Primary and secondary forests

Primary forests are frequently perceived as undisturbed forest in a late stage of succession, but the concept should include the associated younger successional stages. In contrast, secondary forests are defined as forests regenerating largely through natural processes after significant human disturbance of the original forest at a single point in time or over an extended period and displaying a major difference in forest structure and/or canopy species composition with respect to nearby primary forests on similar sites. Because of the large scale and intensity of the disturbance, secondary forests pass through stages in a succession series that is different from the typical forest vegetation of that site.

While conservationists traditionally focus on primary forests for their biodiversity and natural processes, there is much recent attention for secondary forests, not in the last place because primary forests are more and more scarce. This comes with the appearance of large and growing areas of secondary forests in areas formerly used for agriculture or intensive logging. Recent estimates indicate the existence of 500-600 million ha of secondary forests in the tropics. It is widely expected that future goods and services from tropical forests will increasingly be met by secondary forests (in addition to plantations), including biodiversity conservation. There are several types of secondary forest. Among the most significant are the areas created by commercial logging and secondary forests that grow on land that has been used by shifting cultivators. Shifting cultivators often create secondary forest gardens by planting fallow areas with trees that provide fruits, nuts, resins and other products. Fire leads to yet another type of secondary forest succession.

Being the product of a heavy disturbance, secondary forests undergo a succession along the lines described in the section on forest dynamics. In the literature, three general phases of succession in secondary forests are distinguished. In the first phase, colonisation occurs by a dense growth of herbs, shrubs and climbers. Within 1-2 years, these will be replaced by short-lived pioneers: trees with a life span of 10-30 years. These form an even canopy, under which seedlings of long-lived pioneers and climax species slowly grow. After the death of the short-lived pioneers, these will form a canopy which is still relatively uniform and even-aged, and which may transform into a forest resembling mature forest after possibly 75 to 150 years. During this succession, species richness may build up quite rapidly and reach levels comparable to mature forests in 20 to 80 years. However, species composition is quite different from mature forests, mainly through the abundance of short- and long-lived pioneers, which are naturally scarce in mature forest.

TBI sources

Vester, H.F.M. (1995). The trees of the forest. The role of tree architecture in canopy development; a case study in secondary forest (Araracuara, Colombia).

Further reading

GTZ report on the significance of secondary forests
CIFOR secondary forests