Site conditions for regeneration of climax species, the key for restoring moist deciduous tropical forest in Southern Vietnam.

Understanding the requirements and tolerances of the seedlings of climax species is fundamental for tropical forest restoration. This study investigates how the presence and abundance of seedlings of a previously dominant, now threatened species (Dipterocapus dyeri Pierre), varies across a range of environmental conditions. Dipterocapus dyeri seedling abundance and site characteristics were recorded at 122 observation points (4 m2) at nine clusters from two sites. Seedling presence (p = 0.065) and abundance varied significantly (p = 0.001) between the two sites, and was strongly correlated with adult D. dyeri dominance and lower soil pH, and weakly correlated with canopy openness and total stand basal area. Dipterocarpus dyeri seedlings were also grown in shade houses with three light levels on two soils. Seedling survival was significantly lower at the lowest light level (<10% full irradiance) at 13% for the forest soil and 25% for degraded soil. At higher irradiance the seedling survival rates were greater than 99%. Moisture levels remained high at the lowest light level and many seedlings died from fungal infection. We concluded that secondary forests which contain adequate numbers of adult D. dyeri as seed sources, light availability, soil pH of < 5.0, and good drainage strongly favour survival and growth of D. dyeri seedlings. Historically, D. dyeri was dominant in moist deciduous tropical forest across south-eastern Vietnam, but today it is rare. Active management of these recovering forests is essential in order to recover this high-value, climax forest species.