Many species of animals, such as neotropical birds, adapt to the changing circumstances by picking up and moving - at just about the time students migrate to college campuses. Plants lack that option. Take a close look outside the car window, the next time you are trapped in traffic. Notice the different types of plants along the roadside. Do this several times, and you'll begin to see the pattern.
Some places are treeless, such as the mowed edges of the roads or the medians in divided highways.
Look at the forested areas, and notice how few wildflowers are visible on the ground. The trees are capturing all the sunlight, shading out the forest floor beneath. Along Interstate 95 in the summer, you can see the sweetgum and other broadleaved species are the understory, the young trees underneath the overstory of mature pines. In 50 years or so, those pines will have died - and their replacements will not be more pine tress. Look at old fields in suburbia, after farmers quite raising corn or grazing cattle.