The most likely reason for grasslands having grasses rather than trees as their dominant plant species is the combination of environmental factors such as relatively low and seasonal rainfall, frequent fires, grazing by large herbivores, and periodic droughts. These factors hinder the establishment and growth of trees and woody shrubs, while grasses are well adapted to survive and thrive under these conditions due to their growth form, deep and widespread root systems, and ability to regenerate after fire and grazing.
Environmental Conditions Favoring Grasses
- Grasslands typically occur in regions where rainfall is insufficient to support dense tree growth but enough to sustain grasses. The generally limited water availability combined with seasonal droughts limits tree establishment.
- Fires, which are frequent in many grassland ecosystems, especially temperate and tropical grasslands, suppress tree seedlings and enable grasses to regenerate quickly due to their growth adaptations.
- Grazing by large herbivores further prevents the dominance of trees by consuming woody seedlings and promoting grass dominance.
Adaptations of Grasses
- Grasses have extensive fibrous root systems that allow them to access soil moisture efficiently and recover from drought conditions.
- Grasses can regrow rapidly after fire or grazing because their growing points are at or below ground level, which protects them from damage.
Summary
In essence, the combination of limited water availability, disturbance from fire and herbivory, and the physiological and morphological adaptations of grasses make them the dominant plant species in grasslands instead of trees.
This explains why grasslands are characterized by dense grass vegetation and sparse tree presence, which is typically confined to river valleys or other localized favorable conditions.
