C. Levassor

Similarity between seed bank and vegetation in Mediterranean grassland: a predictive model

. The similarity in species composition between seed bank and vegetation was analysed in Mediterranean grasslands in relation to altitude, topography and grazing. Soil samples were collected in permanent plots in autumn at the end of the summer drought period and in spring, before the new seed fall and after the natural winter seed stratification. The seed bank composition was determined by greenhouse germination over a nine-month period. Presence/absence of species in the standing vegetation throughout the complete annual cycle, and the percentage area of bare ground in October, were recorded in the same plots. The species composition of the standing vegetation is clearly determined by altitude, topography and grazing, while the floristic composition of the seed banks is only related to altitude and topography in the case of autumn seed bank and with any of the three factors in the spring seed bank. Relative abundances of grasses, legumes and forbs also show different patterns in vegetation and seed bank data. Sorensen similarity between the autumn seed bank and the vegetation declines as altitude rises, but there are no significant differences for topography and grazing. This similarity decreases in the case of the spring seed bank and does not show any significant relationship with any of the factors. The perennial/ annual ratio and the proportion of bare soil in October are proposed as explanatory variables in a predictive model of similarity between the seed composition of the seed bank and vegetation.

Species diversity patterns in Mediterranean grasslands

. A field experiment involving herbivore exclusion, ploughing, and the combination of both was carried out over a period of 4 - 5 yr in Mediterranean grasslands located along an elevational gradient. The empirical results provide a general hierarchical framework for understanding patterns of plant species diversity in thesegrasslands. In grazed grasslands, plant species density decreased as altitude increased, and this pattern was maintained through time. The reduced seasonality along the climatic gradient is suggested as the extrinsic, indirect control factor. Ploughing caused species loss, but after 4 yr the original diversity was recovered in most grasslands. Our hypothesis is that a negative feedback mechanism regulates species increase towards a characteristic level. A trend of species density reduction was observed in ungrazed grasslands. Plant-herbivore interaction is considered to be essential for maintaining species diversity in grazed grasslands and for the recovery of diversity in mechanically disturbed grasslands.

Seed size, shape and persistence in dry Mediterranean grass and scrublands

Seed size and shape, measured as the variance of the three main dimensions, have been proposed as good indicators for predicting seed persistence. We tested whether these variables were robust predictors of seed persistence in the soil for 58 abundant herbaceous species, primarily annuals, in grass and scrubland of central Spain. Seed persistence was estimated from data on germinable seed banks, while seed weight and shape were measured using fresh seeds collected in the study area. There was a significant tendency for species with persistent seeds to have smaller seeds than species with transient seeds. Seed shape was not, however, related to persistence and we did not find any clear seed weight/shape threshold for predicting persistence. The binary logistic model of seed bank type as a function of seed weight was significant and explained 67% of total variability. Supplementary information about dormancy, environmental conditions of habitat, predation and attack by pathogens has to be used to elaborate more accurate general predictive models of seed persistence.

Adaptation of ecological systems: compositional patterns of species and morphological and functional traits

. The species composition and morphological and functional traits of ecological systems are compared. The significance of the resulting relationship is interpreted considering the underlying spatial environmental variation. Using 60 morphological and functional traits, a gradient of ecological persistence was detected in Mediterranean grasslands along an altitudinal range of ca. 1200 m. This pattern coincides with the one expressed by its taxonomic composition and thus permits ‘styles of adaptation’ of ecological systems to be recognized. Species composition may represent one of the possible combinations which optimises its environmental fit. The relevance to ecological theory of this pattern of grassland organization is discussed in the context of a hypothetical model of evolution.

Commonness and Rarity: Theory and Application of a New Model to Mediterranean Montane Grasslands

The copyright to this article passed from the Ecological Society of America to the Resilience Alliance on 1 January 2000. Published in the Journal Ecology and Society (formerly Conservation Ecology)

Effects of area, environmental status and environmental variation on species richness per unit area in Mediterranean wetlands

We propose a mechanistic model to relate a- and y-diversity to area per se, moisture status and environmental variation (local and total), and explored the effects these abiotic variables have on species richness per unit area (a- diversity) for plant communities in a network of wetland habitats located in a Mediterranean mountainous region of central Spain. In this study, environmental status is measured as actual evapotranspiration (as an expression of energy), slope and soil wetness, and environmental variation refers to slope variation and soil wetness variation. Species richness per unit area was related to soil wetness, soil wetness varia- tion, ground slope and ground slope variation. There were also positive correlations among moisture status and environmen- tal variation variables. There is a joint effect of slope and soil wetness variation in explaining species richness per unit area of these wetland habitats, but area effects and energy are relatively unimportant. We conclude that species richness per unit area of wetland vegetation can be explained by moisture status and local environmental variation, and that habitat area may not have an important effect. Area could affect y-diversity directly through random sampling and/or indirectly through increasing /-diversity, and energy may be important in areas with larger energy ranges. Complete surveys of environmental status, local and total environmental variation, and their asso- ciated species assemblages are needed to explain the processes that give rise to the rule that larger areas have larger species richness.

Vegetation of groundwater discharge sites in the Douro basin, central Spain

The chemical composition of the groundwater and the distribution of phreatophytic vegetation in a relatively homogeneous aquifer in the Douro basin appear to depend primarily on groundwater flow patterns within the aquifer. The concentration of major ions in the water is correlated with flow path length, which indicates that the chemical composition of the groundwater changes with the ageing of the water. The distribution of phre- atophytic plant species is closely related to both the chemical composition of groundwater discharging in seepage areas, and length of the groundwater flow path. Factors such as heterogeneity in lithology, human influ- ences and local climate are of secondary importance.