The Understory

REGROWTH AFTER RAINFOREST CLEARING

(continued)

When Europeans cleared the rainforest for agriculture, they probably assumed that the forests would regrow relatively quickly like the temperate forests of Europe and North America. But rainforest will not readily return on lands with agricultural monocultures that have been devoid of forest for several years and have highly degraded soils. Tropical soils rapidly become inhospitable to growth due to swift leaching of nutrients caused by heavy rains and intense sunlight. In addition, many tropical hardwood trees are dependent on certain animal species for pollination, seed dispersal, and seed processing. The seeds of many tropical rainforest species are large (since they germinate in the shade of the canopy and must have enough food reserves to grow in the low light conditions of the forest floor) and require animal dispersers (wind or other mechanical means often are not sufficient for dispersing seeds of this size). The loss of these dispersal species when forest is leveled, means tree seeds are unlikely to be dispersed into cleared areas. Therefore these important forest tree species will not return.

The seeds and seedlings of those tree species not limited by animal dispersal and pollination agents are often specifically adapted to the light and climate conditions of the shaded rainforest floor. These seeds usually will not germinate in the hot, arid conditions of clearings, and those that do sprout rapidly succumb to the sun and poor soils. Tropical rainforest plants are accustomed to the stability of the rainforest, where they are robust. When they and their seeds are confronted with the entirely different set of conditions presented by forest clearing, they do not fare well.Their seeds have little or no capacity to remain dormant since they have no need under normal forest conditions.

The dry air of the forest clearing also desiccates the leaf litter causing the mycorrhizae to die. The elimination of the symbiotic mycorrhizae reduces the capacity of trees to take up nutrients from the soil. This fungi is especially difficult to replace since each species of tree may have its own symbiotic species of mycorrhizae. Regeneration is further stunted by the rapid encroachment of tough grasses and shrubs after the clearing of forest.

The situation is different when a cleared area is left surrounded by forest and the soils have not be decimated. Under these conditions, forest may rapidly reclaim the barren patch as fast-growing, weedy pioneer plants like forest grasses, bananas (Heliconia), gingers, and vines move into the clearing. Pioneer tree species—which require such conditions of bright sunlight and lower humidity for growth—like cecropia (Neotropics) and macaranga (Asia) quickly colonize forest gaps. After several years, the small number of pioneer plants and trees like kapok and cecropia are gradually replaced with diverse species more characteristic of older-growth primary forest. The necessary mycorrhizae fungi can recolonize from the surrounding forest and facilitate nutrient uptake. After 20 years the formerly cleared land may again support vigorous growth, although this forest is only a shadow of the original primary forest. The new secondary forest, is far less diverse, has a less developed canopy, fewer animals, and thicker ground vegetation. It is unknown how long it takes for secondary forest to return to the complexity of primary forest, but the estimates range from hundreds to thousands of years.

REGROWTH AFTER CLEARING

When Europeans cleared the rainforest for agriculture, they probably assumed that the forests would regrow relatively quickly like the temperate forests of Europe and North America. But rainforest will not readily return on lands with agricultural monocultures that have been devoid of forest for several years and have highly degraded soils. Tropical soils rapidly become inhospitable to growth due to swift leaching of nutrients caused by heavy rains and intense sunlight. In addition, many tropical hardwood trees are dependent on certain animal species for pollination, seed dispersal, and seed processing. The seeds of many tropical rainforest species are large (since they germinate in the shade of the canopy and must have enough food reserves to grow in the low light conditions of the forest floor) and require animal dispersers (wind or other mechanical means often are not sufficient for dispersing seeds of this size). The loss of these dispersal species when forest is leveled, means tree seeds are unlikely to be dispersed into cleared areas. Therefore these important forest tree species will not return.

The seeds and seedlings of those tree species not limited by animal dispersal and pollination agents are often specifically adapted to the light and climate conditions of the shaded rainforest floor. These seeds usually will not germinate in the hot, arid conditions of clearings, and those that do sprout rapidly succumb to the sun and poor soils. Tropical rainforest plants are accustomed to the stability of the rainforest, where they are robust. When they and their seeds are confronted with the entirely different set of conditions presented by forest clearing, they do not fare well.Their seeds have little or no capacity to remain dormant since they have no need under normal forest conditions.

The dry air of the forest clearing also desiccates the leaf litter causing the mycorrhizae to die. The elimination of the symbiotic mycorrhizae reduces the capacity of trees to take up nutrients from the soil. This fungi is especially difficult to replace since each species of tree may have its own symbiotic species of mycorrhizae. Regeneration is further stunted by the rapid encroachment of tough grasses and shrubs after the clearing of forest.

The situation is different when a cleared area is left surrounded by forest and the soils have not be decimated. Under these conditions, forest may rapidly reclaim the barren patch as fast-growing, weedy pioneer plants like forest grasses, bananas (Heliconia), gingers, and vines move into the clearing. Pioneer tree species—which require such conditions of bright sunlight and lower humidity for growth—like cecropia (Neotropics) and macaranga (Asia) quickly colonize forest gaps. After several years, the small number of pioneer plants and trees like kapok and cecropia are gradually replaced with diverse species more characteristic of older-growth primary forest. The necessary mycorrhizae fungi can recolonize from the surrounding forest and facilitate nutrient uptake. After 20 years the formerly cleared land may again support vigorous growth, although this forest is only a shadow of the original primary forest. The new secondary forest, is far less diverse, has a less developed canopy, fewer animals, and thicker ground vegetation. It is unknown how long it takes for secondary forest to return to the complexity of primary forest, but the estimates range from hundreds to thousands of years.

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