Large contribution of recent photosynthate to soil respiration in tropical dipterocarp forest revealed by girdling

Tropical forests are the most productive terrestrial ecosystems, fixing over 40 Pg of carbon from the atmosphere each year. A substantial portion of this carbon is allocated below-ground to roots and root-associated micro-organisms. However, there have been very few empirical studies on the dynamics of this below ground transfer, especially in tropical forests where carbon allocation processes are influenced by high plant species diversity. We used a whole-stand girdling experiment to halt the below-ground transfer of recent photosynthates in a lowland tropical forest in Borneo. By girdling 209 large trees in a 0.48 ha plot, we determined: (a) the contribution of recent photosynthate to root-rhizosphere respiration and; (b) the relationships among the disruption of this below-ground carbon supply, tree species composition and mortality. Mortality of the 209 trees was 62% after 370 days, with large variation among species and particularly high mortality within the Dipterocarpaceae (99%) and Fagaceae (100%) families. We also observed a higher risk of mortality following girdling for species with lower wood density. Soil CO₂ emissions declined markedly (36 ± 5%) over ~50 days following girdling in three of six monitored subplots. In the other three subplots there was either a marginal decline or no response of soil CO₂ emissions to girdling. The decrease in soil CO₂ efflux was larger in subplots with dominance of Dipterocarpaceae. Synthesis. Our results indicate high spatial variation in the coupling of below-ground carbon allocation and root-rhizosphere respiration in this tropical forest, with a closer coupling in forest dominated by Dipterocarpaceae. Our findings highlight the implications of tree species composition of tropical forests in affecting the dynamics of below-ground carbon transfer and its release to the atmosphere.