Abstract
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.
Highlights
Vascular epiphytes form a major component in many vegetation types, in tropical latitudes, representing globally almost ten percent of terrestrial plant diversity [1]
Drag reduction by streamlining as epiphytic bromeliads reorient in wind a key role in the hydrology of many tropical forests [3,4,5], influence the circulation of nutrient fluxes [6,7,8] and provide favourable micro-climates and refugia for other biota [9, 10]
At wind velocity 22 m s-1, drag force was reduced by up to 54% (G. monostachia (54%), V. sanguinolenta (47%), T. fasciculata (36%), T. flexuosa (37%), and T. elongata (22%))
Summary
Vascular epiphytes form a major component in many vegetation types, in tropical latitudes, representing globally almost ten percent of terrestrial plant diversity [1]. By definition, non-parasitic but are structurally dependent on trees, taking advantage of previously unexploited spaces such as tree crotches and branches in the canopy [2] There are two ways in which wind acts as a mechanical disturbance to epiphytes—either by directly dislodging individuals from their substrate or by breaking the substrate itself. In both cases, survival on the ground is transitory and premature death is almost inevitable [20]
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