Smoothing out the misconceptions of the role of bark roughness in vascular epiphyte attachment.

Vascular epiphytes represent almost 10% of all vascular plant species. In epiphytes, attachment is essential for survival throughout consecutive ontogenetic stages of their life, starting with 1) initial propagule attachment to the host, followed by 2) the development of first root-substrate connections, and 3) maintenance of this attachment despite increased size and mechanical disturbances by rain, wind or crossing animals. Although structural dependence on a host is a defining characteristic of an epiphyte, the fundamental mechanism(s) of how these plants initially attach and remain attached to their hosts remain poorly understood. Bark characteristics such as stability and roughness have been highlighted as key to an understanding of this connection. Here, we stress that the understanding of how an epiphyte attaches itself to the substrate is central for a meaningful quantification and interpretation of bark roughness. Without explicit information on the attachment mechanism or the relative sizes of the attaching structures, simply linking a haphazardly chosen index of bark roughness to epiphyte establishment is flawed. This review introduces a conceptual framework to explain the mechanistic link between epiphytes and host in different ontogenetic stages and should guide future work designed to improve our understanding of this vital part of epiphyte ecology.