Environmental factors and vegetation composition, Lefka Ori massif, Crete, S. Aegean

Aim The aim of this study was to explore the environmental factors that determine the spatial distribution of oro‐mediterranean and alti‐mediterranean plant communities in Crete. Location The paper provides a quantitative analysis of vegetation–environment relationships for two study areas within the Lefka Ori massif Crete, a proposed Natura 2000 site. Methods Eleven environmental variables were recorded: altitude, slope, aspect, percentage of bare rock, percentage of unvegetated ground, soil depth, pH, organic matter content and percentages of sand, silt and clay content. Classification of the vegetation was based on twinspan , while detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA) were used to identify environmental gradients linked to community distribution. Results One hundred and twenty‐five species were recorded from 120 plots located within the two study areas. Forty‐seven of the recorded species are endemic, belonging to 35 families. Hemicryptophytes and chamaephytes were the most frequent, suggesting a typical oro‐mediterranean life form spectrum. The samples were classified into five main community types and one transitional. The main gradients, identified by CCA, were altitude and surface cover type in the North‐west site, while in the Central site the gradients were soil formation‐development and surface cover type. Main conclusions The use of classification in combination with ordination techniques resulted in a good discrimination between plant communities and a greater understanding of controlling environmental factors. The methodology adopted can be employed for improving baseline information on plant community ecology and distribution in Mediterranean mountain zones.     

Plant species coexistence in cliff habitats

An ecological analysis of the ledge and crevice vegetation of cliffs in north-east Ireland was carried out using a sampling programme based on multivariate land classification. The composition of the vegetation was shown to be determined primarily by distance from the sea, elevation and rock type. The chemical composition and physical structure of the cliffs, together with environmental gradients associated with aspect, elevation and nearness to the sea provide a wide range of habitats in which plants coexist. Certain types of cliff vegetation contained associations of species with a contrasting ecology, such as arctic–montane, maritime, woodland, calcicole and calcifuge plants. The positions of these species in ordination space were found to be unique but overlapping. A model is described to account for these observations. It proposes that species dominance in cliff habitats is prevented by factors such as physical disturbance and that plants are able to coexist in cliff habitats which meet their common environmental tolerances and preferences. The model predicts that cliff habitats with the greatest diversity of species are likely to be subject to less extremes of environmental variability. These habitats include inland sites with nutrient enrichment by sea spray, north-facing sites, seepage sites, base-rich sites, low elevation sites and sites with a heterogeneous physical structure.     

Regeneration patterns in relation to canopy species composition and site variables in mixed oak forests in the Sierra de Manantlán Biosphere Reserve, Mexico

Regeneration patterns in relation to canopy species composition and site variables were analyzed in mixed oak forests of the Sierra de Manantlán in western Mexico with the aim of establishing an ecological basis for the design of management alternatives. Using ordination (canonical correspondence analysis) and classification (two-way indicator species analysis) methods, five different canopy types and three different seedling associations were revealed according to species composition, all of them dominated by one or more oak species. Red–far red ratio, slope, altitude, topography, canopy type and grazing intensity were the main variables that explained differences in species composition among the seedling associations. Oak seedlings were relatively scarce in the sampling plots, with the lowest frequency values of all species recorded except for those of Quercus crassipes Humb. & Bonpl., and also the lowest density values. The presence of a particular oak seedling species was strongly associated with a particular percentage of canopy openness; Quercus candicans Née, Quercus laurina Humb. & Bonpl. and Quercus rugosa Née were present in the plots with the least-open canopy (6.4%, 2.9 and 6.2%, respectively), while Quercus castanea Née and Quercus crassipes Humb. & Bonpl. were present in the plots with the most-open canopy (13 and 8.1%, respectively). Every oak seedling species was more frequent, although not dependent, on the canopy type where the same oak species dominated. Because of the great heterogeneity in species composition and the physiographical factors of mixed oak forests in the Sierra de Manantlán, we concluded that management alternatives must be prescribed for each ecological situation where the different oak species are growing.