The Earth’s terrestrial surface, or “land,” includes the physical environment, soil, and all flora. Classifying this land cover is essential for managing natural resources, conservation planning, and monitoring ecological changes. The most effective way to categorize land is by its dominant vegetation, which is primarily controlled by regional climate, specifically precipitation and temperature.
The Three Primary Categories of Land
Ecologists classify land into three broad categories based on climate and the resulting dominant plant life. These categories are defined by a gradient of moisture availability, which dictates vegetation structure. The first type is forests, dominated by a dense layer of trees supported by high moisture. Next are grasslands, characterized by continuous herbaceous cover in regions with moderate moisture. The final category is arid and semi-arid lands, defined by water scarcity and sparse vegetation.
Land Defined by High Precipitation and Tree Cover
Land with high annual precipitation and mild temperatures supports the growth of forests, ecosystems defined by a dense canopy of trees. These ecosystems store vast amounts of biomass, making them the largest terrestrial reservoirs of carbon. Trees absorb atmospheric carbon dioxide through photosynthesis, sequestering carbon in their biomass and soil. Tropical forests are estimated to store over 200 gigatonnes of carbon, helping mitigate climate change.
Forests also regulate the global hydrological cycle, influencing water quantity and purity. The dense canopy intercepts rainfall, and extensive root systems enhance soil porosity, promoting water infiltration and recharging groundwater. Evapotranspiration releases moisture back into the atmosphere, contributing to local precipitation. Forest soil acts as a natural filter, trapping sediments and absorbing pollutants.
Land Defined by Moderate Precipitation and Grass Cover
Grasslands, savannas, and rangelands receive moderate precipitation, which is too low to support widespread tree growth but sufficient for continuous grass cover. These ecosystems are often maintained by natural disturbances, such as grazing by herbivores and periodic fires. Grazing and fire prevent woody plants from establishing, allowing grasses to dominate the landscape. The deep, fibrous root systems of grasses are adapted to quickly regrow after a fire, ensuring the ecosystem’s resilience.
The continuous cycle of grass growth and decay creates a distinctive, highly fertile soil known as Chernozem, or “black earth.” This soil is characterized by a deep, dark topsoil layer rich in humus, often containing 4% to 16% organic matter. Chernozem’s granular structure provides excellent aeration and moisture storage capacity, and the high concentration of nutrients makes these soils productive. These fertile lands form the backbone of global grain production in regions like the North American Prairies and the Eurasian Steppe.
Land Defined by Low Precipitation and Sparse Vegetation
The final category includes arid and semi-arid lands, where vegetation is sparse due to a lack of usable water. This category encompasses both hot deserts, which lack liquid water, and cold environments like the tundra, where water is locked in permafrost most of the year. Plants that survive in these conditions are known as xerophytes, which have evolved specialized features to tolerate physiological drought. Their adaptations focus on maximizing water uptake or minimizing water loss.
In hot deserts, xerophytes like cacti possess thick, waxy cuticles, reduced leaf surface area, or leaves modified into spines to curtail transpiration. They may have shallow, widespread root systems to capture light rainfall quickly, or deep taproots to reach groundwater. Tundra plants are typically low-growing with small leaves, which helps them retain moisture and shelter from desiccating wind. The harsh environmental factors in both deserts and tundra result in low species diversity, as only highly adapted organisms can survive.