[en] 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.

[es] En ecosistemas Mediterr?neos, ?ridos y semi?ridos, varios estudios han mostrado el efecto favorable de los matorrales sobre la comunidad herb?cea subyacente, mientras que estas interacciones son menos conocidas en ambientes Mediterr?neos sub-h?medos del centro-oeste de Espa?a, cuyas condiciones abi?ticas son m?s variables tanto espacial como temporalmente. Por lo tanto, se eval?o el efecto de Cytisus multiflorus? la especie de matorral m?s importante en este ?ltimo ecosistema, mediante la cuantificaci?n de la biomasa a?rea y cobertura de las principales familias de herb?ceas a lo largo de un gradiente longitudinal de distribuci?n, desde el centro del matorral le?oso hasta los espacios m?s abiertos fuera de su ?rea de influencia. De tal forma, que se analizaron tres hip?tesis: (1) La inducci?n de cambios del matorral sobre la comunidad de herb?ceas; (2) El cambio de direcci?n del efecto del matorral, de negativo a positivo, debido a un incremento del estr?s abi?tico; y (3) la capacidad del matorral para mantener la biomasa verde de las herb?ceas bajo su cubierta. Estas hip?tesis fueron contrastadas mediante la comparaci?n de dos comunidades con diferentes condiciones ed?ficas muy marcadas durante tres a?os, per?odo de tiempo que present? adem?s diferencias de precipitaci?n anual muy acusadas en la ?poca de primavera principalmente.El efecto de cada matorral aislado de C. multiflorus var?o en relaci?n con un gradiente definido de las condiciones abi?ticas, que dependieron a su vez del tipo de variable cuantificada. El efecto sobre la cobertura total, la cobertura y biomasa por familias (gram?neas, leguminosas, compuestas y otras familias ), la diversidad y riqueza no fueron consistentes con la segunda hip?tesis enunciada. C. multiflorus evidenci? un fuerte efecto negativo sobre la cobertura total y de las leguminosas a medida que el estr?s h?drico aumentaba y, al contrario, un claro efecto positivo sobre la cobertura de gram?neas hacia mejores condiciones h?dricas. En contraste, evaluando el efecto del matorral sobre la biomasa total y aquella de gram?neas, se apoy? la misma hip?tesis pero en un gradiente de estr?s, donde se detect? un mecanismo de facilitaci?n en las peores condiciones de estr?s h?drico (muy seco). Un proceso similar tambi?n se encontr? para las variables de cobertura total, de gram?neas y compuestas. Adicionalmente, C. multiflorus permiti? una postergaci?n en el tiempo de la biomasa verde de las gram?neas pero no de las restantes familias debido a que la mayor?a de ellas estuvieron verdes al momento del muestreo. Asimismo, C. multiflorus gener? un gradiente longitudinal de condiciones abi?ticas que permiti? el establecimiento de diferentes especies bajo su cubierta. Las contribuciones de este estudio se relacionaron con un mayor conocimiento de la ecolog?a de C. multiflorus en Espa?a, que permitir? formular planes de conservaci?n acorde con un manejo de tipo dehesa. Debido a que el matorral establece una segregaci?n espacial entre gram?neas y leguminosas, importantes por su aporte nutricional al ganado, su presencia resultar?a m?s importante en aquellos ambientes con mayor estr?s h?drico. Consecuentemente, la conservaci?n de C. multiflorus permitir?a no s?lo el establecimiento de estas familias de herb?ceas, sino que adem?s mantendr?a su biomasa verde por m?s tiempo. En cambio, sin la facilitaci?n del matorral, el ?xito de estas familias ser?a menor temporalmente.