Accumulation, allocation and dynamics of carbon in slash pine plantations along a management intensity gradient

Mineralization of nutrients from the decomposing forest floor is an important component of nutrient cycling in mature forests. Silvicultural treatments commonly used in plantation management can alter these processes and affect forest productivity. The effects of understory elimination and fertilization at establishment and at age of 12, and the combination of these treatments on litterfall inputs, decomposition and nutrient release of the forest floor were investigated in 18-year-old slash pine (Pinus elliottii Engelm.) plantations in northern Florida. Needlefall rates were increased by both fertilization and understory elimination treatments. Understory elimination was associated with increased forest floor standing mass, the result of increased needlefall and decreased decomposition rate. In contrast, fertilization didn't increase forest floor standing mass. Fertilization increased P concentrations of living needle and needlefall, and K concentration in needlefall. Meanwhile, fertilization decreased Ca and Mg concentrations in needlefall. Fertilization significantly increased N and P concentrations and contents in forest floor, while understory elimination had no effects on forest floor nutrient concentration or content. Fertilization increased the P release from the forest floor. Neither fertilization nor understory elimination had impacts on N release from the forest floor. Net accumulation of N and slow release of other nutrients suggested that the forest floor in these pine plantations played a relatively unimportant role in nutrient cycling at this stage of stand development.; Soil C plays a key role in long-term forest productivity and in global C cycles. The effects of fertilization and understory elimination on soil C storage and dynamics near the end of the rotation were also assessed. Fertilization and understory elimination increased C storage in vegetation and forest floor. Fertilization, understory elimination, and fertilization plus understory elimination treatments increased C storage in above-ground biomass by 39%, 59%, 60%, in below-ground biomass by 27%, 26%, 25%, and in the forest floor by 23%, 42%, 48%, respectively. Fertilization increased total stand production by 8%, mainly through increasing above-ground NPP, while understory elimination reduced total NPP by 34%, mainly by reducing below-ground growth by 66%. Fine root turnover was reduced by understory elimination, while fertilization didn't affect fine root turnover. Increased productivity resulting from fertilization was not associated with increased soil C in these 18-year-old stands. Instead, soil C was lower in fertilized plots than in non-fertilized plots. The decrease of soil C storage was suggested to be due to C loss as DOC leaching. Understory elimination also reduced soil C storage by reducing fine root turnover throughout the rotation and, perhaps, by reducing litterfall inputs early in the rotation. The overall effect of fertilization and understory elimination was to increase ecosystem C sequestration. This increase was due to increased C in vegetation components and the forest floor.