Functional traits explain vascular epiphyte distributional patterns along environmental gradients in tropical canopies

Background/Question/Methods

Plant functional traits determine how species respond to environmental conditions and, thus, can provide a mechanistic understanding of species distributional patterns along environmental gradients. In tropical rain forests, the canopy is host to a large proportion of the tropical diversity in wet tropical forests including up to 35% of the vascular flora, a richness that often exceeds that of the forest floor. However, our understanding of what influences the distribution and floristic composition of vascular epiphytic flora lags far behind that of terrestrial-based plant communities. The steep environmental and resource gradients that exist within large tree crowns may influence the distribution of epiphytic vascular plants and promote coexistence if there is evidence of species sorting and habitat specialization. Here, I examine the importance of functional traits in epiphytic species partitioning along environmental gradients within tree canopies by describing the distributional patterns of epiphytic ferns, aroids, and bromeliads along environmental gradients within large tree canopies and relating them to their functional leaf traits including specific leaf area (SLA), leaf dry matter content (LDMC), succulence, sclerophylly, and the rate of epidermal water loss (EWL). 

Results/Conclusions

I found that vascular epiphytes showed evidence of habitat specialization as species partitioned the canopy habitat along gradients of vapor pressure deficit (VPD), relative humidity (RH), light, and the amount of canopy soil; and concomitantly so did their functional traits. Epiphyte species found in shaded sites with canopy soil, low VPD, and high RH such as aroids and ferns exhibited a resource-conservation strategy with large, tough, thick leaves that had a low SLA, high LDMC, high degree of sclerophylly, and a low rate of epidermal water loss (EWL), reflecting the low and intermittent resource supply found in these microhabitats. Epiphyte species found in more open sites on bare bark without canopy soil had a diversity of drought-avoidance strategies to tolerate extreme water limitation; atmospheric bromeliads had a low SLA, a high degree of succulence, thick leaves, and a low EWL rate to reduce water loss from their leaves, while ferns put little energy into their leaves (i.e., high SLA, low LDMC, low degree of succulence, high EWL rate) but were drought deciduous. Functional leaf traits and their relationships to environmental gradients explained epiphyte distributions and the partitioning of tropical tree canopies, which may promote coexistence in these hyperdiverse communities.