The vast grasslands of the world—known as prairies, steppes, and pampas—share a defining characteristic: a lack of trees across their expansive, flat terrain. This is an ecological puzzle, as the soil in many of these regions is incredibly rich and fertile, a condition that might otherwise support forests. The absence of widespread woodlands is not due to a single factor but a dynamic interplay of climate, disturbance, and competition. This results in a landscape dominated by deep-rooted, resilient grasses, maintained by environmental forces that actively suppress tree growth.
The Primary Constraint of Water Stress
The most significant barrier to tree growth across the plains is a severe water deficit created by the regional climate. While annual precipitation in the Great Plains ranges from approximately 425 mm in the west to 695 mm in the east, this moisture is often insufficient for tree survival. The climate is characterized by high rates of potential evapotranspiration (PET), which often exceed annual precipitation, creating a substantial moisture debt. In semiarid parts of the southern Great Plains, PET can be as high as 1,500–1,650 mm annually, far surpassing the actual rainfall.
This gap between water supply and atmospheric demand is exacerbated by intense solar radiation and persistent, high winds typical of the open plains. Wind dramatically increases the rate of water loss from plant leaves through transpiration, desiccating young, shallow-rooted tree saplings. Grasses exploit surface moisture aggressively during brief wet periods, thriving on immediate gains. Trees require a long-term water supply to sustain a large woody structure and are quickly overwhelmed by the drought conditions that grasses are adapted to endure.
The Active Role of Fire and Herbivory
Beyond climatic limitations, the treeless nature of the plains is actively maintained by two major historical disturbances: fire and herbivory. Natural wildfires, often ignited by lightning, historically swept across the grasslands with high frequency, moving quickly across the continuous fuel of dried grass. These fast-moving, low-intensity surface fires destroy the above-ground woody tissue of tree seedlings, which are poorly protected by thin bark. Grasses, having most of their biomass and vulnerable growth points safely underground, are merely top-killed and rapidly regrow from their deep root crowns.
Herbivory by large grazing animals, such as the North American bison, further suppressed the survival of tree saplings. Grazers selectively browsed on the tender shoots and leaves of woody plants, pruning and destroying the young trees before they could grow tall enough to escape the animals’ reach. This interaction created pyric herbivory, where grazers were attracted to recently burned patches due to the flush of nutritious new grass growth. The intense grazing pressure helped prevent the establishment of woody vegetation, reinforcing the dominance of the grassland ecosystem.
Soil Structure and Root Competition
The physical characteristics of the plains soil impose constraints that favor grasses over trees. Grasses develop dense, fibrous root systems that form a thick mat within the top layers of the soil, often extending several meters deep. This extensive network rapidly absorbs nearly all available moisture as soon as it penetrates the surface, monopolizing the water before deeper-rooted tree seedlings can access it. Tree roots, which typically consist of a single, slow-growing taproot system in saplings, are outcompeted for the limited surface water.
In many semiarid plains regions, the soil profile contains a hardened layer called caliche, or calcrete. Caliche is a dense layer of calcium carbonate that physically impedes the downward growth of tree taproots. This hard layer forces tree roots to grow laterally in the shallow, drier topsoil, preventing them from accessing deeper, more stable water reserves. This restriction confines the tree’s root system to the same zone where grass roots are most competitive, intensifying the water stress and reducing the tree’s physical stability.
Localized Exceptions to the Treeless Landscape
The presence of trees in localized areas highlights the importance of the environmental constraints that govern the treeless landscape. Along rivers and streams, trees readily form dense stands known as riparian zones, often dominated by species like cottonwoods, willows, and hackberries. These areas bypass the primary constraint of water stress by having constant access to groundwater, nullifying the regional moisture deficit. The reliable water source allows tree roots to establish and grow without competition for surface rainfall.
In modern times, human intervention has created other pockets of tree growth, most notably shelterbelts or windbreaks. These long, narrow strips of planted trees reduce the desiccating effect of the strong plains winds on crops and homesteads. By reducing wind speed, the trees lower the rate of evapotranspiration for themselves and the plants in their lee, locally mitigating a major climatic stressor. These micro-environments demonstrate that if the primary limiting factors—water deficit, fire, and wind exposure—are removed or substantially reduced, trees can survive and flourish in the fertile soils of the plains.