INFLUENCE OF PLANT SIZE ON THE ECOPHYSIOLOGY OF THE EPIPHYTIC FERN ASPLENIUM AURITUM (ASPLENIACEAE) FROM COSTA RICA

Premise of the study: A central goal of plant ecophysiological studies is to generate patterns of physiological behavior that are applicable to a species, which can be complicated when plant size is considered. Studies indicate that plant size can influence numerous ecophysiological parameters, especially in vascular epiphytes. The few studies that have included ferns in their analyses suggest that plant size is less important in ferns than angiosperms. This study investigates this apparent disparity by examining the relationship between plant size and an array of ecophysiological parameters in the epiphytic fern Asplenium auritum, especially the role of plant size in determining responses to water stress. Methods: Plants were classified according to size and measured for a variety of functional traits, including maximum photosynthetic rate, stomatal conductance, water-use efficiency, stomatal density, chlorophyll content, chlorophyll a/b ratio, specific leaf area, whole plant drying rates, and desiccation tolerance. Results for all traits were compared across size classes to determine size-related differences. Key results: Plant size significantly influenced most traits examined, most notably photosynthetic rate, stomatal density, stomatal conductance, whole plant drying rates, and recovery from desiccation. We report the first evidence of size-mediated shifts in desiccation tolerance in plants: small individuals tolerated water loss, whereas larger individuals avoided desiccation. Conclusions: Our findings indicate that size-mediated ecophysiological shifts are more important than previously thought in ferns, particularly in regard to water relations. Desiccation tolerance may allow young fern sporophytes to establish in stressful environments and is subsequently lost in older individuals.