Gender variation and primary succession in a tropical woody plant, Antirhea borbonica (Rubiaceae)

Summary1 Ecological context can significantly affect plant reproduction: in particular, differences in the relative allocation of resources to male and female function can contribute to gender variation. 2 We quantified variation in sex expression among populations of Antirhea borbonica , a sexually dimorphic woody pioneer species found both on young lava flows and in later stages of primary succession on the volcanic island of La Réunion. We also tested whether environmental conditions influence maternal fertility by experimentally manipulating resource levels in natural populations. 3 The polliniferous morph showed significant gender variation, from almost zero fruit production in pioneer populations to high fruit set, albeit of less than females, in late‐succession. Repeated observations over 4 years showed that, even in pioneer populations, most polliniferous plants are capable of producing fruits. 4 A significant increase in fruit production by the polliniferous morph was observed after 2 years of resource supplementation in a pioneer population. 5 Sex ratios were close to 1 : 1 in all but one of 12 populations. Seeds and seedlings produced by the polliniferous morph in a late‐succession population had significantly lower viability than those from females. These results indicate that there is strong selection against the maternal offspring produced by polliniferous plants and that, regardless of successional status, A. borbonica possesses a functionally dioecious sexual system. 6 The combination of multi‐year surveys and experimental manipulation of resource availability provides evidence of phenotypic gender plasticity. This provides a novel illustration of how the heterogeneous environment of primary succession can influence trait variation.