Water availability and evolutionary similarity shape the global distribution of ferns with chlorophyllous spores

Abstract

About 14% of all fern species have chlorophyllous spores, which lack dormancy, have thin walls, and have a shorter viability (only a few days in some species). These spores should have limited dispersal distances and be more susceptible to harsher climatic conditions, raising questions about the evolutionary and ecological significance of this trait. Here, we assemble the global distribution of chlorophyllous‐spored ferns and assess the underlying environmental and evolutionary factors. We first evaluated the environmental predictors of the proportional representation of 1387 chlorophyllous‐spored species (CSS) across 577 geographical regions using generalized linear mixed models. We then estimated the phylogenetic signal of spore type and assessed the relative importance of environmental factors in the phylogenetic structure of fern assemblages. Species richness of CSS peaked in the tropics, while their proportional representation was highest in temperate and island floras. The proportion of CSS was positively associated with water availability and less seasonal climates. Spore type was strongly conserved phylogenetically, and CSS assemblages were phylogenetically clustered towards higher latitudes. Our study provides strong evidence that chlorophyllous spores do not limit the geographical distribution of fern species and that their latitudinal distribution patterns can be explained by a combination of environmental and evolutionary factors.