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FP5

TRANSPLANT Résumé de rapport

Project ID: EVK2-CT-1999-00004
Financé au titre de: FP5-EESD
Pays: United Kingdom

Measuring / modeling dispersal

In 2001 and 2002, Workpackages 2-6 collected seeds of five species from five countries (UK, The Netherlands, Germany, Sweden, the Czech Republic). The species were chosen from those assigned to different cells of a colonisation/longevity matrix previously established by TRANSPLANT partners. The species were Tragopogon pratensis (ssp. pratensis and orientalis), Hypochoeris radicata, Carlina vulgaris, Succisa pratensis and Pimpinella saxifraga. A standard sampling scheme was used to sample seeds from each of three small and three large populations in each country, for all species for which this was possible. Seeds were transported to the UK under controlled conditions, to ensure that they suffered no damage or change to their natural characteristics, and stored under standard conditions.

Measurements were carried out on seed weights, lengths, pappus widths (in the cases where a pappus was present), and terminal velocity, using a machine designed specifically for this part of the project. Data were analysed for within-species differences in the measured characteristics that could be referred to country of origin, population size and year of collection. The assumptions that seed size is correlated with dispersal distance, and that lighter seeds are more prone to be incapable of germination, were also tested.

For the latter test, germination was compared for seeds from the two extremities, and from the centres, of the individual seed weight distributions for each species. For Hypochoeris and Pimpinella, all seed weight categories exhibited >80% germinability. For Carlina and Tragopogon, heavy and medium seeds displayed >70% germinability, whereas small seeds exhibited just over 50% germinability. The germinability of Succisa seeds fell as seed weight declined, to a value of 30-50% for the lightest seeds. Nevertheless, many of the lightest seeds dispersed by all species are clearly capable of germination, and thus these seeds must be considered when dispersal and colonisation potentials of dry calcareous grassland species are investigated. These light seeds have the potential to establish in suitable habitat at the most extreme distances to which seeds can disperse.

Deterministic models have been utilised, using measured individual seed and plant characteristics, to establish the seed dispersal kernels for the five species. Particular attention was paid to the long distance dispersal tail of the distributions. Characteristics measured at the species' field sites, including wind speeds and their frequency distributions, were used to create realistic scenarios of the conditions under which seeds disperse at each of the sites of collection. In addition, extreme wind conditions and standard wind profiles were used to model dispersal kernels. Using standard wind profiles, the characteristics of the dispersal kernels for the five species were found to be affected by different characteristics.

For Pimpinella, average dispersal distances differed significantly between seeds obtained from different countries and between seeds from small and large populations (seeds from small populations had larger average values). Dispersal of Succisa seeds also differed significantly between countries, and seeds from small populations showed smaller dispersal distances. Carlina dispersal differed for seeds collected in different years. Tragopogon dispersal did not differ significantly between country of origin and size of population, although for seeds collected from different countries, significantly different proportions were uplifted by the wind profiles used in the models. Dispersal kernels for Hypochoeris seed showed significant variation due to country of origin, size of source population and year of collection.

When extreme wind profiles were used in the model, the overall dispersal distances increased for all species. Ultimately, the dispersal kernels demonstrate that the majority of populations in fragmented patches of calcareous grassland habitats in Europe are effectively isolated, since seeds cannot disperse across the intervening countryside, even under the most favourable conditions. It can therefore be confirmed that many populations in fragmented calcareous grassland habitats are already effectively ecologically and evolutionarily isolated.

Detailed examination has also been undertaken by WP5 of seed dispersal under field conditions using sticky trapping methods, study of soil seed banks by soil coring methods, and investigations of seed dispersal via dog-walking activities. Results indicate that 80% of seeds collected by dogs' coats are deposited within 0.25km, although 7% can be dispersed at least 3km. Seeds carried for such long distances would be capable of being moved between isolated fragments of calcareous grassland. Although most of the more widely dispersed seeds possessed structures obviously promoting adhesive dispersal, this was not always the case.

Informations connexes

Contact

Michael HUTCHINGS, (Professor of Ecology)
Tél.: +44-1273-872761
Fax: +44-1273-678433
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