Dutch ecologist Marijke van Kuijk has studied the regeneration of the tropical forest in Vietnam. Abandoned agricultural land does regenerate to tropical forest, but only slowly. Two procedures are used to help nature along: pruning of foliage to free up space for trees and planting the desired tree species. Van Kuijk used the PHOLIAGE model to calculate the appropriate measures.
People in the tropics depend heavily on the products and services the forest supplies. However, the natural regeneration process from agricultural land to forest often stagnates at the scrub stage. Some plants and shrubs grow vigorously and become dominant as a result of which young trees do not receive enough light to grow.
Cutting free
Cutting young trees free generally results in increased growth. Van Kuijk discovered that the response of trees to an opening in the vegetation varied among species. This was related to the tree height, the leaf surface, the dimensions of the crown and the amount of light the trees needed. The ideal size for the opening in the surrounding vegetation varies for each species and depends on the height and density of the vegetation. The PHOLIAGE model can predict tree growth accurately. This makes it possible to determine per tree and per forest the best timing, the best opening and the effects of cutting free.
Planting
The PHOLIAGE model can also be used when scheduling the planting of new trees. The success of planting depends on factors such as exposure to light by the existing vegetation, tree species, et cetera. In general, the calculations indicated that shade-tolerant species achieve maximum growth faster (with less intervention) than photophilic species. However, it is not always desirable to open up the vegetation to such an extent that all tree species can reach their maximum growth. That can be at the expense of the existing forest and requires a lot of work. The PHOLIAGE model calculates the amount of growth increase per planting, given a particular opening size.
Secondary forests
Forests start to regenerate after agricultural land has been abandoned. The resulting vegetation is termed "secondary forest". The vegetation is dominated initially by non-ligneous plants and shrubs, which are replaced within a few years by pioneer trees. After several decades, the pioneers die off, giving the climax species the opportunity to grow and later form a forest. This process, where species replace each other over time, is called "succession" and is the natural process by which a forest regenerates. Often this regeneration process stagnates during the early stages of succession. Non-ligneous plants and shrubs grow vigorously and become dominant, with young trees not receiving sufficient light to grow. What remains is scrubland of little biological, economic, social or cultural value.