Phenology and fecundity in 11 sympatric pioneer species of Macaranga (Euphorbiaceae) in Borneo

Reproductive traits of tropical tree species vary predictably in relation to successional stage, but this variation may be due to the species' phylogenetic histories rather than selective pressures imposed by regeneration requirements. Reproductive phenology, tree size at the onset of reproduction, and fecundity of 11 sympatric, closely related Macaranga species were studied to investigate within-species variation in reproductive traits in relation to resource availability, and among-species variation in relation to other life-history traits (shade tolerance, seed size and maximum tree size, H-max) and consequently the requirements for forest-gap colonization. Nine species reproduced in synchronous episodes, and two species reproduced continuously over 32 mo. Episodic reproduction was most intense in 1992 following a severe drought. For several species, reproductive trees had greater light availability, lower fecundity in lower light levels, and lower growth rates than nonreproductive trees, reflecting resource-limited reproduction. Among species, H-max was negatively correlated with shade tolerance and seed size. Tree size at the onset of reproduction and fecundity was strongly linked to this axis of life-history variation, but phenological pattern was not. Absolute tree size at the onset of reproduction was positively correlated with H-max and negatively correlated with shade tolerance. Relative size at reproductive onset was not correlated with shade tolerance or H-max. Fecundity ranged four orders of magnitude among species and was correlated positively with H-max and negatively with seed size and shade tolerance. The interrelationships among these reproductive and other life-history traits are strongly correlated with the species' requirements for gap colonization.