One important reason is that the complex ecology of the prairie environment is formed by a cycle of fires and regrowth of a succession of fire-tolerant plants, and trees do not survive the periodic fires.
The other limiting factor is a dry climate, affecting trees more than grassland plants, which hoard water.
The fires sustained the prairies by clearing dead thatch, fertilizing the soil, and destroying trees. Perennial grasses, with their extensive root systems forming an underground sponge, survived both fires and dryness.
In efforts to restore remnants of the tallgrass prairie of the Middle West, ecologists have employed controlled burning to destroy invading trees that suppressed the original vegetation. These fire-adapted species included goatgrass, rattlesnake fern, ebony spleenwort, lady’s tresses, and foxtail barley.
Before European settlement, the tallgrass ecosystem covered more than 220,000 square miles, from Canada into Texas and from Nebraska to the Great Lakes.
In the dry season of midsummer, most of the fires there were probably caused by lightning. In spring and fall, most were probably set by Indians.
The quick new growth was grazed upon by the great herds of buffalo, who left behind dead grasses and stalks that fed the next round of fires.