Sowing time and direct seeding success of native tree species for restoring tropical forest ecosystems in northern Thailand

Abstract

Direct seeding (sowing seeds directly into ground) is potentially a cost-effective method of forest restoration that could replace or complement conventional tree planting, under certain conditions. However, the effects of timing on both the success and practicability of direct seeding has received little attention. Therefore, this study determined the effect of seed sowing time on direct seeding efficiency in terms of yield (number of established seedlings per 10 seeds sown) and seedling growth. We tested the hypothesis that seeds, stored and sown at the start of the rainy season, have higher and more rapid germination and that the resultant seedlings perform better, compared with those sown immediately after seed collection. Seeds of 17 native tree species, typical of seasonally-dry, upland, evergreen forest, were collected. Triplicates (50 seeds per replicate) were sown directly into degraded land, shortly after collection and compared with the same for stored seeds, sown at the start of the rainy season. Control seed batches were also germinated in a nursery for comparison. Seed germination was recorded weekly and growth and survival of resultant seedlings were recorded periodically. Our results did not support the above hypothesis. Differences in mean yield (number of seedlings established per 100 seeds sown) and growth between the two sowing times were not significant. Germination percentage and median length of dormancy (MLD) did not differ significantly between the sowing times, except for Artocarpus lacucha and Horsfieldia amygdalina, whose seeds germinated better immediately after seed collection than after storage (p < 0.01). Storage shortened median length of dormancy of Hovenia dulcis, Melia azedarach, Phyllanthus emblica, Prunus cerasoides and Spondias pinnata seeds (p < 0.01). Seedling growth of a few species rivalled reported values of planted saplings (P. cerasoides, M. azedarach and B. variegata), regardless of sowing time. Timing of the direct seeding did not appear to be critical. This provides forest restoration project managers with flexibility in their use of this technique as a restoration tool. The advantages and disadvantages of direct seeding, immediately after seed collection or at the start of the rainy were compared with those of conventional tree planting. We conclude that species selection matters more than timing, when direct-seeding in forest restoration projects (except for recalcitrant species, which can only be direct seeded soon after seed collection). From this study, Adenanthera microsperma, Bauhinia variegata, Melia azedarach, Phyllanthus emblica and Prunus cerasoides are recommended, for direct seeding, to restore seasonally dry upland evergreen forest ecosystems in northern Thailand and at other sites within their natural species ranges.