Phenotypic plasticity and biomass allocation pattern in three Dryopteris (Dryopteridaceae) species on an experimental light-availability gradient

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.