Reliable estimates of forest productivity at the site scale can guide site-specific forest management. However, the best methods to evaluate productivity of natural forest sites, and in particular secondary natural forests, remain undefined. In this study we: (1) applied an indirect method for estimation of the potential productivity of a forest site (‘site form’) in Dong Nai Biosphere Reserve. In addition, we: (2a) investigated the influence of a range of environmental conditions on site form, (2b) built models to estimate site form from the corresponding environmental variables and (3) generated site form maps for the region. We found that Michaelis–Menten models provided the most consistent representation of tree height–diameter relationships, with reliable anamorphic curves at both plot-scale (50 m × 20 m) and multiplot-scales. Predicted tree height at 25-cm diameter (H25) was selected as the best indicator for site form as 25 cm was both a tree size, which was common in the forests, and for which height could be accurately predicted. H25 was significantly correlated with site environmental conditions, with the dominant influences being factors related to water availability. We generated site form maps for Dong Nai province based on the relationship between known environmental variables and H25. We conclude that maps such as these could have broad application in guiding tropical forest restoration measures elsewhere, particularly where there is limited knowledge of tree ages and species.

No abstract

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 m 2) 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 southeastern Vietnam, but today it is rare. Active management of these recovering forests is essential in order to recover this high-value, climax forest species.