Temporal and Environmental Patterns of Reproduction in Zamia Skinneri, A Tropical Rain Forest Cycad

(1) Zamia skinneri is a long-lived cycad inhabiting the shaded understorey of tropical rain forest. To evaluate the effects of habitat and plant size on reproductive performance of this dioecious species, 200 individuals were studied for 6 years at the La Selva Biological Station, Costa Rica. (2) The ability to reproduce and the sex of reproductive plants were related to plant size (females > males > non-reproductives) and to the amount of incident light, as indicated by the openness of the surrounding canopy. (3) For both female and male plants, several indices of potential reproductive success increased with plant size. For females, cone size, length of the receptive period and frequency of cone production were greater for larger plants. Larger males produced more numerous and larger cones, released pollen over a longer period and produced cones more frequently. (4) Males began releasing pollen before the first female was receptive, and individual males were reproductively active for longer periods than individual females were receptive. (5) In primary forest, potential reproductive success of males was related to a temporal aspect of reproductive effort. Males with more cones shed pollen over a longer period and, therefore, overlapped temporally with more females. (6) In primary forest, there were only two major reproductive episodes in 6 years. Of the plants which had reached the minimum of ten leaflets per leaf needed for reproduction (adult-sized plants), fewer than 20% produced cones in each episode. Only 26% of the adult-sized plants produced cones during the 6 years. (7) The sex ratio of reproductively-active plants in primary forest was strongly malebiased on a daily basis, over a total reproductive episode, and for the entire 6-year period. (8) Reproductive behaviour was very different in secondary forest, where incident light was higher. A much higher proportion of the adult-sized individuals produced cones during the 6-year period (73% compared to 26% in primary forest), and there were three major reproductive episodes. (9) For this long-lived dioecious plant in a very light-limited environment, reproduction is strongly influenced by light availability and by plant size. The greater resource demands for female reproduction contribute to the strongly male-biased sex ratios during episodes of reproductive activity in primary forest.