The effect of environmental variables on the presence and abundance of tree species in a tropical lowland undisturbed limestone and non-limestone forest in Ben En National Park, Vietnam was investigated. The relationships between 13 environmental variables and 29 tree species with a DBH ≥10 cm, as well as between six 6 physical variables with 26 species of seedling and sapling communities were assessed by canonical correspondence analysis (CCA). Data concerning all tree species ≥10 cm DBH were collected from eighteen 400 m 2 sample plots, while the abundance of regeneration (all individuals ≤5 cm DBH) was counted in fifty 2x20 m strip-plots. The significance of species-environments correlations were tested by distribution-free Monte Carlo tests. The CCA of the 29 examined tree species and 13 environmental variables indicated that the presence and abundance of the tree species were closely related to topographic factors. We may confirm that soil properties including pH, soil moisture content, and soil textures, were the most crucial factor in tree species composition and their distribution. Several species including Pometia pinnata, Amesiodendron chinense, Gironniera cuspidate, Cinnamomum mairei, and Caryodaphnopsis tonkinensis were not controlled by soil properties and topographic variables. The CCA also indicated that the abundance of regeneration tree species at all sites had positive and significant correlations with soil depth, while the occurrence of several other tree species (such as Koilodepas longifolium and Aglaia dasyclada) was positively correlated with a higher slope and rocky outcrop.

Forest plantation growers in Vietnam commonly burn residues after harvesting and often apply suboptimal amounts of nutrients during plantation establishment. We examined whether the retention of forest residue, and application of phosphorus fertiliser at higher rates, can increase rates of growth. A factorial combination of residue management (burning vs retention) and phosphorus fertiliser application at planting (15 vs 100 kg ha −1 ) treatments were applied at a steeply sloping site in northern Vietnam. Two adjacent experiments were established, one with Acacia mangium and the other with a Eucalyptus hybrid (Eucalyptus urophylla × Eucalyptus pellita). Standing volume and leaf area index in A. mangium were greater following burning; this was mostly attributable to the significantly higher survival rate of seedlings. Burning of residues was associated with increases in the number of large branches per tree, and a higher crown damage index (CDI). In the Eucalyptus hybrid, diameter and height responses to the higher rate of fertiliser were observed at age 6 and 12 months, but not beyond. High phosphorus application also led to higher CDI. Standard fertiliser treatment, applied in amounts equivalent to 17, 15 and 8 kg ha −1 of nitrogen, phosphorus and potassium, respectively, was adequate to meet the early growth requirement of eucalypt and acacia plantations at this site. The relatively low amounts of harvest residue and high fertility levels at the site may have masked more significant responses of trees to the silvicultural treatments applied in this study. On steep slopes, especially if soil is poorly fertile, harvest residue retention with adequate weed and termite control may be preferential to burning as it is closely correlated with reducing factors that negatively impact productivity, i.e. water run-off and soil erosion.

Biotic and abiotic factors control aboveground biomass (AGB) and the structure of forest ecosystems. This study analyses the variation of AGB and stand structure of evergreen broadleaved forests among six ecoregions of Vietnam. A data set of 173 1-ha plots from 52 locations in undisturbed old-growth forests was developed. The results indicate that basal area and AGB are closely correlated with annual precipitation, but not with annual temperature, evaporation or hours of sunshine. Basal area and AGB are positively correlated with trees [ 30 cm DBH. Most areas surveyed (52.6%) in these old-growth forests had AGB of 100-200 Mg ha-1 ; 5.2% had AGB of 400-500 Mg ha-1 , and 0.6% had AGB of [ 800 Mg ha-1. Seventy percent of the areas surveyed had stand densities of 300-600 ind. ha-1 , and 64% had basal areas of 20-40 m 2 ha-1. Precipitation is an important factor influencing the AGB of old-growth, evergreen broadleaved forests in Vietnam. Disturbances causing the loss of large-diameter trees (e.g., [ 100 cm DBH) affects AGB but may not seriously affect stand density. Keywords Aboveground biomass Á Carbon storage Á Climatic variables Á Ecoregion Á Edaphic variables Á Oldgrowth forest Project funding: This word is funded by Vietnam Ministry of Science and Technology under Grant number ÐTÐL.XH.10/15, Vietnam National Foundation for Science & Technology Development (106-NN.06-2016.10), and International Foundation for Science (J-1-D-4602-3).

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