Growth, demography and stand structure of Scaphium macropodum in differently managed forests in Vietnam

Scaphium macropodum is an important multipurpose timber species in tropical rain forests of Vietnam. It was once abundant in natural forest and local farmers significantly benefited from its valuable fruits. However, due to unsustainable management, the species is now endangered. We quantified plant species diversity of S. macropodum forests at four sites: Cattien, with high disturbance due to destructive harvesting of fruits, Bachma and M’drak with medium to high disturbances, and Dakuy with least disturbance. Across these sites, Shannon diversity index (H’) ranged from 1.19 to 5.08, with its highest values at Bachma, the medium disturbance site and lowest values at Cattien, the high disturbance site. H’ peaked at moderately disturbance (SDI of 0.45) in our study sites. We analyzed population dynamics of the species in the three sites: Cattien, Bachma and Dakuy using matrix modeling. Among study sites, population growth rate (λ) was lowest at Cattien (0.981) but its 95 % confidence limits include the value of 1, so that we cannot demonstrate a statistically significant decline of that population. λ values of Bachma and Dakuy are higher (1.022 and 1.017 respectively), indicating their populations are stable. The fruit harvesting pattern by cutting branches in Cattien had negative effects on seedling height growth, survival and on tree DBH of S. macropodum, and as a result negative impact on its λ. Our analysis indicated that the natural regeneration and future prospect of S. macropodum was worrisome in Cattien, rather good in Dakuy, and good in Bachma. We conducted greenhouse experiments to study the effects of light availability on growths and response to top breakage of S. macropodum. The effects of light availability on height and diameter growth of S. macropodum seedlings showed two clearly different stages: during the first stage (month 1 to 8) height growth was strongest at 25% irradiance and lowest at full light, but the pattern reversed in the second stage (as from month 8). Diameter growth was strongest at full sunlight and lowest at the lowest light level (12.5 %) in the second stage. Across the light levels, Relative Growth Rate (RGR) was maximal at 50 % sunlight (0.199) and just a bit lower at full irradiance 100 % (0.198), Net Assimilation Rate (NAR) increased (from 1.105 to 3.789) with increase of light levels, while Leaf Area Ratio (LAR) and Specific Leaf Area (SLA) both decreased with the increase in light level. Apparently top breakage does not significantly affect the survival rate of S. macropodum juveniles. All topped off plants that survived grew at least one new shoot; less than one tenth grew with two new shoots at 50 % and full light, respectively. From our enrichment planting trial with S. macropodum in a secondary poor forest, we concluded that at about 50 % light availability, created in a belt cut at about 4 m wide (at a cost of US $ 579.7 per ha), RGR of S. macropodum is highest, but this RGR is much lower than that in the greenhouse.