Question: Do different competitive abilities of three fern species explain their different regional abundances? Location: Estonia, Europe. Methods: The factorial pot experiment, in which single individuals of three fern species were grown in natural soil with 0, 2,4, and 8 neighbouring individuals of Deschampsia flexuosa. Results: The response patterns of different biomass fractions and morphological parameters of fern species were similar to each other. The diploid D. expansa was the most vulnerable to competition, while tetraploid D. carthusiana and D. dilatata were more tolerant. D. carthusiana allocated relatively more to below‐ground parts than the other two species and allocation to roots increased when neighbour density increased. For D. expansa and D. dilatata, allocation to below‐ground parts decreased at high neighbour density, while in D. dilatata also the relative length of the stipe increased. Thus, the response of D. carthusiana corresponds to a ‘persistence type’ and that of D. dilatata to a ‘foraging type’. Conclusions: Lower vulnerability of D. carthusiana to competition, compared to D. expansa, may explain the higher regional and local abundance of the former. The rarity of D. dilatata in Estonia, however, could rather be explained by the impact of climatic factors, since this species is near its northeastern distribution limit in Estonia.

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. 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.

We were interested in whether the contrasting regional distribution patterns of three congeneric, frequently co-occurring fern species (Dryopteris carthusiana, D. dilatata and D. expansa) could be explained by differential biomass allocation strategies and different phenotypic plasticities to light availability. The morphology and habitat preference of these ferns are known to be very similar, but in Estonia, their frequencies of occurrence differ sharply--Dryopteris carthusiana is common, D. expansa grows in scattered localities, and D. dilatata is rare. We grew the species under different levels of illumination (100, 50, 25 and 10% of full daylight) in an experimental garden to compare their autecological responses to shading. After one growing season there were clear interspecific differences in total plant biomass accumulation--D. carthusiana > D. expansa > D. dilatata--indicating the possible competitive inferiority of the latter at the young sporophyte stage. D. expansa was the least shade-tolerant, with biomass decreasing sharply under less than 50% illumination; D. dilatata was the most shade-tolerant, with similar growth at all illumination levels. In relative biomass allocation patterns, the most notable differences among species were in the relative shares of biomass stored in rhizomes. In D. carthusiana and D. expansa this share was nearly constant and independent of the illumination conditions. D. dilatata allocated very little biomass into rhizome in deep shade, but was able to increase this share more than twofold in 50% light. Dryopteris dilatata was clearly shown to be morphologically the most plastic of the three. In four traits--rhizome mass, frond:below-ground biomass ratio, stipe length and specific leaf area--its degree of ontogenetic plasticity to light was significantly higher than that of D. expansa and D. carthusiana. While the general performance (biomass production) of species in the experiment coincided with that observed in nature, the results of plasticity estimation were somewhat surprising--it is difficult to explain the inferior performance of a species (D. dilatata) through high morphological plasticity. Probably, the species is rare either because of certain climatic restrictions, or because it is presently expanding its distribution and is in the phase of invading Estonian understory communities.

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