify the temperature pattern first, then the precipitation pattern, then the seasonality. Do not jump straight to vegetation.

### Other controls: latitude, altitude, and air circulation

Temperature and precipitation are the best global predictors, but they are not the only controls. **Latitude** matters because the equator receives more direct solar radiation than the poles. As you move away from the equator, temperatures usually decrease and seasonality becomes stronger.

**Altitude** can mimic latitude. Higher elevations are cooler, often with shorter growing seasons. That is why mountain slopes can show biome changes over short distances. A warm lowland forest may grade into conifer forest and then tundra-like conditions near the summit.

**Global air circulation** explains major dry and wet belts. Near the equator, warm air rises, cools, and releases moisture, helping create tropical wet biomes. Around 30° north and south, dry air descends, suppressing rainfall and creating many subtropical deserts.

Mountains also create **rain shadow** effects. Air rises on the windward side, cools, and loses moisture. The descending air on the leeward side is drier, so the far side often supports a drier biome.

> **Key Term: rain shadow**
> A dry region on the leeward side of a mountain range caused by air losing moisture on the windward side before descending.

## Major terrestrial biomes and how to distinguish them

### Warm biomes: tropical rainforest, tropical seasonal forest/savanna, and subtropical desert

**Tropical rainforest** is warm year-round and wet year-round. On a climograph, the temperature line stays high and fairly flat, while precipitation stays high in nearly every month. This biome has very high biodiversity and high productivity.

A key APES trap is soil. Students often assume lush vegetation means rich soil. In tropical rainforests, soils are often **nutrient-poor** because heavy rainfall causes leaching and because nutrients are rapidly taken up and stored in biomass. The system is productive, but much of the nutrient capital is in living tissue, not in the soil.

**Tropical seasonal forest/savanna** is also warm all year, but rainfall is strongly seasonal. The key difference from tropical rainforest is the presence of a distinct wet season and dry season. Savannas usually have grasses with scattered trees. Fire and grazing help maintain this open structure. If rainfall were reliable all year, tree cover would usually increase.

**Subtropical desert** is very dry, usually with sparse vegetation and low productivity. Many are found around 30° latitude because of descending dry air from Hadley cell circulation. A desert is defined mainly by low precipitation, not by heat alone. Some deserts are hot, but some are cooler in winter.

Students often confuse desert and tundra because both can have low precipitation totals. The distinction is temperature. Desert is dry and generally warmer; tundra is dry but much colder and often underlain by permafrost.

> **Key Term: leaching**
> The removal of dissolved nutrients from soil by percolating water, often leaving soil less fertile.

### Temperate biomes: temperate rainforest, temperate seasonal forest, woodland/shrubland, and temperate grassland/cold desert

**Temperate rainforest** has mild temperatures and very high precipitation. It is wetter than temperate seasonal forest and usually supports large trees, mosses, ferns, and epiphytes. Biodiversity is high, though usually not as high as in tropical rainforest.

**Temperate seasonal forest** has stronger seasonal temperature change: warm summers and cold winters. Precipitation is moderate and spread more evenly through the year. Broadleaf deciduous trees are common, and soils are generally fairly fertile because leaf litter decomposes and returns nutrients.

**Woodland/shrubland**, often called **chaparral**, has hot, dry summers and mild, wet winters. This biome is strongly linked to fire. Shrubs often have waxy leaves, deep roots, and other adaptations that reduce water loss. Some species resprout after fire or germinate better after heat exposure. Fire is natural here, but too-frequent fire can still damage the system by not allowing enough recovery time.

**Temperate grassland** has hot summers, cold winters, and moderate precipitation, but not enough to support widespread forests. Fire, drought, and grazing help keep trees from taking over. The major APES soil point here is that grasslands usually have **deep, fertile, organic-rich topsoil** because dense root systems die, decompose, and enrich the soil.

**Cold desert** is often grouped near temperate grassland in APES materials because both have relatively low precipitation and larger seasonal temperature ranges than tropical biomes. Cold deserts have sparse shrubs and grasses, lower productivity than grasslands, and often salty or alkaline soils.

> **Test Tip:** If a climograph shows four seasons and moderate, fairly even precipitation, think temperate seasonal forest. If it shows dry summers and wetter winters, think woodland/shrubland.

## Cold biomes, soils, and productivity

### Boreal forest (taiga), tundra, and the link to NPP

**Boreal forest (taiga)** is a cold northern coniferous biome with long winters and short growing seasons. Precipitation is low to moderate, often with much falling as snow. Coniferous trees dominate because needle leaves reduce water loss and can photosynthesize quickly when conditions improve.

Its soils are usually **acidic and nutrient-poor**. Decomposition is slow because cold temperatures reduce microbial activity. Organic matter can build up as partially decomposed material, but nutrient cycling remains slow. So productivity is lower than in warmer, wetter forests.

**Tundra** is colder still and is treeless. The defining features are low-growing vegetation, short growing season, and often **permafrost**, which is permanently frozen subsoil. Permafrost limits root penetration, drainage, and decomposition. Water may collect above the frozen layer, producing wet surface conditions in summer even though annual precipitation is low.

The best distinction between taiga and tundra is tree presence. Taiga supports coniferous forest; tundra does not. The reason is not just “colder weather” in a simple sense. Permafrost, shorter growing season, and lower temperatures together prevent tree establishment.

> **Key Term: permafrost**
> Permanently frozen subsoil that restricts drainage, root growth, and decomposition, common in tundra regions.

Biome productivity can be linked to **net primary productivity**. Warm and wet biomes generally have higher NPP because photosynthesis can occur for longer periods and water is less limiting. Cold or dry biomes usually have lower NPP because water shortage, low temperature, or short growing season limits growth.

> **Key Term: net primary productivity (NPP)**
> The rate at which producers store energy as new biomass after subtracting the energy they use in respiration.

The APES formula is:

$$
\text{NPP} = \text{GPP} - \text{respiration}
$$

You do not usually need heavy calculations for biome questions, but you do need to explain patterns. Tropical rainforest tends to have high NPP. Deserts and tundra tend to have low NPP. Boreal forest is also relatively low compared with temperate or tropical forests because cold slows decomposition and nutrient cycling.

### A compact comparison of major soil patterns

Students often lose marks by discussing climate well but saying little about soil. These are the most useful soil links:

- **Tropical rainforest:** thin, heavily leached, often nutrient-poor soil
- **Temperate grassland:** deep, dark, fertile, humus-rich topsoil
- **Temperate seasonal forest:** generally fertile soil enriched by leaf litter
- **Boreal forest (taiga):** thin, acidic, nutrient-poor soil
- **Tundra:** poorly developed soil, often over permafrost
- **Subtropical desert:** low organic matter, often coarse-textured soil
- **Woodland/shrubland:** relatively thin soils, often drought- and fire-stressed

This matters because AP questions may ask why one biome is good for agriculture and another is not. Grasslands are often farmed because their soils are fertile. Tropical rainforests may look productive, but once vegetation is removed, the soil often loses fertility quickly.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Terrestrial biomes are determined mainly by long-term patterns of temperature and precipitation
- Climographs are used to identify biomes from monthly temperature and rainfall data
- Tropical rainforest is warm and wet year-round, with high biodiversity but often nutrient-poor, leached soil
- Tropical seasonal forest/savanna is warm year-round but has distinct wet and dry seasons
- Subtropical deserts are very dry and are common near 30° north and 30° south because of descending dry air
- Temperate rainforest is wetter and milder than temperate seasonal forest
- Woodland/shrubland has hot, dry summers, mild wet winters, and strong fire adaptation
- Temperate grasslands have deep, fertile soils and are maintained by drought, fire, and grazing
- Boreal forest (taiga) has long cold winters, conifer trees, and acidic nutrient-poor soils
- Tundra is treeless, cold, dry, and often underlain by permafrost
- Higher altitude can produce biome changes similar to moving toward higher latitude
- Warm, wet biomes generally have higher NPP than cold or dry biomes

## Key Terms and Concepts

- terrestrial biome
- climograph
- rain shadow
- leaching
- permafrost
- net primary productivity (NPP)


Terrestrial Biomes - Major terrestrial biomes