The Spatial Separation of Callistopteris baldwinii (Hymenophyllaceae) Sporophytes and Gametophytes Along Elevational Gradients in Hawai‘i

Abstract. Ferns and lycophytes are unique among land plants in having two independent life stages, the haploid gametophyte generation and the diploid sporophyte generation. While in most species the sporophyte is the dominant, long-lived portion of the life cycle, in some ferns the gametophyte is capable of sustained vegetative growth, and a number of species produce sporophytes only in parts of their geographic range (a pattern known as the separation of generations). One such species is the Hawaiian filmy fern Callistopteris baldwinii, whose growth form varies across elevational gradients. This species occurs as independent gametophytes near sea level, produces dwarfed sporophytes at mid elevations, and at the highest elevations—where precipitation is greatest due to the orographic uplift of trade winds—it grows as large, mature sporophytes. We measured temperature, relative humidity, and precipitation for several populations of this fern on the island of O‘ahu in Hawai‘i to determine whether these environmental factors may influence the spatial separation of generations exhibited by C. baldwinii on the island. Our results indicate that temperature and precipitation vary across life stages of C. baldwinii, underscoring the key role environmental conditions play in the completion of the fern life cycle.