Forest regeneration and restoration in Vietnam

Abstract

It is generally thought that asymmetric competition for light and asymmetric growth, determines the course of succession in a regenerating secondary tropical forest, but it is hardly ever quantified. In this study a shrub and three woody tree species were monitored in time during the first five years of succession. Light competition and light-use were related to the success of species. It appeared that in a slow growing vegetation the importance of asymmetric competition for light and growth can be much less than often assumed. The species composition that was present from the very early beginning of succession remained for at least the first five years. Nitrogen-use efficiency may partly determine a plant’s success during forest regeneration. Whole canopy nitrogen-use efficiency and its underlying traits were compared among pioneer species over the first five years of succession. Nitrogen-use efficiency was largely determined by leaf life span and resorption and differed twofold among species. It was however not related to growth rates and only partly to species height. Nitrogen-use efficiency was slightly different within a species between successional stands that differed in height, leaf area index and resource availability, but an increase in competitive pressure did not result in major changes in the use of nitrogen. Forest regeneration is often slow or stagnates due to the excessive growth of non-woody plants and shrubs. One often used method to accelerate succession, called liberation, is opening up the vegetation canopy around young target trees which increases their growth. A 3D-model is used which enables us to examine how stature, crown structure and physiological traits of target trees and characteristics of the surrounding vegetation together determine the growth of trees. The model was applied to a liberation experiment that was conducted on pioneer species in a young secondary forest. Species responded differently to the treatment depending on their height, crown structure and their light demands. The responses were also dependent on the height and density of the vegetation and the gap radius from which it was removed. Another method to enhance regeneration is by planting tree species in the existing vegetation. Lines are cut and overstory thinning is applied to increase light levels. The line width and the degree of overstory thinning affect growth of target species differently depending on the characteristics of the surrounding vegetation and the light demands of the target trees themselves. The same model approach was used to test for the effects of line width, the degree of overstory thinning, planting trees in different successional stands of the same forest and planting trees in a forest stand with different light levels than the stand in which they were originally grown. It was shown that the optimal stand for each species, the optimal line width in a stand and the optimal degree of overstory thinning, can be predicted. With the model approach presented here it is possible to simulate different management activities aimed at improving forest regeneration and it has therefore a large potential in forest restoration ecology.