Numerical syntaxonomy of the Asplenietalia petrarchae in the Iberian Peninsula

Abstract. A numerical syntaxonomic study of rupicolous vegetation of the Asplenietalia petrarchae in the Iberian Peninsula is described. The study is based on 37 local tables (OSUs) of different rupicolous communities from limestone cliffs found in the eastern half of the Iberian Peninsula. Three classification approaches were followed: (1) ‘hard partition’ techniques, (2) ‘fuzzy partition’ and (3) ordination partition. In order to find a direct solution, we tried an evaluation system based on the inertia associated with a CCA, Canonical Correspondence Analysis, and its relationships with the inertia of a similar CA, Correspondence Analysis. The most effective solutions were found through spatial partitions, and the best partition was obtained with CA considering five segments per axis. Among the fuzzy partitions the best solution was obtained with two clusters and a fuzziness value of 1.20. The best hierarchical partition was Complete Linkage with Chord Distance as a resemblance function. With the GRID procedure applied to the CA diagram, three different groups were distinguished. The first group is related to the Saxifragion boisseriano-reuterani, including five OSUs from Baetic ranges; the second group is very close to the Cosentinio-Lafuenteion rotundifoliae and includes 11 OSUs from thermo-mediterranean areas; the third group, including 21 OSUs basically related to meso-mediterranean areas, is strongly related to the Teucrion buxifolii.     

Evidence for lottery recruitment in Mediterranean old fields

Abstract. The hypothesis of lottery establishment ( Sale 1977 ) explains coexistence of species with similar niches through processes of stochastic recruitment. This initial idea forms the basis for a variety of mathematical models, but has not been tested empirically. This study is a field investigation of lottery establishment for plants with a seed bank, using Canonical Correspondence Analysis to compare the compositions of the vegetation and the seed bank according to different hypotheses on the mechanisms of establishment. This method was used for a data set from old fields from southern France. The weighted lottery (i.e. a random draw from the seed pool, weighted by the frequencies of each species) appeared as the best suited hypothesis to explain the high degree of similarity between the vegetation and the seed bank and the relative spatial distributions of the species. Several mechanisms are probably interacting, depending on the life histories of the species. Modelling and experimental approaches are needed to further test the hypothesis of lottery recruitment.     

Spatio-temporal patterns in post-burn succession on Brittany heathlands

Heathlands in the Paimpont area, Brittany, were ravaged by numerous fires in the summer of 1976. The period in which the fires occurred, and their intensity had important consequences for recolonization processes. In the tall Ulex europaeus heathland described in this paper, fire occurred relatively late (mid-July), and was of high intensity. Colonisation was mainly from propagules, often from species foreign to the original heathland. Plant succession together with vertical and horizontal structure was studied in permanent plots over 13 yr. Three stages were distinguished. Vertical structure varied greatly among the three stages. It was very simple in the colonisation phase, became more complicated during the competition phase, but became simpler again in the stabilisation phase. This was particularly evident for the moss layer, which, 13 yr after the fire, was made up of Polytrichum commune alone. Mosses showed interspecific competition with size as the decisive factor: the large mosses gradually eliminated the smaller ones. Horizontal structure was studied using spatial autocorrelation, which provided some indication of the distinctive -ness and scale of spatial pattern. Some species were weakly aggregated such as Ceratodon purpureus, Polytrichum pili-ferum, and P. juniperinum, whileothers, like the P. formosum-P. commune complex, Agrostis curtisii and Ulex minor were strongly aggregated. In the current community, 13 yr after the fire, a mosaic can be distinguished with three components: (1) a moss layer of Polytrichum commune on which a herbaceous layer, dominated by Agrostis curtisii has developed; (2) a birch wood (both Betulapéndula and B. pubescens), which established on the Polytrichum - Agrostis combination; (3) patches of Ulex minor heathland, with a few Frica and Calluna plants. Extreme simplicity and stability are the two prominent features of the present vegetation state.     

Spatial variation in a grassland on soil rich in heavy metals

Abstract. The vegetation in a grassland area in the prefecture of Kilkis (N. Greece), known for its surface Cu-mineralization, was studied. 43 quadrats were established along a transect through an area where the vegetation formed patches of different size. Cover-abundance and frequency estimates for all species were made. Normal Association Analysis revealed five quadrat groups characterized by Trifolium scabrum, Linaria pelisseriana, Anthoxanthum ovatum, Gypsophila muralis and Minuartia hirsuta ssp. falcata. 43 soil samples were analysed for pH, organic C, CaCO₃, total content of Fe, Zn, Pb, Cu, Mn, Ca, Mg, K, Na and soil texture. From a discriminant analysis performed on soil data five soil groups resulted, which are highly related to the five vegetation groups. Thymus sibthorpii, Minuartia hirsuta ssp. falcata and Rumex acetosella are the most important taxa with respect to physiognomy and patchi-ness of the vegetation. The number of species in each group of quadrats is affected by the relative favourableness or severity of the soil conditions. Metal contents, mainly Zn, Cu, Mg and Na, and soil texture are considered to be among the main factors controlling the structure and physiognomy of the vegetation.     

Mediterranean pasture dynamics: the role of germination

Abstract. Meteorological patterns have a decisive influence on the inter-annual dynamics of therophyte pastures under Mediterranean climatic conditions. The germination behaviour of annual pasture species was studied by subjecting two collections of seeds taken from plants and soil-seed banks to two phytotron-simulated weather patterns: early and late autumn rains. Species from these pastures were arranged along a gradient of sensitivity to temperature on the arrival of the first persistent rain. This sensitivity was manifested in both the total germination success of the species and the germination time profile. The different germination patterns of the species can provide competitive advantages depending on the autumn weather conditions.     

Do rivers function as corridors for plant dispersal?

Abstract. We evaluated the importance of dispersal for species frequencies and distribution by comparing dispersal properties of vascular plant species with their frequencies along river banks. We assumed that species with long-floating seeds would be more frequent than species with short-floating seeds. We compiled data on frequencies of vascular plants and their dispersal properties from ten rivers in northern Sweden and compared these with boreal forests and grasslands in the same region. In all rivers, but in none of the reference areas, there was a positive relationship between floating capacity and frequency of species. A comparison of floating capacity between species with and without certain dispersal devices showed that seeds of vegetatively dispersed species had higher floating capacities than other seeds. For other dispersal categories (animal and wind dispersal), floating time did not differ from contrast groups. The results indicate that water dispersal has a certain role in structuring the riparian flora, and provide a basis for explaining species distribution patterns from dispersal characteristics. They also suggest that continuous river corridors are important for maintaining regional biodiversity.