Substrate texture and natural removal processes mediate vascular epiphyte establishment: Experimental evidence from a Panamanian cloud forest1, 2

Abstract

Epiphytes are a unique group of plants that live nonparasitically on other plants (“hosts”) and constitute approximately one-fifth of Neotropical vascular plant diversity. However, the processes governing early epiphyte community assembly are poorly understood and have scarcely been experimentally tested. Here, we use an in situ experiment in the cloud forest of Santa Fé, Panama, to evaluate the extent to which host substrate texture regulates early epiphyte establishment. We experimentally varied the surface roughness of native wood substrates, applied bromeliad and orchid seeds to the substrates, and monitored emergence of epiphyte seedlings and their persistence for a year. Rougher substrates facilitated higher initial abundance of epiphyte seedlings; after two months, 81% of the 1,934 total germinated epiphytes occurred on the substrates with experimentally added roughness. Via photo analysis, we also show that epiphytes disproportionately established early on in the experimental grooves, wherein 71% more epiphytes per unit area occurred within 1.5 mm of the grooves than on nearby smooth surfaces. While epiphyte cohort survival rates differed between rough and smooth substrates in the first six months, more than 99% of all seedlings died after one year, regardless of experimental roughness treatment. Only 10 seedlings survived through the end of the experiment. Our results suggest that while substrate texture explains some variation in early epiphyte emergence, roughness alone is not sufficient to explain epiphyte persistence to adulthood. Moreover, our results highlight the importance of removal processes (e.g., wind, rain, animals) in structuring early epiphyte community assembly. Variation in substrate texture may contribute to differences in epiphyte diversity and community composition within- and among-host tree species, but more experiments are needed to disentangle removal processes from substrate-mediated host affinity.