How does Dryobalanops aromatica supply carbohydrate resources for reproduction in a masting year?

Abstract

The effect on reproduction of the dynamics of resource allocation was studied in an emergent and masting tree species, Dryobalanops aromatica (Dipterocarpaceae), in a lowland dipterocarp forest in Sarawak, Malaysia. Girdling of the reproductive shoots (5 mm diameter) caused an increase in abortion during the flowering period, but did not affect the fruit set at the middle or final stages of seed maturation. In contrast, 50% defoliation significantly affected fruit setting, but had little effect on flowering. The total leaf area of reproductive shoots was significantly correlated with final fruit set and total fruit mass. Control of the carbohydrate supply to reproductive shoots by girdling and defoliation made no difference to fruit size, but the fruit number was highly sensitive to carbohydrate availability. Total non-structural carbohydrate (TNC) decreased during the flowering period mainly in the branch (P<0.05), but fluctuated little in any organs during fruit maturation. Leaf nitrogen and photosynthetic capacity of the reproductive shoots were not significant variables for reproduction. Our results suggest that D. aromatica uses current photosynthates in the leaves of reproductive shoots as a carbon source during fruit development, but requires stored assimilates in the branch for flowering. However, since TNC was still present in all organs even after flowering, our study also suggests that storage of carbohydrate resources might not be the decisive factor in the occurrence or frequency of flowering in this species.