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Titre : Efectos de Cytisis multiflorus (L’Hér) Sweet sobre las comunidades herbáceas, ante diferentes niveles de estrés hídrico en la penillanura salmantina

Auteur : Echavarria Pedraza, Monika Cristina

Université de soutenance : Universidad de Salamanca

Grade : Doctoral Tesis 2008

Résumé
Favorable effects of shrubs on the understorey herbaceous community have been shown for arid and semi-arid Mediterranean areas but poorly known in sub-humid Mediterranean areas of CW Spain that, in turn, have more uneven abiotic conditions for both spatial and temporal scales. Since Cytisus multiflorus is the most important shrub of these latter Mediterranean areas, therefore it was used as a model to evaluate its effect upon the underlying herbaceous vegetation. The interaction between C multiflorus and different herbaceous family plants was quantified by several variables such as total aerial biomass, total cover and herbaceous plants cover along a longitudinal gradient drawn from the centre of individual shrubs towards more open spaces outside the influence of the shrub cover. In this way, three different hypotheses were analysed separately : (1) the shrub induces any changes in the underlying herbaceous community ; (2) the shrub grade effect varies from negative to positive as the abiotic stress increases and (3) the herbaceous plants biomass remains green longer when sheltered under the shrub cover. These hypotheses were contrasted by comparing both two communities differing in their degree of abiotic stress and three years differing in their annual spring rainfall.The effect of isolated C. multiflorus individuals varied in relation to a gradient of stressful conditions that, in turn, depended more on the type of the variable being quantified. The shrub effect was not consistent with the second former hypothesis when some variables were taking into account such as plant diversity, species richness, total plant cover or, the biomass and cover of different herbaceous plants (grasses, legumes, forbs and other families herbs). Consequently, C. multiflorus had a strong and negative effect on both total and legumes cover as water stress conditions increased but, on the contrary, a clear positive effect on grasses cover towards better conditions of water availability. In contrast, the shrub effect on both total and grasses biomass was consistent with the hypothesis related to the abiotic stress, where it was detected a facilitation effect but only under extreme water stress conditions (very dry conditions). A similar pattern was found when comparing either total cover or grasses and forbs covers. Additionally, C. multiflorus was capable to keep green the grasses biomass through longer periods of time but not other herb biomass which were green at the moment of sampling. Nevertheless, C. multiflorus created a spatial gradient associated with abiotic conditions that allow the establishment of underlying herbaceous plants under its shelter.This study showed a better understanding of the ecology of C. multiflorus in sub-humid areas of CW Spain that will contribute towards the conservation efforts of dehesa zones. As this shrub species was effective to promote the establishment primarily of underlying legumes and grasses in extreme water stress environments, it therefore implied to be crucial for maintaining these species pools along with its biomass green longer in time. Therefore, the conservation of C. multiflorus will permit achieve species-rich semi-natural grasslands capable to cope with the intensification of farming industry that are pushing the dehesa systems towards more degrading practices.

Descripteurs : Cytisus multiflorus Dehesa Estrés hídrico Facilitación Competición Especies herbáceas Influencia del matorral Modelos de HOF Gradientes ambientales Hipótesis de gradiente-estrés Water stress Facilitation Competition Herbaceous species Shrub influence HOF models Environmental gradients Stress-gradient hypothesis

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