How Epiphytic Are Epiphytes? Field Evidence for Flexible Substrate Use in Tropical Cloud Forest Plants

Substrate use in vascular plants is commonly treated as a categorical trait where a species is either epiphytic or terrestrial. Yet many species grow on both substrates, and the degree of epiphytism varies continuously within and among species. We quantified substrate use in 27 species traditionally classified as epiphytic across two tropical montane cloud forests in Colombia and Peru, recording 483 instances of substrate use, and compared field-derived Epiphytic Values (EV) with literature-derived estimates. In the field, 17 species grew exclusively epiphytically, while 10 also grew terrestrially, with EVs ranging from 43% to 97%. Literature-derived EVs consistently overestimated substrate fidelity relative to field observations, particularly single-source classifications, with mean absolute differences of up to 32%. Our results challenge treating 'epiphytic/terrestrial' as a categorical trait and support a continuous 'Epiphytic Value' per species.