Why haven't weeds overtaken the entire planet?

Question

Given how rapidly weed plants spread and grow, choking out all other plant life, how come after millions of years we haven't ended with forests full of thistle or pokeweed, as opposed to pines or oak trees?

Answer 1

Well, each forest is different. Often, there are invasive species (what some might consider weeds) that thrive in a new environment that they have infiltrated. Numerous factors allow for a weed to thrive or not to thrive. Like weed control or forest management, human interference can be major prevention for non-native species' success. Weeds are often so successful because of how adaptive they are to their environment and the fact that they tend to be more generalists in their environmental requirements. Both animal and plant species rely on the area around them to allow for further growth. So really the fact that plants like thistle or pokeweed have not overtaken a majority of the world's forests is not such a surprise.