Epiphytes and wind in a changing world
Final Report Abstract
Vascular epiphytes are a major component of tropical vegetation and make up almost ten percent of the global vascular flora on land. A defining characteristic of epiphytes is their structural dependence on the host. In the trees and particularly in the higher regions of the tree crowns epiphytes face abiotic challenges like higher temperature, radiation and wind intensity compared to the understorey, but also different biomechanical challenges. The successful completion of an epiphyte’s life cycle begins with the successful attachment of their propagule to the host and throughout their life the attachment has to withstand the increasing weight of the plant itself and mechanical disturbances from factors such as heavy rain, strong winds or by crossing animals. So far, our understanding of biomechanical aspects of epiphytic life is very slim. However, ongoing forest fragmentation not only leads to habitat loss for epiphytes, but also creates abrupt edges that can exacerbate wind impact within the forest and previously sheltered epiphytes could be exposed to higher wind stress, risking dislodgement and falling to the ground where chances of survival are low. It has been repeatedly reported that epiphytes were blown off during storms, whereas only one study investigated if fallen down epiphytes fell down with their substrate or without it, concluding that most times substrate failure was the cause. It is essential for epiphytes to stay attached to their host, but we lack a basic understanding of the attachment mechanism itself. We explored two biomechanical aspects 1) the attachment mechanism of epiphytes to their hosts and 2) the stress responses of epiphytes to external mechanical disturbances. Focusing exclusively on bark epiphytes (they attach to the host via direct contact to the host, without rooting in accumulated organic material), we investigated the attachment mechanism and associated attachment strength of epiphytes to their host. Two main attachment mechanisms were identified, 1) adhesion, a glue-like substance is exuded from the root/root hairs for initial contact, and 2) mechanical interlocking, root hairs grow into crevices on the surface and thus promote attachment. Furthermore, the actual attachment mechanism of the root hairs to the substrate varies with substrate roughness, shifting from adhesive to interlocking as substrate topography becomes increasingly coarser. We also explored the thigmomorphogenic responses of epiphytes to external mechanical disturbances. Contrary to the commonly recorded thigmomorphogenic responses in ground-rooted herbaceous plants, little to no effect on the growth, and shoot and root properties was recorded for the two studied species. In a wind tunnel experiment we investigated the drag forces on five bromeliad species, exposed to wind velocities to up to 22 m s^-1. Streamlining by structural reconfiguration of the leaves reduced the overall frontal area of the plant perpendicular to the wind flow. Hence, passive reconfiguration reduced the extent of drag force increment on the plant at higher wind velocity. Although bromeliads are rather limited in their streamlining ability, none of the bromeliads detached from their substrate when subjected to hurricane-grade winds. In a field study, instead of streamlining by structural reconfiguration, tank bromeliads subjected to winds (max. 13 m s^-1) in their natural environment hardly displayed any movement. This lack of movement was due to the swaying of the host that reduced the direct drag forces on the epiphytes. In conclusion, the main findings pertaining to mechanical disturbances on epiphytes imply that the host is the bottleneck in the epiphyte-host system in windy and storm conditions. Mechanical perturbation by wind is unlikely to pose as a threat to the attachment of epiphytes to their hosts because factors such as structural reconfiguration and swaying of the host can help to reduce direct drag force on the epiphytes. In a review, we summarised recent progress towards understanding the biomechanics of bark epiphytes, addressing two related aspects 1) any correlation of substrate roughness with epiphyte diversity and abundance is meaningful only if the attachment mechanism and relevant size scales of the attaching structures involved are correctly defined, and 2) the impact of mechanical disturbances on epiphytes is highly plant-size dependent.
Publications
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Go with the flow: the extent of drag reduction by reconfiguration of epiphytes in wind. British Ecological Society, Festival of Ecology. Online conference. (Poster) 2020
Tay J. Y. L., Zotz G., Puczylowski J. & Einzmann H. J. R.
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Getting a Grip on the Adhesion Mechanism of Epiphytic Orchids – Evidence From Histology and Cryo-Scanning Electron Microscopy. Frontiers in Forests and Global Change, 4.
Tay, Jessica Y. L.; Zotz, Gerhard; Gorb, Stanislav N. & Einzmann, Helena J. R.
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Getting a grip on the adhesion mechanism of epiphytic orchids – evidence from histology and Cryo-SEM. British Ecological Society: Ecology Across Borders 2021 – Joint Annual Meeting with the French Society for Ecology and Evolution, Liverpool, UK. (Vortrag)
Tay J. Y. L., Zotz G., Gorb S. & Einzmann H. J. R.
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Go with the flow: epiphytic bromeliads are unlikely to be detached by laminar wind flows of hurricane force. British Ecological Society: Ecology Across Borders 2021 – Joint Annual Meeting with the French Society for Ecology and Evolution, Liverpool, UK. (Vortrag)
Einzmann H. J. R., Zotz G., Puczylowski J. & Tay J. Y. L.
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Go with the flow: The extent of drag reduction as epiphytic bromeliads reorient in wind. PLOS ONE, 16(6), e0252790.
Tay, Jessica Y. L.; Zotz, Gerhard; Puczylowski, Jaroslaw & Einzmann, Helena J. R.
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Thigmomorphogenic responses of epiphytic bromeliads to mechanically induced stress. Plant Ecology, 223(1), 1-11.
Tay, Jessica Y. L.; Zotz, Gerhard & Einzmann, Helena J. R.
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Holding on or falling off: the attachment mechanism of epiphytic Anthurium obtusum changes with substrate roughness. 10th International Plant Biomechanics Conference, Lyon, France (Vortrag).
Tay, Jessica Y. L.; Kovalev, Alexander; Zotz, Gerhard; Einzmann, Helena J. R. & Gorb, Stanislav N.
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Holding on or falling off: The attachment mechanism of epiphytic Anthurium obtusum changes with substrate roughness. American Journal of Botany, 109(6), 874-886.
Tay, Jessica Y. L.; Kovalev, Alexander; Zotz, Gerhard; Einzmann, Helena J. R. & Gorb, Stanislav N.
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What happens to epiphytic bromeliads in a windy spot?. Journal of Tropical Ecology, 1-6.
Einzmann, Helena Julia Regina; Zotz, Gerhard & Tay, Jessica Ying Ling
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Smoothing out the misconceptions of the role of bark roughness in vascular epiphyte attachment. New Phytologist, 238(3), 983-994.
Tay, Jessica Y. L.; Zotz, Gerhard & Einzmann, Helena J. R.
