Direct Seeding for Restoring Tropical Lowland Forest Ecosystems in Southern Thailand

ABSTRACT: Deforestation is a major cause of loss of biodiversity globally, resulting in losses of forest products and ecological services. It also accounts for 20-25% of anthropogenic carbon emissions into the atmosphere. Expansion of monoculture plantations and urbanization has resulted in biodiversity depletion in southern Thailand, particularly due to loss of lowland tropical forest that was formerly widespread, but has become sparse and fragmented. Planting trees is the most intensive and expensive of forest restoration techniques but has been conducted worldwide, due to lack of alternatives. Low cost restoration techniques that can be applied on a large scale must therefore be developed to begin to reverse the effects of tropical deforestation.

Direct seeding has been suggested as a way to accelerate forest regeneration on degraded areas at minimal cost. However, its success is hindered by low germination rates, susceptibility of seeds to predation, and poor survival and growth of established seedlings. This study investigated which tree species and which techniques are most effective and suitable for restoring lowland tropical forest in S. Thailand by direct seeding. It also identified general factors and tree species characteristics that contribute to success, so that direct-seeding techniques may be more easily developed for other forest ecosystem types.

Forty-three tree species were monitored in their natural forest habitats, for one year on the west coast and 35 tree species on the east coast of southern Thailand. Twelve and 15 species, respectively, bore ripe fruits prior to the rainy season and should be targeted for seed collection for direct seeding. Analysis of seed characteristics revealed that seed size, shape and moisture content can be used as guidelines for selecting potential candidate species for direct seeding. Species having oval to round, large or intermediate-sized seeds and with low or medium moisture contents are more likely to be suitable for direct seeding at about 70% confidence.

Tree species of the families Leguminosae and Moraceae were the most successful colonizers of degraded areas by direct seeding, showing excellent growth performance. Other families with high performance included Ebenaceae, Tiliaceae and Verbenaceae. Moreover, species with fleshly fruits and mostly dispersed by animals could be identified of particularly suitable candidates for seed sowing in the early rainy season. These studies show that seed availability and species selection are the important factors linked to the success of direct seeding.

Direct seeding was compared with conventional tree planting, using nursery-raised planting stock from seed. Thirty species were studied (20 on the west coast and 14 on the east coast, with 4 species overlapping between both sites). Seeds were sown in the nursery and the field at the same time. The effects of mulching, soil nutrient supply and seed-predator exclusion on the germination, establishment and growth performance of direct-seeded species were investigated. None of these treatments significantly increased seedling establishment and growth over the first year after sowing, suggesting that low soil moisture and nutrients, and seed predation are not limiting factors for early establishment and growth of direct-seeded plants, in the study sites. One year after sowing, 26 tree species nursery-grown plants were transplanted into the field, randomly next to the 25 surviving species of direct-seeded tree seedlings, 50 cm apart, and growth performance and costs were compared. Excellent growth performance of direct seeded plants was demonstrated. Their growth parameter means were, in general, twice as high as those of transplanted, nursery-raised plants, 18 months after early sowing.

Direct seeding could reduce tree establishment costs by about 30% (33 and 45 baht per tree for direct-seeded and nursery-grown seedlings respectively), whilst also achieving a comparable density of successfully recruited seedlings (about 2700 seedlings ha-1). This study shows that direct seeding has great potential for restoring tropical lowland forests, on a large scale, at much lower costs than by conventional tree planting. However, in order to realize these benefits a broader range of tree species must be tested and optimal species composition determined.