Understanding and assisting natural regeneration processes in degraded seasonal evergreen forests in northern Thailand

Abstract

The Thai government has recently embarked upon a nation-wide project to restore degraded forests. One approach could be to assist natural regeneration (ANR) by counteracting particular limiting factors, such as insufficient dispersal of tree seeds into cleared areas, lack of beneficial shade or excessive competition from weeds. This paper describes part of a 2-year project in northern Thailand which analysed, stage by stage, the regeneration from seed of a range of tree species in an abandoned agricultural clearing to identify limiting factors and develop appropriate ANR techniques to overcome them. Fruit production, seed dispersal, seed germination and seedling survival were monitored in the field. Experiments on selected species were carried out in the field and nursery to determine the effects of high light and low moisture on seed germination and the effect of above-ground weed interference on seedling performance in the first year. Three species are compared here to demonstrate how systematic study of regeneration processes can be of use in devising strategies to accelerate tree regeneration in deforested areas. Despite high levels of production of Beilschmiedia sp. seeds, the low rate of seed dispersal limited seedling recruitment in the clearing. In addition, seed germination was sharply reduced by lack of rainfall and the seedlings were highly susceptible to scorching by direct sunlight. Raising seedlings in nurseries and planting them out in degraded areas under the shade of existing herbaceous vegetation may be a suitable method of accelerating the regeneration of this species. Prunus cerasoides seeds were produced abundantly in both years of the study but seedling recruitment in the clearing was limited mainly by insufficient dispersal of its seeds into the cleared area. Under experimental conditions seeds germinated and seedlings established readily, so direct seed sowing in degraded areas may be appropriate. Alternatively, natural seed dispersal could be encouraged by improving the habitat for birds. Engelhardia spicata seeds were widely dispersed by wind and its regeneration was limited at the germination and early establishment stages. Physical obstruction by thick stem and leaf litter appeared to be a limiting factor. This barrier could be overcome by cutting back weeds (particularly grasses and ferns) or by shading them out with nurse trees.