Developing Techniques for Direct-seeding for Forest Restoration in Northern Thailand

ABSTRACT: Forest restoration by direct seeding is potentially more cost-effective than tree-planting, especially for upscaling restoration of tropical forest ecosystems. Unfortunately, its success is limited by seed predation, low seed germination and low seedling establishment, due to the harshness of environmental conditions on restoration sites, and particularly by lack of information about species suitability. Consequently, the main objective for this study was to test the suitability of 23 native forest tree species for direct seeding, to restore biodiversity-rich, upland, evergreen forest in northern Thailand and to find effective coating materials to prevent seed predation and promote seedling establishment.

Experiments were carried out in two-degraded areas at Mon Cham (MC) and Ban Mae Khi (BMK), and under controlled conditions in a tree nursery. Three replicate seed batches with 20 seeds for each species were sown randomly on each site. Five of the 23 seeds species were selected for testing two different seed-coating treatments: three treatments of thick-layer seed coating (or “seed balls”); biochar, soil mixture and polysaccharide mixture and two treatments of thin-layer seed coating or microbial seed coating: Streptomyces antibioticus and S. thermocarboxydus isolate S3, then sown using the same method. Seed removal and germination were monitored weekly, until germination had ceased for three weeks. Seedling yield, growth and species-performance scores were also monitored at appropriate intervals. Moreover, various species traits were also recorded.  

Nine months after sowing, the intensity of seed predation was low, seed removal decreased with increasing seed size. Among 23 tree species, eight species failed to germinate, two species including Adenanthera microsperma and Alangium kurzii, were ranked as having high germination. After the first dry season, two germinating species failed to establish. Germination and establishment were influenced by seed size, seed storage behavior and successional status. Thus, the species recommended for direct seeding, based on their high species-performance index, were A. microsperma, Spondias pinnata and Choerospondias axillaris. The study also suggested that opting for desiccation-tolerant seeds, with medium to large seeds, could enhance the likelihood of successful seedling establishment. To maintain seed viability, especially for recalcitrant seeds, a potential solution would be to sow them immediately after collection.

Biochar seed balls were the most effective treatment at reducing seed removal compared to non-coated seeds. However, seed germination of the coated seeds was less than that of non-coated seeds, probably because the thick coating reduced permeation of water, oxygen and light to the embryo. Microbial seed coatings also did not promote seed germination, seedling yield and growth. Therefore, the balance between predation reduction and seed coat permeability must be considered when developing treatments to enhance overall direct-seeding success.