Plains are largely defined by their lack of continuous tree cover, maintained by environmental forces. While a plain is geographically a region of low relief, tree presence is determined by specific climate and disturbance regimes. Plains are home to large ecosystems, such as the North American prairie and the Eurasian steppe, dominated by herbaceous plants. Exceptions occur only where localized conditions overcome these ecological limits.
Defining Plains and Grassland Biomes
A plain is a broad, flat expanse of land with minimal changes in elevation. The ecological biome covering these plains is typically grassland, defined by vegetation consisting predominantly of grasses and other non-woody plants. These biomes cover approximately one-fifth of the Earth’s land surface and are known regionally as the North American prairie, the South American pampas, and the African veld. Grasses thrive here because their unique life history allows them to outcompete woody plants.
Grasses have growth points located near the soil surface, which protects them from grazing animals and environmental disturbances. Their extensive, fibrous root systems rapidly capture water and nutrients from the upper soil layers. This allows grasses to complete their life cycles quickly, ensuring survival in regions with seasonal or limited moisture. Tree seedlings struggle to establish themselves against this dense, competitive root mat.
Ecological Constraints on Tree Survival
Tree survival across plains is primarily limited by low annual precipitation combined with high rates of evaporation. Grasslands typically receive 10 to 30 inches of rain annually, which is insufficient to support the deep, sustained water needs of large woody plants. Trees require significantly more water over a longer growing season, and the high winds common on open plains further increase water loss through transpiration.
Frequent fire is a powerful, naturally occurring disturbance that actively prevents tree encroachment. Grasslands burn quickly and intensely; the fire moves fast enough not to destroy deeply rooted perennial grasses but kills the above-ground parts of woody saplings. Tree seedlings and young trees cannot develop the thick, protective bark necessary to survive the frequent fire return intervals common to these biomes. This dynamic favors fire-resistant grasses and suppresses succession to a forest.
Soil structure restricts the growth of large trees. Many grassland soils feature a dense, interwoven mat of grass roots that effectively blocks the downward growth of tree taproots, which are necessary for accessing deeper water reserves. The high organic matter content sequestered in the topsoil by decaying grass roots creates a complex soil environment. This dense upper layer can inhibit the initial establishment of tree roots, contributing to the dominance of herbaceous vegetation.
The Role of Waterways and Topography
Trees can flourish in specific microclimates that overcome the limitations of the surrounding plain. These exceptions often occur along waterways, creating riparian zones. Rivers, streams, and associated floodplains provide a consistently high water table, guaranteeing the reliable, shallow water access that trees require.
These water-fed areas support dense stands of drought-intolerant species, such as cottonwood, willow, and ash, forming “gallery forests” that contrast sharply with the treeless plain. The alluvial soils deposited by flooding are often richer and deeper, giving tree roots a further advantage. Localized changes in topography can also offer protection from the elements.
Ravines, steep slopes, or sheltered depressions provide microclimates that shield young trees from the full force of the wind and fire. Reduced wind speed lowers the rate of evaporation, conserving soil moisture long enough for saplings to establish. These sheltered areas may also act as natural firebreaks, allowing trees to reach a height where their crown is above the flame zone, escaping the destructive effects of a surface fire.
Distinguishing Between Grasslands, Steppes, and Savannas
The term “plain” encompasses several distinct biomes that vary in tree density. True grasslands, such as the North American prairie and the Eurasian steppe, are characterized by extremely low tree cover, often featuring fewer than one tree per acre outside of riparian zones. Steppes are typically drier and colder, with precipitation dispersed throughout the year, which severely limits woody plant growth.
In contrast, savannas are a type of plain that explicitly includes a scattered tree population, making them a mixed woodland-grassland ecosystem. Savannas receive higher total rainfall than steppes, but this moisture is concentrated in a distinct wet season followed by a prolonged dry season. This seasonal water availability supports isolated, often fire-resistant trees, but does not permit the closed canopy and continuous forest growth seen in wetter biomes.
Tree density in savannas can range widely, from areas with only a few widely spaced trees to areas where density is high, provided the canopy remains open. While most plains (prairies and steppes) are ecologically treeless, the savanna confirms that some plains support a significant population of trees. This illustrates that the ecological limits are a matter of degree, not an absolute rule.