Diverse spore rains and limited local exchange shape fern genetic diversity in a recently created habitat colonized by long-distance dispersal.

BACKGROUND AND AIMS: Populations established by long-distance colonization are expected to show low levels of genetic variation per population, but strong genetic differentiation among populations. Whether isolated populations indeed show this genetic signature of isolation depends on the amount and diversity of diaspores arriving by long-distance dispersal, and time since colonization. For ferns, however, reliable estimates of long-distance dispersal rates remain largely unknown, and previous studies on fern population genetics often sampled older or non-isolated populations. Young populations in recent, disjunct habitats form a useful study system to improve our understanding of the genetic impact of long-distance dispersal. METHODS: Microsatellite markers were used to analyse the amount and distribution of genetic diversity in young populations of four widespread calcicole ferns (Asplenium scolopendrium, diploid; Asplenium trichomanes subsp. quadrivalens, tetraploid; Polystichum setiferum, diploid; and Polystichum aculeatum, tetraploid), which are rare in The Netherlands but established multiple populations in a forest (the Kuinderbos) on recently reclaimed Dutch polder land following long-distance dispersal. Reference samples from populations throughout Europe were used to assess how much of the existing variation was already present in the Kuinderbos. KEY RESULTS: A large part of the Dutch and European genetic diversity in all four species was already found in the Kuinderbos. This diversity was strongly partitioned among populations. Most populations showed low genetic variation and high inbreeding coefficients, and were assigned to single, unique gene pools in cluster analyses. Evidence for interpopulational gene flow was low, except for the most abundant species. CONCLUSIONS: The results show that all four species, diploids as well as polyploids, were capable of frequent long-distance colonization via single-spore establishment. This indicates that even isolated habitats receive dense and diverse spore rains, including genotypes capable of self-fertilization. Limited gene flow may conserve the genetic signature of multiple long-distance colonization events for several decades.

Similar cation exchange capacities among bryophyte species refute a presumed mechanism of peatland acidification.

Fen-bog succession is accompanied by strong increases of carbon accumulation rates. We tested the prevailing hypothesis that living Sphagna have extraordinarily high cation exchange capacity (CEC) and therefore acidify their environment by exchanging tissue-bound protons for basic cations in soil water. As Sphagnum invasion in a peatland usually coincides with succession from a brown moss-dominated alkaline fen to an acidic bog, the CEC of Sphagna is widely believed to play an important role in this acidification process. However, Sphagnum CEC has never been compared explicitly to that of a wide range of other bryophyte taxa. Whether high CEC directly leads to the ability to acidify the environment in situ also remains to be tested. We screened 20 predominant subarctic bryophyte species, including fen brown mosses and bog Sphagna for CEC, in situ soil water acidification capacity (AC), and peat acid neutralizing capacity (ANC). All these bryophyte species possessed substantial CEC, which was remarkably similar for brown mosses and Sphagna. This refutes the commonly accepted idea of living Sphagnum CEC being responsible for peatland acidification, as Sphagnum's ecological predecessors, brown mosses, can do the same job. Sphagnum AC was several times higher than that of other bryophytes, suggesting that CE (cation exchange) sites of Sphagna in situ are not saturated with basic cations, probably due to the virtual absence of these cations in the bog water. Together, these results suggest that Sphagna can not realize their CEC in bogs, while fen mosses can do so in fens. The fen peat ANC was 65% higher than bog ANC, indicating that acidity released by brown mosses in the CE process was neutralized, maintaining an alkaline environment. We propose two successional pathways indicating boundaries for a fen-bog shift with respect to bryophyte CEC. In neither of them is Sphagnum CE an important factor. We conclude that living Sphagnum CEC does not play any considerable role in the fen-bog shift. Alternatively, we propose that exclusively indirect effects of Sphagnum expansion such as peat accumulation and subsequent blocking of upward alkaline soil water transport are keys to the fen-bog succession and therefore for bog-associated carbon accumulation.

The role of petioles in light acquisition by Hydrocotyle vulgaris L. in a vertical light gradient.

In natural herbaceous vegetation plants are exposed to a vertical light gradient. In experiments, however, morphogenetic responses of stoloniferous plants to shade have nearly always been tested under homogeneous shade conditions. In this study we simulated a vertical light gradient and found that the response of Hydrocotyle vulgaris in this gradient differed considerably from the responses to homogenous shade. Petioles grew longer while at the same time the specific weight of petioles increased. The elongated petioles raised leaf-blades into better-lit places resulting in higher biomass. Though leaves in the light gradient started their growth under low-light conditions, the size of the leaf-blade was the same as in high light. Internodes were longer than in homogeneous shade conditions but specific weight decreased, probably due to increased allocation to the fast-growing petioles.

Recent developments in bryophyte population ecology.

Bryophytes abound in a wide variety of habitats, and despite their low stature play a significant role in many ecosystems. Earlier views of bryophytes as being 'evolutionary failures' are being questioned since the discovery of high genetic variability in those species and populations which have been studied. At the same time, there is growing evidence that in many taxa the maintenance of populations is almost completely dependent on asexual propagation; sexual reproduction may result in an enormous spore output, but establishment from spores in the field seems to be very difficult. The remarkably rapid fine-scale dynamics found in many bryophyte populations may play a role in the maintenance of genetical variability; it may also partly determine community diversity.

The reproductive biology of Polytrichum formosum: clonal structure and paternity revealed by microsatellites.

Using highly polymorphic microsatellite markers, we assessed clonal structure and paternity in a population of the bryophyte species Polytrichum formosum. Identical multilocus genotypes of individual shoots were almost never observed in spatially separated cushions, but were found to be highly clustered within moss cushions. Therefore, asexual reproduction through dispersal of gametophyte fragments is not very important in P. formosum. However, asexual reproduction on a very localized scale through vegetative growth of genets (branching of gametophytes via clonal growth of rhizomes) is very extensive. The patchy spatial distribution of genets and the absence of intermingling among genets suggest that this species follows a 'phalanx' clonal growth strategy. Vegetative proliferation of genets will increase their size, and, consequently, will have considerable fitness consequences for individuals in terms of increased genet longevity and reproductive output. Although paternity analysis of sporophytes confirmed male genet size, i.e. gamete production, to be an important determinant of male reproductive fitness, it also showed that the spatial distance to female genets is the predominant factor that governs male reproductive success. Moreover, we showed that male gamete dispersal distances in P. formosum are much further than generally assumed, and are in the order of metres rather than centimetres. Combining the findings, we conclude that the high genotypic diversity observed for this facultatively clonal species is most likely explained by a preponderance of sexual reproduction over clonal reproduction.