## Learning Outcomes

- Understand how **temperature**, **precipitation**, and **seasonality** control terrestrial biome distribution
- Identify how **latitude**, **altitude**, winds, and mountains affect resource availability on land
- Explain how **soil characteristics** and **topography** modify plant growth within the same broad biome
- Apply **ecological tolerance** and **limiting factors** to species distribution
- Interpret a **climograph** to identify likely terrestrial biomes
- Link climate and resource distribution to **net primary productivity (NPP)**

## AP Environmental Science Syllabus

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

- Know that terrestrial biomes are determined primarily by **temperature** and **precipitation**
- Identify the role of **seasonality** in separating biomes with similar annual temperature or rainfall totals
- Explain how **latitude** and **altitude** influence temperature and growing season length
- Understand how **topography**, including **rain shadow effects** and **aspect**, changes local resource distribution
- Explain how **soil depth**, **drainage**, **nutrient content**, **pH**, and **water-holding capacity** affect vegetation
- Apply **ecological tolerance** and **limiting factors** to explain why species occur in some biomes but not others
- Interpret a **climograph** using temperature pattern, precipitation pattern, and wet-dry seasonality
- Relate warm, wet conditions to higher **NPP** and cold or dry conditions to lower **NPP**

## 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. Why can two places on different continents have very similar biomes?
2. How does a mountain create a rain shadow?
3. What is the difference between a desert and a tundra if both receive little precipitation?
4. How can soil make a grassland and a forest differ even under fairly similar climates?
5. What clues on a climograph would suggest tropical rainforest rather than savanna?

## Introduction

Terrestrial biomes are not distributed randomly. If you know the long-term climate of a place, especially its temperature and precipitation, you can make a strong prediction about its vegetation, and from that, about the animals likely to live there. That is the main APES idea behind terrestrial resource distribution.

For exam questions, you need to go beyond naming biomes. You must explain _why_ certain resources are available in some regions but limited in others. Climate sets the broad pattern, but soils, slope, drainage, and mountain effects refine it at the local scale.

> **Key Term: terrestrial biome**
> A large land-based ecological region defined primarily by climate, dominant vegetation, and characteristic organisms.

### Climate is the main control on biome distribution

The first rule of terrestrial resource distribution is simple: **temperature** and **precipitation** matter most. A biome exists where long-term climate allows certain plants to survive, and those plants then shape the rest of the community.

Why plants come first matters. Plants are the primary producers in terrestrial systems. If water is too scarce, trees cannot maintain leaves and woody tissue. If temperatures are too low, growing seasons are short and enzyme activity slows. If both warmth and moisture are high, plants can photosynthesize for more of the year and grow more biomass.

This gives broad patterns:

- **Warm + wet** tends to support forests
- **Warm + strongly seasonal rainfall** tends to support savanna or tropical seasonal forest
- **Moderate rainfall but not enough for trees** tends to support grassland
- **Very low precipitation** tends to support desert
- **Cold + short growing season** tends to support boreal forest or tundra
  > **Key Term: limiting factor**
  > An environmental condition that restricts an organism’s survival, growth, abundance, or distribution.

A common mistake is to assume temperature alone determines the biome. It does not. APES questions often compare biomes that overlap in temperature but differ in rainfall, or overlap in rainfall but differ in temperature.

For example:

- A **temperate grassland** and a **temperate seasonal forest** may both have moderate temperatures, but the forest gets enough reliable precipitation to support trees.
- A **desert** and a **tundra** can both be dry, but desert is usually hot or warm while tundra is very cold and often has **permafrost**.
  > **Key Term: permafrost**
  > Ground that remains frozen for at least two consecutive years.

Permafrost changes everything in tundra. Even though tundra gets little precipitation, summer surface water may remain because frozen ground blocks drainage. This is why tundra can be cold and wet at the surface while still counting as a low-precipitation biome.

> **Test Tip:** When a question compares two biomes, do not stop at “one is wetter” or “one is colder.” Add the effect on vegetation, soil, or growing season to earn full credit.

### Geographic controls: latitude, altitude, winds, and mountains

Climate patterns do not appear by chance. They are shaped by geography.

**Latitude** affects the angle and intensity of incoming solar radiation. Near the equator, sunlight is more direct year-round, so temperatures are higher and seasonal variation is often smaller. Toward the poles, sunlight arrives at a lower angle, spreading energy over a larger area and lowering temperatures. This is why tropical rainforests are near the equator and tundra occurs at high latitudes.

> **Key Term: latitude**
> Distance north or south of the equator; it strongly affects solar energy input and therefore temperature.

**Altitude** also matters because temperature generally decreases with elevation. A mountain can therefore show biome changes over a short horizontal distance that resemble traveling from the equator toward the poles over a much longer distance.

> **Key Term: altitude**
> Height above sea level; increasing altitude usually decreases temperature and shortens the growing season.

This is important for APES because students often forget that a cold biome does not have to be near the poles. **Alpine tundra** occurs at high elevations for the same temperature reason that Arctic tundra occurs at high latitude.

Mountains also affect precipitation through the **rain shadow effect**. Moist air is forced upward on the windward side of a mountain. As it rises, it cools, and water vapor condenses and falls as precipitation. By the time the air crosses the peak and descends the leeward side, it is drier. Descending air warms, which further lowers relative humidity and suppresses rainfall.

> **Key Term: rain shadow effect**
> A pattern in which mountains cause wetter conditions on the windward side and drier conditions on the leeward side.

The cause-and-effect chain matters:

1. Moist air moves toward a mountain
2. Air rises and cools
3. Cooling causes condensation and precipitation
4. Air loses moisture
5. Air descends leeward, warms, and dries
6. Leeward side becomes much drier

This can create sharp biome contrasts over short distances. One mountain slope may support forest, while the other supports shrubland or grassland.

**Prevailing winds** and nearby water bodies also affect how much moisture reaches land. Regions receiving moist maritime air tend to be wetter than inland regions where air masses have already lost much of their moisture.

**Aspect**, the direction a slope faces, changes sunlight exposure. In the Northern Hemisphere, south-facing slopes usually receive more direct sunlight than north-facing slopes. That often makes them warmer and drier, which can change local vegetation even within the same broader biome.

> **Key Term: aspect**
> The direction a slope faces; it affects sunlight exposure, temperature, and moisture.

A useful APES summary is: climate sets the broad biome, while topography fine-tunes local resource availability.

### Soil and surface conditions determine what can grow locally

Students often remember climate and forget soil. That loses marks, because soil is one of the main ways resource distribution is expressed on land.

Plants need more than light and rain. They also need rooting space, oxygen around roots, water retention, and access to mineral nutrients. Soil controls these.

Important soil features include:

- **Depth**: deep soils allow larger root systems
- **Drainage**: poorly drained soils may become waterlogged
- **Nutrient content**: affects plant growth rate
- **pH**: controls nutrient availability
- **Texture**: influences water movement and retention
- **Organic matter**: improves fertility and moisture retention

Examples by biome are especially useful:

- **Temperate grasslands** often have deep, dark, fertile soils because grasses have dense root systems and large organic inputs below ground.
- **Tropical rainforests** often have nutrient-poor soils even though plant growth is high. Heavy rainfall causes **leaching**, and many nutrients are stored in living biomass rather than in the soil.
- **Taiga** soils are often acidic and nutrient-poor because decomposition is slow in cold conditions and conifer needles add acidic litter.
- **Tundra** soils are shallow and poorly developed, with slow nutrient cycling due to cold and permafrost.
- **Desert** soils are often dry and low in organic matter, though some may contain mineral nutrients that are not easily used because water is scarce.
  > **Key Term: leaching**
  > The removal of dissolved nutrients from soil by percolating water.

This explains an important APES paradox: **high productivity does not always mean fertile soil**. Tropical rainforests have very high plant productivity, but much of the nutrient supply is rapidly taken up and held in biomass. Once forest cover is removed, the soil often loses fertility quickly.

By contrast, grasslands may look less lush than forests, but their soils are often excellent for agriculture because of deep roots and organic matter buildup.

Surface moisture and drainage also matter. If water runs off quickly, plants may struggle despite moderate rainfall totals. If water stays too long, roots may lack oxygen. That is why local hydrology can produce patches of different vegetation within the same region.

> **Test Tip:** If an FRQ asks why vegetation differs between two nearby places, include at least one soil or topographic factor, not only climate.

### Ecological tolerance explains species patterns within biomes

A biome describes a broad region, but individual species within that biome still differ in where they can live. That is where **ecological tolerance** matters.

> **Key Term: ecological tolerance**
> The range of environmental conditions within which a species can survive, grow, and reproduce.

Under **Shelford’s law of tolerance**, each species has minimum, maximum, and optimum ranges for factors such as temperature, water, light, and nutrients. Outside those limits, survival or reproduction declines.

> **Key Term: Shelford's law of tolerance**
> The idea that each species has minimum, maximum, and optimum levels for environmental conditions that control its distribution and abundance.

This helps explain why not all species in a biome are spread evenly through it. A plant may tolerate the overall climate of a temperate forest, but fail on dry ridges, acidic soils, or shaded north-facing slopes.

It also explains biome boundaries. If average winter temperature drops below what a tree species can tolerate, that tree cannot dominate beyond that point. If annual rainfall falls below what tree seedlings need, grassland may replace forest.

A strong APES example is comparing **desert** and **tundra**. Both are low-precipitation systems, but species adapted to desert heat are outside their tolerance range in tundra cold. Species adapted to tundra can survive freezing and short growing seasons, but would fail in hot arid conditions.

Disturbance can also help maintain certain resource patterns. In grasslands and savannas, **fire** and **grazing** often prevent trees from taking over. Even where rainfall might support some woody vegetation, repeated disturbance can favor grasses instead.

This means biome distribution is not only a climate map. It is climate interacting with species tolerance, soil, and disturbance.

### How to read a climograph for biome questions

A **climograph** shows average monthly temperature and precipitation for one location. APES uses climographs because they let you infer biome from climate pattern rather than memorizing labels.

> **Key Term: climograph**
> A graph showing average monthly temperature and precipitation for a location, used to identify climate patterns and likely biome.

A good method is step-by-step:

1. Look at overall temperature level  
   Is it warm year-round, strongly seasonal, or cold most of the year?

2. Look at total precipitation  
   Is it very wet, moderate, or very dry?

3. Look at seasonality  
   Is precipitation spread through the year or concentrated in a wet season? Is temperature range narrow or wide?

4. Match likely vegetation pattern  
   Warm and wet year-round suggests forest; warm with a long dry season suggests savanna; cold with a short summer suggests tundra or boreal forest.

Common distinctions:

- **Tropical rainforest**: warm year-round, high precipitation all year, little temperature variation
- **Savanna**: warm year-round, clear wet and dry seasons
- **Temperate seasonal forest**: moderate to high precipitation, warm summers and cold winters
- **Temperate grassland**: moderate precipitation, often less than nearby forests, with drought or fire
- **Tundra**: very cold, low precipitation, short cool growing season
- **Subtropical desert**: very low precipitation, generally warm to hot

A frequent trap is confusing **tropical rainforest** with **savanna**. Both are warm year-round. The difference is rainfall pattern. Rainforest stays wet across the year; savanna has a strong dry season.

Another trap is confusing **desert** with **tundra** because both can be dry. Temperature and growing season solve that problem.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Terrestrial biomes are controlled mainly by **temperature** and **precipitation**
- **Seasonality** helps distinguish biomes with similar annual totals
- Climate determines dominant vegetation, and vegetation helps shape animal communities
- **Latitude** affects biome distribution by changing solar energy input
- **Altitude** lowers temperature and shortens the growing season
- Mountains create **rain shadow effects**, making windward slopes wetter and leeward slopes drier
- **Aspect** changes local sunlight, temperature, and moisture conditions
- Soil properties such as **depth**, **drainage**, **pH**, and **nutrient content** affect local vegetation
- Tropical rainforests can have high productivity but relatively nutrient-poor soils because of **leaching**
- Grasslands often have deep, fertile soils due to dense root systems and organic matter
- **Ecological tolerance** explains why species survive in some conditions but not others
- **Climographs** are interpreted using temperature pattern, precipitation amount, and seasonal distribution
- Warm, wet biomes usually have higher **NPP**, while cold or dry biomes usually have lower **NPP**

## Key Terms and Concepts

- terrestrial biome
- limiting factor
- permafrost
- latitude
- altitude
- rain shadow effect
- aspect
- leaching
- ecological tolerance
- Shelford's law of tolerance
- climograph


Terrestrial Biomes - Terrestrial resource distribution factors