Abiotic factors play a key role in determining the distribution of terrestrial biomes.

Water is essential for photosynthesis, transpiration, and maintaining cell turgidity.

Sunlight is required for photosynthesis.

Since photosynthesis is a chemical process, temperature influences its rate of progression.

The rate of photosynthesis impacts an ecosystem's productivity (NPP), and more productive biomes tend to have higher NPP.

Rainfall, temperature, and insolation regulate the rate of photosynthesis, which in turn shapes the structure, function, and distribution of biomes.

For specific temperature and rainfall patterns, a particular natural ecosystem type is likely to develop.

Tropical rainforests experience consistent high temperatures (around 26°C) and high rainfall (over 2500 mm per year) year-round.

Located in a band around the equator, between the Tropics of Cancer and Capricorn (23.5° N and S), tropical rainforests enjoy high sunlight levels throughout the year.

There is little seasonal variation in sunlight and temperature, though the monsoon season may reduce insolation, providing an all-year growing season.

The rainforest's location at low latitudes, with the Sun directly overhead, results in optimal climatic conditions for high levels of photosynthesis and NPP.

Tropical rainforests are responsible for around 40% of the total NPP of all terrestrial ecosystems.

These rainforests are broad evergreen forests with exceptionally high biodiversity, hosting up to 480 tree species per hectare, compared to just six species in temperate forests.

High plant diversity results from year-round high rainfall and sunlight, leading to high productivity.

The forest's complex structure, with multiple layers and various niches, supports a diverse range of animals.

Despite their productivity, the soil in tropical rainforests is low in nutrients, as most inorganic nutrients are locked in the trees.

Nutrient cycling in the rainforest is rapid, with nutrients sourced from decaying matter on the forest floor. If decay rates are high, the forest can maintain growth.

Leaching, caused by heavy rainfall, can wash nutrients from the soil, limiting primary production.

Tropical rainforest soils are thin, so trees have shallow roots, with one long taproot and wide buttresses for support.

The canopy helps protect soils from heavy rainfall, but logging and deforestation cause rapid soil erosion, making it difficult for forests to recover.

It may take around 4000 years for a logged area to regain its original biodiversity.

Temperate forests are located between 40° and 60° N of the equator and are found in seasonal climates with cold winters and warm summers, unlike tropical rainforests, which have consistent conditions year-round.

These forests feature two types of trees: evergreens (retain leaves year-round) and deciduous (shed leaves in winter).

Evergreen trees are adapted to cold winters with thicker leaves or needles, while deciduous trees shed their leaves to prevent frost damage during winter.

Temperate forests may contain only one type of tree (evergreen or deciduous) or a mix of both.

The amount of precipitation in these regions (between 500 and 1500 mm per year) determines whether forest or grassland develops.

Insolation variation throughout the year, caused by Earth's tilt and rotation around the Sun, results in a limited growing season, reducing productivity compared to tropical rainforests.

Due to the mild climate, lower average temperatures, and lower rainfall, photosynthesis and productivity in temperate forests are less than in tropical rainforests.

Temperate forests have the second highest NPP (after tropical rainforests) among biomes.

While biodiversity is lower than in tropical rainforests, temperate forests are simpler in structure and often dominated by one species, with up to 90% of the forest consisting of just six tree species.

The vertical stratification in temperate forests is limited, with trees typically not growing taller than 30 meters.

The simpler structure of temperate forests offers fewer niches, resulting in lower species diversity compared to tropical rainforests.

The forest floor has a thick leaf layer that breaks down quickly when temperatures rise, providing adequate nutrients.

The less dense canopy in temperate forests allows more light to reach the forest floor, supporting a variety of shrubs and plants like brambles, grasses, bracken, and ferns.

Deserts are located in bands around 30° N and S latitudes, covering 20–30% of the land surface.

These areas are dry because air at these latitudes descends after losing moisture over the tropics.

Hot deserts experience high temperatures, often reaching 45–49°C in the early afternoon, and have very low precipitation, typically under 250 mm per year.

Rainfall is often unevenly distributed, and the lack of water limits photosynthesis, resulting in low NPP.

Temperature fluctuations are significant, with nighttime temperatures dropping as low as 10°C, sometimes even to 0°C, making survival difficult.

Due to low productivity, vegetation is sparse, and soils are typically rich in nutrients because they aren’t leached away by water.

Decomposition is slow due to the dry conditions and lack of moisture.

Species in deserts are highly adapted with xerophytic adaptations to minimize water loss in dry conditions.

• For example, cacti have reduced surface area for transpiration by converting leaves into spines and store water in their stems, which can expand to hold more water.

• The surface area: volume ratio of cacti is low, further reducing water loss.

• Spiny leaves deter animals from consuming the plants and accessing stored water.

• Xerophytes also have thick cuticles to minimize water loss.

• Their roots are both deep (to access underground water) and extensive near the surface (to absorb precipitation before it evaporates).

Animals in deserts are also adapted to the harsh conditions:

• Snakes and reptiles are common, as their cold-blooded metabolism helps them conserve water.

• Mammals are adapted to live underground and emerge during the cooler parts of the day.

Tundra is found at high latitudes with low insolation and short day lengths, limiting sunlight.

Water availability is restricted as it may be locked in ice for months, and combined with low rainfall, it becomes a limiting factor.

Low light intensity and rainfall result in low rates of photosynthesis and productivity.

Temperatures remain very low for most of the year, affecting photosynthesis, cellular respiration, and decomposition, as these processes slow down in cold conditions.

Soil may be permanently frozen (permafrost), limiting water availability.

Low temperatures also slow nutrient recycling, leading to the formation of peat bogs, which store large amounts of carbon.

Vegetation consists mainly of low scrubs and grasses.

Most of the world’s tundra is located in the North Polar region, known as Arctic tundra, with small areas in parts of Antarctica (ice-free zones) and high-altitude mountains (known as alpine tundra).

Winter temperatures can reach as low as -50°C, causing life activity to slow significantly.

In summer, the tundra experiences nearly 24 hours of sunlight, increasing insolation and temperatures, promoting plant growth.

The growing season is short, lasting only about 6 weeks before temperatures drop again, and sunlight hours decline.

Vegetation is mainly composed of small plants, as the soil is too shallow for trees to grow and permafrost remains only a few centimeters below the surface.

During the summer, animal activity increases with higher temperatures and primary productivity.

Plants are adapted with leathery leaves or underground storage organs to conserve water and endure cold conditions.

Animals have adaptations for cold, such as thick fur, and are often larger than their southern counterparts (e.g., the Arctic fox is larger than the European fox) to reduce heat loss.

Tundra is the youngest biome, having formed after the retreat of glaciers between 15,000 and 10,000 years ago.

Grasslands are found on every continent except Antarctica, covering about 16% of the Earth’s surface.

They develop in regions with insufficient precipitation to support forests but enough to prevent the formation of deserts.

There are different types of grasslands:

• Temperate grasslands, such as the Great Plains and Russian Steppes.

• Tropical grasslands, like the savannahs of East Africa.

Grasslands occur at the intersection of the Polar and Ferrel cells, where mixing of cold polar air and warmer southerly winds increases precipitation compared to polar and desert regions.

Rainfall is generally balanced with evaporation rates in grasslands.

Decomposing vegetation forms a nutrient-rich mat, but decomposition is slow due to the cooler climate.

Grasses grow beneath the surface and can remain dormant during cold periods (especially in more northern grasslands), until the ground warms up.