EVALUATING THE ROLE OF PLANTATIONS AS CARBON SINKS - AN EXAMPLE OF ANINTEGRATIVE APPROACH FROM THE HUMID TROPICS

Despite their fast growth, tropical plantations are a small sink of at mospheric carbon because they occupy only a small area in relation to other land uses world-wide. Proper design and management of plantation s can increase biomass accumulation rates, making them more effective C sinks. However, fast-growing plantations can extract large amounts o f nutrients from the soil, and site fertility declines may limit susta ined plantation forestry after a few rotations. We measured abovegroun d biomass accumulation, carbon sequestration, and soil chemistry in th ree young plantations of 12 indigenous tree species in pure and mixed designs in the humid lowlands of Costa Pica. Annual biomass increments for the three mixed plantations ranged from 10-13 Mg/ha. The mixtures of four species gave higher biomass per hectare than that obtained by the sum of one fourth hectare of each species in pure plots. At this early age of the plantations, estimated annual C sequestration values were comparable to other reports from young plantations of exotic spec ies commonly grown in the tropics. Four years after planting, decrease s in soil nutrients were apparent in pure plots of some of the fastest growing species, while beneficial effects on soils were noted under o ther species. The mixed plots showed intermediate values for the nutri ents examined and, sometimes, improved soil conditions. A mixture of f ast and slower growing species yields products at different times, wit h the slower growing species constituting a longer term sink for fixed carbon. Examination of the role of tropical plantations as C sinks ne cessitates integrative approaches that consider rates of C sequestrati on, potential deleterious effects on ecosystem nutrients, and economic , social, and environmental constraints.