Diversity and evolution of pteridophytes, with emphasis on the Neotropics

Abstract

Pteridophytes, understood as taxonomic group containing ferns, horsetails, and clubmosses, are especially diverse and abundant in the tropical mountain ranges of South America. My work aims to understand better the diversity of these plants, how they have evolved, and how they function in the ecosystems. Tree ferns are conspicuous elements of tropical montane rainforest. About 200 species of scaly tree ferns of the family Cyatheaceae occur in the Neotropics, ranging from small trunkless ferns of rocky outcrops to almost 20 m tall giants in dense forests. My work on these plants led to the recognition of 19 new and several formerly synonymized species in the genera Alsophila and Cyathea. The confusing nomenclature of the family was partially clarified by the correction of the typification of Cyathea pallescens (Sodiro) Domin. A checklist with keys to the Bolivian tree ferns was also accomplished during these studies. The revision of the genus Melpomene revealed 27 species with 10 varieties, most of them small ferns with deeply pinnatifid fronds, which are mostly epiphytes in upper montane forests or characteristic terrestrial elements of treeless páramo vegetation. The phylogenetic analysis based on morphology and chloroplast DNA shows that this genus as currently morphologically circumscribed is monophyletic and originated in South America. The radiation of the core group is apparently directly connected with the uplift of the northern Andes, which is the center of diversity of this genus. My ecologically orientated field studies of fern communities in southern Ecuador initiated several projects that aimed on different aspects. I found 248 different species of pteridophytes in our main study area, the Reserva Biológica San Francisco (RBSF), Prov. Zamora-Chinchipe, but this number is likely to increase in the future. The study area is part of the Amotape-Huancabamba zone, a stretch of low elevation in the Andes located a the overlap of several biogeographic subunits and thus rich in endemic and widespread species alike. I found that the upper limit in the elevational distribution of most of the widespread ferns follows this dent in the mountain range, indicating that probably a downward shift of all vegetation belts may be found. The mountain ridges in the RBSF support a unique heath forest dominated by the otherwise rare tree Purdiaea nutans Planch., but these peculiarities are not reflected in the fern composition. Ridge habitats in the study area, including two comparative sites close to the RBSF, are less diverse than adjacent slopes, and there was no higher representation of localized species on ridges. Overall, widespread species were weakly but significantly more abundant than localized species, and terrestrial but not epiphytic species were more abundant on ridges compared to slopes. The observed influence of soils on the distribution of ferns in the Ecuadorian study area, where terrestrial and phylogenetically more derived taxa increase in diversity along a nutrient gradient caused by a downhill flux from the ridges to the gorges, encouraged me to look at the soil preferences of pteridophytes worldwide and see if it contains a phylogenetic signal. It seems that more derived lineages are better represented on rich soils, but have also a higher percentage of epiphytes. Looking at the mediator between soils and plant roots, the mycorrhizal fungi, I found that the published reports cover only a small fraction of the fern diversity and often give contradicting results. Focusing on neglected taxa, like the epiphytic Hymenophyllaceae, grammitid ferns (Polypodiaceae), and the genus Elaphoglossum (Dryopteridaceae), the investigation of root samples gathered in the Ecuadorian study area increased the known number of fern species with ascomycete infection considerably. The finding of this more derived type of mycorrhiza is in concordance with the phylogenetic position and life form of the host plants. Terrestrial and especially phylogenetically basal groups of pteridophytes have predominantly vesicular-arbuscular mycorrhiza, which is a very common and supposedly old form symbiosis.