Grasslands, expansive areas characterized by their dominant cover of grasses, exist across every continent except Antarctica. These ecosystems typically form transitional zones between forests and deserts. While forests support dense tree growth and deserts sustain sparse vegetation, grasslands present a landscape where trees are notably absent or scattered. This lack of woody plants raises a question about the environmental factors that shape these grassy plains.
Rainfall and Temperature
Rainfall patterns significantly influence the distribution of trees within grasslands. Grasslands receive moderate precipitation, ranging from 500 to 950 millimeters (approximately 20 to 35 inches) annually. This amount supports grasses but often falls short of the higher water requirements for extensive tree growth. Rainfall in these regions is also frequently seasonal, with concentrated wet periods followed by prolonged dry spells. This limits the consistent moisture supply trees need to establish and thrive.
Temperature extremes further challenge tree survival. Temperate grasslands, for instance, can experience summer temperatures exceeding 38°C (100°F) and winter lows plunging to -40°C (-40°F). Such wide temperature fluctuations can stress or kill young tree saplings, which are vulnerable to desiccation during hot, dry periods or frost damage in severe cold. Grasses, conversely, are well-adapted, often going dormant during unfavorable periods and quickly rebounding when conditions improve.
Soil and Root Systems
The characteristics of grassland soils and the specialized root systems of grasses also exclude trees. Grassland soils are often deep and dark, enriched with nutrients from decaying grass roots. Grasses develop dense, fibrous root systems that form a thick, interwoven mat, typically extending a few inches to a foot deep. This intricate network efficiently captures moisture and nutrients near the surface.
Trees, especially young saplings, struggle to compete with this dense grass root mat for essential resources like water and nutrients. Most tree feeder roots are concentrated within the top 12 inches of soil, directly competing with the efficient grass roots. This intense competition often leaves insufficient resources for tree seedlings to establish and grow. Some grasses release chemicals that inhibit the growth of other plants, including tree saplings, through allelopathy.
Fire and Herbivore Influence
Natural disturbances like fire and grazing animals are important for maintaining grassland ecosystems. Grasslands are fire-prone environments, with fires frequently ignited by lightning or, historically, by indigenous populations. These fires typically burn rapidly, consuming woody vegetation and preventing the encroachment of trees and shrubs.
Grasses are resilient to fire because their growth points are located underground, allowing them to quickly regenerate after a burn. Fire also stimulates new grass growth and cycles nutrients back into the soil, benefiting the ecosystem. Large grazing animals, such as bison and various antelope species, further contribute to keeping grasslands treeless. These herbivores selectively browse on young tree saplings, pruning them and preventing their maturation. Their grazing also stimulates grass growth and helps maintain the open, grassy structure of the landscape.