Does Tree Management Affect Biomass and Soil Carbon Stocks of Acacia mangium Willd. Stands in Kerala, India?

Abstract

Initial spacing and tree pruning are silvicultural strategies that influence tree growth and productivity, which determine the potential of tree stands to store C in the vegetation and soil. A field experiment was conducted at Thiruvazhamkunnu, Kerala, India in a 6.5 year-old Acacia mangium stand to evaluate the changes in vegetation and soil (0–15 cm) C pools as a function of four planting densities (625, 1,250, 2,500, and 5,000 stems ha⁻¹), with and without 50% crown pruning. Both tree planting density and crown pruning significantly (p  <  0.01) influenced the C stocks of A. mangium trees. Total vegetation (aboveground  +  roots) C was highest for the 5,000 trees ha⁻¹ (81.82 Mg ha⁻¹) and lowest in the 625 trees ha⁻¹ treatment (41.39 Mg ha⁻¹). Soil C stocks also co-varied among the density regimes with the lowest values for treeless control plots. Overall, denser stands promote C storage, but very high stand densities (e.g., 5,000 stems ha⁻¹) may adversely affect tree growth and productivity, reducing vegetation C pools. Likewise, intense pruning may depress the vegetation C pool and would release CO₂ from the pruned biomass, especially if the slash are burnt or decomposed. Pruning effects are, however, dependent on stand density, implying the need for optimizing crown pruning regimes in conjunction with stand density levels. By extension, stand thinning exerts negative or positive feedbacks on biomass accretion depending on stand density, which may also influence the amount of C sequestered by the trees. Information reported in the literature confirms this. Irrigating the trees during water scarcity periods also may promote soil and vegetation C sequestration. But inorganic fertilization may have positive, negative, or neutral effects depending on site fertility, species, fertilizer doses, and the stage of stand development.