High but not dry: diverse epiphytic bromeliad adaptations to exposure within a seasonally dry tropical forest community

Vascular epiphytes have developed distinct lifeforms to maximize water uptake and storage, particularly when delivered as pulses of precipitation, dewfall or fog. The seasonally dry forest of Chamela, Mexico, has a community of epiphytic bromeliads with Crassulacean acid metabolism showing diverse morphologies and stratification within the canopy. We hypothesize that niche differentiation may be related to the capacity to use fog and dew effectively to perform photosynthesis and to maintain water status. Four Tillandsia species with either tank or atmospheric lifeforms were studied using seasonal field data and glasshouse experimentation, and compared on the basis of water use, leaf water d18O, photosynthetic and morphological traits. The atmospheric species, Tillandsia eistetteri, with narrow leaves and the lowest succulence, was restricted to the upper canopy, but displayed the widest range of physiological responses to pulses of precipitation and fog, and was a fog-catching nebulophyte. The other atmospheric species, Tillandsia intermedia, was highly succulent, restricted to the lower canopy and with a narrower range of physiological responses. Both upper canopy tank species relied on tank water and stomatal closure to avoid desiccation. Niche differentiation was related to capacity for water storage, dependence on fog or dewfall and physiological plasticity.