Biological Flora of the British Isles: Dryopteris carthusiana, D. dilatata and D. expansa

Summary1. This account presents information on all aspects of the biology of Dryopteris carthusiana (Vill.) H. P. Fuchs, D. dilatata (Hoffm.) A. Gray, and D. expansa (C. Presl) Fraser‐Jenk. & Jermy that are relevant to an understanding of their ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, reproductive characteristics, herbivores, history, and conservation.2. All three species are native deciduous ferns that are morphologically similar and genetically interrelated. Dryopteris dilatata, one of the commonest ferns in the British Isles, is found in many different habitats, but is above all a woodland species. Dryopteris carthusiana is less widely distributed, being mostly a species of wetlands and wet woodlands. Dryopteris expansa is the least common, mostly found in mountains, but also in wet woodlands at lower altitudes.3. Dryopteris dilatata is mainly a species of semi‐shade, and in the British Isles is considered to be the most shade‐tolerant of the three. Dryopteris carthusiana occurs in a wide range of habitats, from exposed, well‐illuminated to moderately shaded ones. Dryopteris expansa mainly grows in better illuminated habitats, often in the shade of sparse canopies or rocks. D. expansa is thus the most light‐demanding species of the three.4. The response to competition from neighbouring herbs has been shown to differ among the three species; D. expansa is clearly more vulnerable to competition than D. carthusiana and D. dilatata.5. Dryopteris dilatata and D. carthusiana are both tetraploid, whereas D. expansa is diploid. Natural hybrids among all three species in the British Isles, as well as hybrids of D. carthusiana with D. filix‐mas and D. cristata. The hybrid D. carthusiana × D. dilatata = D. × deweveri is the most common. The hybrids D. carthusiana × D. expansa = D. × sarvelae and D. cristata ×D. carthusiana = D. × uliginosa are extremely rare and at high risk of extinction.6. Although not currently threatened, the distribution of all three species may be susceptible to continued habitat loss arising from changes in land use, management for control of Bracken and predicted climate change.