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

Vascular epiphytes represent c. 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 keys 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.

Holding on or falling off: The attachment mechanism of epiphytic Anthurium obtusum changes with substrate roughness.

PREMISE: For vascular epiphytes, secure attachment to their hosts is vital for survival. Yet studies detailing the adhesion mechanism of epiphytes to their substrate are scarce. Examination of the root hair-substrate interface is essential to understand the attachment mechanism of epiphytes to their substrate. This study also investigated how substrate microroughness relates to the root-substrate attachment strength and the underlying mechanism(s). METHODS: Seeds of Anthurium obtusum were germinated, and seedlings were transferred onto substrates made of epoxy resin with different defined roughness. After 2 months of growth, roots that adhered to the resin tiles were subjected to anchorage tests, and root hair morphology at different roughness levels was analyzed using light and cryo scanning electron microscopy. RESULTS: The highest maximum peeling force was recorded on the smooth surface (glass replica, 0 µm). Maximum peeling force was significantly higher on fine roughness (0, 0.3, 12 µm) than on coarse (162 µm). Root hair morphology varied according to the roughness of the substrate. On smoother surfaces, root hairs were flattened to achieve large surface contact with the substrate. Attachment was mainly by adhesion with the presence of a glue-like substance. On coarser surfaces, root hairs were tubular and conformed to spaces between the asperities on the surface. Attachment was mainly via mechanical interlocking of root hairs and substrate. CONCLUSIONS: This study demonstrates for the first time that the attachment mechanism of epiphytes varies depending on substrate microtopography, which is important for understanding epiphyte attachment on natural substrates varying in roughness.

Go with the flow: The extent of drag reduction as epiphytic bromeliads reorient in wind.

Vascular epiphytes represent almost 10% of all terrestrial plant diversity. Being structurally dependent on trees, epiphytes live at the interface of vegetation and atmosphere, making them susceptible to atmospheric changes. Despite the extensive research on vascular epiphytes, little is known about wind disturbance on these plants. Therefore, this study investigated the wind-epiphyte mechanical interactions by quantifying the drag forces on epiphytic bromeliads when subjected to increasing wind speeds (5-22 m s-1) in a wind tunnel. Drag coefficients (Cd) and Vogel exponents (B) were calculated to quantify the streamlining ability of different bromeliad species. Bromeliads' reconfiguration occurred first via bending and aligning leaves in the flow direction. Then leaves clustered and reduced the overall plant frontal area. This reconfiguration caused drag forces to increase at a slower rate as wind velocity increased. In the extreme case, drag force was reduced by 50% in a large Guzmania monostachia individual at a wind velocity of 22 m s-1, compared to a stiff model. This species had one of the smallest Cd (0.58) at the highest wind velocity, and the largest negative mean B (-0.98), representing the largest reconfiguration capacity amongst the tested bromeliads. The streamlining ability of bromeliads was mainly restricted by the rigidity of the lower part of the plant where the leaves are already densely clustered. Wind speeds used in this study were generally low as compared to storm force winds. At these low wind speeds, reconfiguration was an effective mechanism for drag reduction in bromeliads. This mechanism is likely to lose its effectiveness at higher wind speeds when continuous vigorous fluttering results in leaf damage and aspects such as root-attachment strength and substrate stability become more relevant. This study is a first step towards an understanding of the mechanical bottleneck in the epiphyte-tree-system under wind stress.