How Much Water is in the Tank? Model Calculations for Two Epiphytic Bromeliads

Abstract

Water-impounding foliage (=tank) characterizes many bromeliad species but, to date, a quantitative evaluation of the effectiveness of these structures to bridge rainless periods is not available. We present a model that analyses the capacity of these phytotelmata to supply moisture to the bromeliad as a function of plant size, and parameterize it for two epiphyte species,Tillandsia fasciculataandGuzmania monostachya. All parameters related to the recharging of a tank (e.g. catchment area, or tank capacity) and those related to water loss from the tank (e.g. tank water surface, or water absorption from the tank by the plant) changed with plant size. For both species a similar qualitative size-related pattern emerged: smaller individuals dried out much faster than larger conspecifics. Modelling the changes in tank water content for an entire year revealed that in the seasonal climate of Barro Colorado Island, Panama, tanks of smaller plants (1 g d. wt) are expected to lack water for up to 110 d (inT. fasciculata). Regardless of plant size, individuals ofG. monostachyawere consistently more efficient thanT. fasciculatain maintaining moisture in their tanks. Our results provide insight into the limitations of water-impounding tanks as an adaptation to cope with an intermittent water supply typical for epiphytic habitats. These limitations are particularly pronounced for smaller individuals.