Here's a closer-up (but more blurry) picture of the nurse tree. Because of the rich nutrients and heavy rainfall on the forest floor, there's not much room for a seed to germinate. The fallen trees provide a bit of growing space, sort of the forest high rise nursery. (In fact, the Park Service has a nursery to start plants that they use to cover bare spots at the high altitudes, but none of that is needed in this fertile lowland forest).