Abstract. The spatial organization of Mediterranean grassland in Spain is described, based on samples from 71 sites covering the existing variation in slope exposition and inclination.The whole set can be regarded as representing a trophic gradient, along which gradual variation in soil, species composition, biomass, and coverage were quantified. Corresponding to other studies from varying habitats, maximal species richness, diversity, and heterogeneity were observed on moderately infertile sites. Maximum species richness, over 60 species, occurred on sites with biomass values from 150–350 g / m2. Species richness values are much higher and biomass values are much lower than those found in temperate grasslands. The decrease in diversity towards the mosteutrophic communities is stronger than expected, but can be easily explained by the high grazing pressure here. The variation in diversity observed runs parallel with that in heterogeneity. Zones with a high species richness also have a high heterogeneity, meaning a low amount of dominance. Grazing is understood as abiotic form of disturbance. Differences in grazing pressure may modify the relation between richness and fertility. While the greatest grazing pressure coincides with the most eutrophic communities, decreasing progressively towards the oligotrophic ones, the trend predicted by the resource availability theory is maintained; species diversity will be maximal at intermediate levels of disturbance. Absence of grazing in the eutrophic communities would lead to an investment in the soil of the unconsumed organic matter.

Communities of plants determine nonrandom spatial patterns defined by the intervention of abiotic and biotic factors acting at different spatial scales. We consider the influence of shrubs as one of the most important factors (biotic) affecting these spatial patterns at microscale. The macroclimate could be considered one of the most important factors (abiotic) at regional scale. To study the role and the floristic implications of each factor on the global patterns of herbaceous communities, we have developed a stratified sampling design that integrates both micro and macroscale on a 100 Km-long transect (east-west) in western central Spain. The results suggest that macroclimate could be one of the most important factors in determining herbaceous spatial patterns. Moreover, shrubs create a microspatial environmental heterogeneity that could alter such global climate patterns, modifying the spatial affinities established among species. This implies that environmental heterogeneity related to microhabitat could play a key role in spatial patterns at broad spatial scales, and consequently in the dynamics of the distribution and establishment of herbaceous species.

Two grassland slopes showing a markedly different degree of homogeneity were investigated. In the classic sector sequence of exportation, transport and sedimentation diversity decreases from the highest to the lowest locations. The exportation sector is relatively homogeneous, while the transport sector is more heterogeneous but only under specific circumstances and over short distances (local level). By contrast, the sedimentation sector appears heterogeneous only over greater distances (regional level).The sedimentation sector can be subdivided into two subsectors: wet and relatively dry. Between these, a third subsector can be observed, forming a transition band with intra-and inter-annual fluctuations of the ground water level, leading to very high diversity and moderate or low heterogeneity. As small changes in the availability of soil water can determine the spatial movement or disappearance of some of these subsectors, it is not surprising that the lower parts of the slopes show great variation in structure, which may explain frequent contradictions in the literature.

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Dehesa ecosystems on granite and on slate clearly differ in soil characteristics. Our aim was to find out whether floristic composition differed too.We selected an equal number of plots, on both granite and slate, in different successional stages of development and at various ages after field abandonment (the youngest 15-20 years). Plots with rock-outcrops, which had never been cultivated, were also included in the study.The results showed that floristic differences between granite and slate communities were not detectable in the younger successional stages. However, they showed up with time and on stabilized grasslands became significant. But on plots with rock-outcrops the differences were not observable. This was undoubtedly influenced by the peculiar features of these plots, characterised by poor arid soils and surface parent material. In fact they did not show significant differences in the analysed soil parameters either.Species diversity and biomass showed a similar pattern of differences to floristic composition. The highest species diversity was found on plots with lower biomass (the youngest ones). The lowest biomass in conjunction with quite low species diversity was found on plots with rock-outcrops, which again is consistent with their peculiar features.

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Pour la réalisation du travail, on part de la vérification d'une série successionelle, en considérant le temps écoulé depuis la dernière culture de céréale jusqu'au développement d'un pâturage semi-aride (Centre-Ouest espagnol) . Après avoir vérifié que les résultats sont valables, on observe à partir de l 'enchaînement par séquences des différents groupes d'âges, les variations structurales qui ont lieu pendant le processus. Dans ces variations il y a des aspects d'interprétation facile (la structure biologique) et d’autres aspects à discussion plus complexe (la structure spatiale). De ce fait, pour une compréhension plus effective de ces derniers, l'application des formules d'hétérogénéité et des résultats obtenus, doit être complétée par une longue expérience dans les écosystèmes traités, car au contraire l'interprétation serait incomplète ou ambigile. Dans le déroulement du travail on considère les niveaux suivants : unités d'échantillonage, communautés, groupes de communautés et paires de groupes successifs. Après avoir vérifié quelques aspects suffisament constatés dans la bibliographie (le rythme décroissant de la succession ; la difficulté pour l'installation de nouvelles espèces dans les derniers stades, dûe à la competition) on peut établir que la structure biologique dépend surtout de l'équitabilité ; la richesse intervient beaucoup moins. La structure spatiale, représentée au début de la succession par une distribution homogène (microstructure) dont le degré est d’autant plus fin au fur et à mesure que la pression de l'élevage est plus forte, et avec également une dominance élevée, vers la fin du processus. Entre les deux extrêmes, l'adoucissement des conditions du milieu permet l'observation de groupements plus ou moins monospécifiques, distribués sous forme de "rodales" (taches) dont l'amplitude progresse avec le temps jusqu’à ce que l'effet du pâturage devienne évident. Ces "rodales" faute d’un facteur d’uniformisation (tropeaux) présentent une composition spécifique différente dans des communautés du même âge, ce qui fait augmenter l ’hétérogénéité entre les communautés, tandis que l’ hétérogénéité de la communauté elle-même dépend du degré de développement atteint par les "rodales" et décroît lorsqu'un ou plusieurs d'entre eux dominent nettement sur les autres.

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