An ecosystem approach to biodiversity effects: Carbon pools in a tropical tree plantation

This paper presents a synthesis of experiments conducted in a tropical tree plantation established in 2001 and consisting of 22 plots of 45 m x 45 m with either one, three or six native tree species. We examined the changes in carbon (C) pools (trees, herbaceous vegetation, litter, coarse woody debris (CWD), and mineral topsoil at 0-10 cm depth) and fluxes (decomposition of CWD and litter, as well as soil respiration) both through time and among diversity levels. Between 2001 and 2009 the aboveground C pools increased, driven by trees. Across diversity levels, the mean observed aboveground C pool was 7.9 +/- 2.5 Mg ha(-1) in 2006 and 20.4 +/- 7.4 Mg ha(-1) in 2009, a 158% increase. There was no significant diversity effect on the observed aboveground C pool, but we found a significant decrease in the topsoil C pool, with a mean value of 34.5 +/- 2.4 Mg ha(-1) in 2001 and of 25.7 +/- 5.7 Mg ha(-1) in 2009 (F-1,F-36 = 52.12, p <0.001). Assuming that the biomass C pool in 2001 was negligible (<1 Mg ha-1), then the plantation gained in C, on average, 20 and lost 9 Mg ha-1 in biomass and soil respectively, for an overall gain of similar to 11 Mg ha(-1) over 8 years. Across the entire data set, we uncovered significant effects of diversity on CWD decomposition (diversity: F-2,F-393 = 15.93, p < 0.001) and soil respiration (monocultures vs mixtures: t = 15.35, df =11, p < 0.05) and a marginally significant time x diversity interaction on the loss of total C from the mineral topsoil pool (see above). Monthly CWD decomposition was significantly faster in monocultures (35.0 +/- 24.1%) compared with triplets (31.3 +/- 21.0%) and six-species mixtures (31.9 +/- 26.8%), while soil respiration was higher in monocultures than in mixtures (t = 15.35, df = 11,p <0.001). Path analyses showed that, as diversity increases, the links among the C pools and fluxes strengthen significantly. Our results demonstrate that tree diversity influences the processes governing the changes in C pools and fluxes following establishment of a tree plantation on a former pasture. We conclude that the choice of tree mixtures for afforestation in the tropics can have a marked influence on C pools and dynamics. (C) 2010 Elsevier B.V. All rights reserved.