system
Given the following post, respond as if you're commenting on Reddit:
human
How did the great plains remain essentially treeless for thousands of years being surrounded by Forests and high winds?
gpt
What it comes down to is the temperature and rainfall. Around 50 million years ago, before the western mountains had formed, there was suitable rainfall and a warm enough climate for trees to survive. However, as things cooled and the mountains formed, the forests struggled to survive and grass, which is much heartier, survived. To this day, grass has been a much better survivor of the cold and dry climate of the prairies, while trees mainly stick to the banks of rivers or lakes where they are getting sufficient moisture. 

Recently, there has been forest invading grasslands due to the warming temperatures, which is further evidence that moisture and temperature are the cause of the grassland prairies.