Pioneers live on the bare terrain, not many species but a dense tangle of stems, almost impossible to penetrate. The colour of this vegetation is light green, due to its rapid growth. Such a blanket may become too heavy for the weak, secondary tree species which carry it, whereupon they die . Under the blanket, branches die because they do not have light. Where first there was a dense mass of stems and leaves, there is now a hollow place with an even microclimate of shade and humidity.
Litter drops and is again turned into humus, and other species begin to grow.
After some centuries, the old secondary forest begins to resemble the surrounding primary forest so closely that only an expert, by looking carefully at the species in the secondary forest, can tell the difference. We know how long this takes by studying forests abandoned in historical times, such as the forests surrounding Maya and Aztec ruins in Central America, in Angkor in Cambodia, and in the former kingdom of Blambangan in eastern Java. Learning to recognise when a forest is 'primary' or 'secondary' is, of course, vital in deciding which rainforest areas should be conserved. One might say that the secondary forest acts like a scab on a healing wound in the canopy. Any bigger gap will first fill with pioneers, which are the first stage of the 'scab'.