Although tropical rainforests can regrow under natural conditions, where clearings are caused by floods, storms, or treefalls, they cannot regenerate under many current land-use practices. Forests regenerate poorly if at all when large areas are logged, the soil is damaged or removed, or erosion is severe. Rainforest ecosystems are vulnerable to disruption because of their internal complexity and interdependence. Because there are few individuals of any one species, removal of even small numbers of them has a substantial effect on species composition and interrelationships in the forest by depressing reproduction, and long periods of time will be required to reconstruct viable populations. Forests are also dependent upon their closed nutrient cycles; disruption of these cycles by exposure and extraction of trees can cause their destruction.
If the land has been cleared for a long period of time, reforestation occurs slowly or not at all, due to the lack of suitable seeds and seedlings, increased ground temperatures, decreased humidity, and the like. Near the temples at Angkor Wat in Cambodia, the forest, which has been regrowing since the destruction of the Khmer empire in 1431 A.D., is still not the same as the surrounding original forest. The same is true of the forest which has regrown in the aftermath of the collapse of the Mayan and Aztec civilizations more than 500 years ago. It may take one thousand years for a rainforest to recover its original level of biomass after clearing and burning, since few seeds and seedlings of rainforest plants can survive burning. This is in marked contrast to temperate forests, which may regenerate within 100 years. In areas where the soil is compacted, as occurs when heavy logging machinery is used, there is little recovery of rainforest trees, even after decades, since compacted soil inhibits root growth. In Malaysia, a field which had been abandoned in 1944 was not colonized by any dipterocarp (the huge hardwoods which dominate Southeast Asian tropical rainforests) until 1976. Also, in Brazil, abandoned cattle pastures in Para state reforested to some degree, but climax forest was not attained, since there were no seed trees remaining in the vicinity (Whitmore, 1998).
Southeast Asian forests are dominated by many species of dipterocarps. Dipterocarps are quite varied, but many species produce seeds synchronously every three to six years (“mast” fruiting). Since dipterocarps are highly desired for timber, logging of these forests has been intense and largely uncontrolled. Fragmentation has caused changes in tree density, distribution, seed production, and climate, and has greatly reduced the ability of the dipterocarps to reproduce. In a recent study in a national park in Borneo (a protected area surrounded by logged forest), fewer species than previously were able to produce viable seeds, and no new seedlings were found several months after fruiting. This is in comparison with 155,000 seedlings per hectare found in 1991 (Hartshorn & Bynum, 1999; Curran, et al., 1999). Obviously, reproductive failure of the major tree species in the forest will substantially alter the proportions of species and entire ecosystems dependent upon them.