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Common garden experiment

The objective of these experiments was to study offspring performance and quantitative genetic variation in Pimpinella saxifraga, Carlina vulgaris, and Hypochoeris radicata in relation to population size and isolation, and plasticity in Carlina. In a first set of experiments, plants were raised from seeds that had been collected in populations of a wide range of size and degree of isolation in different European regions (see 9379, 9380). Fragmentation reduces the size of populations and increases their isolation, and this may result in genetic erosion in the remaining populations.

Poorly dispersed species like Carlina and Pimpinella would be expected to suffer more strongly from the negative effects of reduced population size and increased isolation than well-dispersed species like Hypochoeris. We examined several traits, both morphological and fitness-related, in the plants grown in a common garden over two years and analysed the distribution of quantitative genetic variation among regions, populations within regions, and families. There was strong variation in most of the investigated characters among regions, among populations within regions, and among seed families within populations in all three species, although regional differences were not expressed in all characters in Pimpinella and Hypochoeris.

A partitioning of variation revealed higher differentiation among regions and among populations within regions in Carlina than in Hypochoeris, which was expected because of the higher dispersal ability and thus higher gene flow among populations in Hypochoeris. Population size and isolation did not influence the variability of morphological traits within populations in any of the species, but affected fitness-related traits in the two poorly dispersed species. In Pimpinella many reproductive traits and thus total fitness increased strongly with population size, but population isolation had little effect. In Carlina the size of flowering plants and multiplicative fitness increased with the size of the population of origin and decreased with increasing isolation. In contrast, there were almost no effects on the traits measured in Hypochoeris.

In a second study, we studied phenotypic plasticity in Carlina to investigate whether plants of Carlina vulgaris originating from different European regions and from populations of different size and degree of isolation differed in their degree of plasticity. Seeds were collected in populations in Czechia, Germany, Switzerland, Luxembourg, The Netherlands, and Sweden that differed in size and isolation. From the seeds plants were raised in a common garden at the University of Marburg, Germany. Overall, more than 1200 plants from more than 50 populations were grown. During two growing seasons, each plant was subjected to one of four treatments: (1) control, (2) drought, (3) fertilizer, (4) drought and fertilizer.

Drought is a common feature of the habitats of the species and eutrophication a problem that is affecting many populations. Fertilizing had a positive effect on vegetative growth and on reproduction, but reduced survivorship until the flowering period as well as flowering probability in surviving plants. Drought had weaker but overall favourable effects on plant performance, suggesting that this treatment was closer to optimal conditions for this species than were control conditions. For some traits the response to fertilizing differed significantly among regions, but the geographical pattern differed among traits. Fertilizing influenced the probabilities of survival and flowering of plants from the various populations within regions differently, but population size or isolation had virtually no effect on the reaction norms of plants. The results of the first study indicate that fragmentation is a serious problem for plants and that efforts should be made to avoid further reductions of population size.

As expected, the negative genetic effects of fragmentation were stronger in poorly dispersed species like Carlina and Pimpinella than in well dispersed like Hypochoeris. Thus, knowledge of the dispersability of species may be useful for the prediction of the effects of fragmentation on different plant species. The strong genetic differentiation found among different European regions is consistent with the results of the transplant studies (see 9380) and indicates that it is important to conserve large populations of plants in different European regions to preserve the genetic variability of a species.

The strong genetic variation among populations indicates very limited gene flow among the remaining fragmented populations within the regions. From the second study we conclude that for Carlina there was no indication that habitat fragmentation already had a negative effect on the phenotypic plasticity of this species. However, populations differed in their plasticity, which is consistent with the overall pattern of genetic isolation among the populations.

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