Terrestrial ferns and understory palms are key components of tropical forest understories. Because they occupy the same forest strata, and achieve high local densities, they may compete for similar resources. Their similarities are not restricted to habitat. Ferns and palms are exceptionally diverse in the Neotropics and their distributions are influenced by soil and rainfall patterns. However, they also occupy opposite ends of a spectrum of reproductive traits. Ferns produce many wind-dispersed spores, while understory palms produce a small number of vertebrate-dispersed seeds. We analyzed the importance of habitat filtering and dispersal limitation in structuring these contrasting groups of plants. We censused terrestrial ferns and palms in forests at Fortuna, western Panama. These forests support humid - superhumid lower montane tropical forests, where canopy tree composition is correlated with the four types of parent material (rhyolite, andesite, dacite and basalt) that vary in soil nutrient availability. Community sampling was done in fifteen 5x5 m subplots in each of 12 one-hectare plots across the four soil types. Light, precipitation and soil variables were measured in each plot. Variation in the composition of both plant groups were accessed by ordination techniques; local co-occurrence and distance-decay relationships were assessed through regression models.
Results/Conclusions
77 species of ferns and 30 of palms were recorded. There was a significant effect of parent material on compositional similarity of ferns and palms (ADONIS: F = 1.62, p = 0.006 and F = 2.23, p = 0.01). Compositional variation for palms and ferns was related to similar soil environmental variables: N:P, Total P, moisture, Mg ( p < 0.05). Ferns were also correlated with R:FR ratio of light, and palms with bulk density, pH and effective cation exchange capacity. Across all plots, fern abundance was weakly negatively correlated with palm abundance (p < 0.001). Compositional similarity declined linearly with log interplot distance both for ferns and palms (r2 = 0.17, P < 0.001; r2 = 0.20 P < 0.001 respectively). After rarefaction, intercepts but not slopes differed for distance-decay relationships between the species groups. These results show that fern and palm communities are structured by similar environmental variables, likely in some circumstances compete for space, and show similar patterns of compositional turnover despite differences in reproductive traits.