Demography of the epiphytic orchid, Dimerandra emarginata

Abstract

Vegetative growth, reproductive effort, seedling establishment, and mortality of the bark epiphyte, Dimerandra emarginata (Orchidaceae), were studied over a 3-y period in the moist lowland tropical forest on Barro Colorado Island, Panama. The study included more than 350 individuals growing on 12 branches of five different host tree species. Mortality was highest among small individuals. Most deaths occurred during the dry season and were probably related to drought. Mortality among older individuals, on the other hand, was always related to the instability of the substrate, i.e. flaking bark, breaking branches or falling trees. Vegetative growth was slow. The average increase in size (= height of the most recent, fully developed shoot) was 2.7 cm over 3 y (not considering ‘stemless’ seedlings). This increase was negatively correlated with initial plant size. Growth was highly seasonal, with little variation between years. The reproductive effort increased strongly with plant size. Larger individuals produced fruits more frequently, in larger numbers and of larger size. After reproduction, plants showed reduced vegetative growth in the following year (in 1994). There was no negative effect on future reproduction. Compared to seed production, annual recruitment was very low and showed large year-to-year variation. Fewer than 50% of the seedlings survived the first dry season after germination. After 3 y, their average size was 0.5 cm. The results suggest that slow-growing epiphytes such as D. emarginata can attain considerable age and that their longevity – after an initial vulnerable juvenile stage – is almost exclusively limited by substrate durability.