Introduction

Plant functional traits impact a plant’s fitness and constrain its environmental niche. Epiphytes are a prominent life form in humid tropical forests and many adaptations have been described as characteristic. We question if the many case studies indeed reflect “typical” adaptations of epiphytes, but generalizations are limited by a lack of comparable and unbiased data on functional traits.

Objectives

We aim to broaden the data on functional traits available for epiphytes. Using these, we formulate hypotheses how functional traits should reflect adaptations to specific constraints of the epiphytic habitat to cope with type, availability and stability of the substrate, and the availability of light, water and nutrients.

Methods

These hypotheses were tested by collecting an unprecedented dataset of > 100 000 trait observations from 2601 species of epiphytes and comparing these with traits of ground-rooted herbs and trees. We test if epiphytes differ in individual traits, whether they occupy a distinct region in the global trait space and whether their trait network differs from ground-rooted plants. In addition, we distinguish orchids, bromeliads and ferns, the three most species-rich groups of epiphytes.

Results

Epiphytes differ from ground-rooted plants mainly in traits related to water relations, but surprisingly do not have lower leaf nutrient concentrations, except for nitrogen. Mean rates of photosynthesis are much lower than in ground-rooted plants and lower than expected from the nitrogen concentrations. The combination of traits clearly distinguishes epiphytes from trees and also from most herbs.

Orchids differ from bromeliads and ferns mainly by having smaller and more numerous stomata and ferns differ from bromeliads by having thinner leaves, higher nutrient concentrations, and lower water content and water use efficiency.

In epiphytes, herbs and trees, the central nodes in the trait network are represented by specific leaf area, mass-based photosynthesis together with nitrogen concentration. In epiphytes, traits related to plant water relations have stronger connections and cations have weaker connections to the remainder of the trait network.

Conclusions

Although this work substantially adds to the available traits for epiphytes, currently mainly leaf traits have been collected. Important gaps are root, shoot and whole plant traits, demographic traits and gas exchange traits and we suggest how future research might use available data and fill data gaps to advance our understanding of epiphyte ecology. This will also help to characterize the ecological niches occupied by the numerous species of epiphytes and understand their responses to environmental changes.