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Fineroots (≤ 2 mm diameter) are dynamic components of the forest ecosystems and play important role in water and nutrient acquisition in forests. These roots are sensitive to forest fertilization and therefore, the response of fineroots to nutrient application would provide better understanding of the forest carbon and nutrient dynamics that will be helpful in sustainable forest management plans. Two fertilization treatments, including (1) F400: 400 g P2O5 (16.5%)/tree and (2) F600: 600 g P2O5 /tree, and F0: control (without fertilization), were applied in an Acacia mangium plantation with a planting density of 1100 trees/ha (3 m × 3 m). The evaluation of fineroot growth across seasons showed that fertilization significantly increased production and subsequent mortality and decomposition. The total decomposition associated with F600 application was 7.95 tons ha−1 year−1, equaling 121% of F400 and 160% of the control. Mortality in F600 was 8.75 tons ha−1 year−1, equaling 111% of F400 and 198% of F0, while production in F600 was 10.40 tons ha−1 year−1, equaling 127% of F400 and 143% of F0. Fineroot production, mortality, and decomposition are seasonally dependent, with higher values measured in the rainy season than in the dry season. Stand basal area increment was significantly correlated with fineroot production (R2 = 0.75), mortality (R2 = 0.44), and decomposition (R2 = 0.48). This study showed that fertilization could facilitate fineroot production, which can then lead to a higher turnover of carbon and nutrients through the decomposition of the greater mass of the fineroots.

Net primary production (NPP) is an important index for understanding carbon cycling in forest ecosystems. In this study, aboveground NPP at Acacia mangium plantation was estimated basing on allometry for aboveground biomass increment (ΔM) and litter trap technique for litterfall (Lf). The experiment was conducted in two plots of 300 m2 each (15 × 20 m), established at a 21-month old plantation. Data were collected five times of 3-month intervals in a total duration of 357 days. The results indicated that Lf and ΔM were seasonal-dependent. Litterfall was highest (4.06 g m-1 day-1) during Sep-Jan (late rainy season, early winter) and lowest (1.10 g m-1 day-1) during Mar-Jun (early rainy season, early summer). While, ΔM was highest (13.51 g m-1 day-1) during Jun-Sep (rainy season, summer) and lowest (3.10 g m-1 day-1) during Jan-Mar (dry season, winter). Total Lf in a duration of 357 days was 9.69 tons ha-1 and ΔM was 27.71 tons ha-1, leading to total aboveground NPP of the present study plantation of 37.40 tons ha-1. It is concluded that aboveground NPP of acacia plantation was much higher than other forests of different types and ages around the world. Such difference indicates the importance of acacia plantation in soil nutrient cycling through litterfall decomposition and carbon sequestration through aboveground biomass increment.

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 m2 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.