Natural Establishment of Tree Seedlings in Forest Restoration Trials at Ban Mae Sa Mai, Chiang Mai Province

ABSTRACT: The Forest Restoration Research Unit (FORRU) has successfully adapted the framework species method of forest restoration to accelerate natural forest regeneration on deforested sites in northern Thailand. This method involves planting 20-30 indigenous forest tree species, selected for fast growth, resilience to weed competition and fire and attractiveness to seed-dispersing animals. Trial plots, to test the technique, have been established annually in Doi Suthep-Pui National Park, northern Thailand, since 1998. The objectives of the research presented here were i) to determine if forest restoration encourages recruitment of non-planted tree species in the planted areas, thus increasing tree species diversity ii) to determine the effectin s of the tree species planted, planting density, plot-age and fire on natural tree-seedling establishment. The study was carried out using two survey techniques. To determine the effects of planting density on natural seedling establishment, rectangular sample units, measuring 30x10m, were established in the centre of plots, planted in 1999 at 3 different densities (2.3, 1.8 and 1.5 m between trees at planting time). To determine the effects of plot age on natural tree seedling establishment, circular sample units 10 m in diameter were laid out across plots, planted in 1998 and 2002 and non-planted control plots. In all sample units, the following measurements were made on all naturally established seedlings observed: height, root collar diameter (using Vernier calipers), canopy width, health, weed cover, shade. Furthermore, the species of any tree crowns immediately above the naturally established seedlings were record. This enabled associations between establishing tree species and planted tree species to be determined.

The population density of naturally established tree seedlings and proportion of climax species increased with age of planted plots. Spacing framework tree 1.8 m apart (3,125 trees per hectare) at planting time, resulted in optimal natural seedling establishment. Most seedlings grew from seeds that had been dispersed into the planted plots by animals (rather than by wind). Mortality of seedlings in the control sites was significantly higher than in planted plots, and the highest mortality occurred in the rainy season. Seventy-three tree seedling species in the planted plots were recruit species (non-planted species). Previous fires in the forest restoration areas inhibited seedling establishment and increased mortality rate, resulting lower species diversity of the seedling community. The 57 framework tree species planted fostered considerable seedling recruitment beneath their crowns. The top three framework tree species for fostering natural regeneration were Ficus glaberrima Bl. var. glaberrima, Prunus cerasoides D. Don and Erythrina subumbrans (Hassk.) Merr. Most of the seedling species recorded growing beneath their crowns grew from animal-dispersed seeds. In conclusion, this study shows that the framework species method is effective at enhancing natural forest regeneration.