Basically, pioneering species create the conditions for others to grow. Usually they are hardy varieties that can withstand direct sun and poor soil. They start to grow, creating shade for the plants below and transforming the soil with their needles/leaves. They usually eventually die off when the other species of the area have the right ecosystem and become strong enough to survive completely on their own. Sometimes you see a few remain or in areas where the poor soil is a result of the location itself, not an external event like a fire, they remain the dominant species.
There are a number of pioneering species that have been identified. Here is a nice article about them.
Great! thanks so much for the info!