In the southeast US long leaf pine trees have also adapted to frequent fires. The bark is fire resistant and they keep all their needles/branches high above ground, smothering everything that tries to compete with them with dry flammable needles. It is believed that frequent fires were needed to keep the long leaf pine forests stable.
So why are there no long leaf pine forests in the midwest? It sounds like we’re left with decreased precipitation as the only remaining explanation. And the long leaf pines are pretty good at resisting drought with the help of tap roots that extend to underground water sources. Are there no such sources of water in the plains?