Limitations to Sexual Reproduction in Encyclia krugii (Orchidaceae)

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Abstract
In the Guanica Forest Reserve, Puerto Rico, Encyclia krugii lacks pollinator rewards, is visited infrequently and matures few fruits. Hand-pollinations showed that flowers are self-incompatible. In 22 censuses, I monitored the number of flowers, fruits, vegetative shoots, and active inflorescences that were produced over two years. During the first 20 censuses, all open flowers of 43 randomly assigned, experimental plants were cross-pollinated. Fruiting of these plants was compared with 37 open pollinated controls to determine if plants were pollination limited. Fruit set estimates were 4% for controls and 8% for experimentals. Fruit production was limited by several factors. More than half the fruits initiated by both groups either aborted or were eaten. At least some of the abortions were probably caused by incompatible pollinations. Inflorescences that produced fruits subsequently produced fewer flowers than those that produced no fruits. This short-term adjustment of reproductive effort suggests that resources do limit the number of flowers and fruits produced. Moreover, experimentally augmented fruit production had some long-term effects on subsequent growth. Thus, sexual reproduction by E. krugii is limited by a combination of factors: incompatible pollinations, resource constraints, predation, and paucity of pollinations. In most orchids studied thus far, paucity of pollinations and resource availability are the dominant constraints and combine to limit sexual reproduction.