[en] Old-field succession was studied during seven years of field experiments, under the CLUE project (CHANGING LAND USAGE, Enhancement of biodiversity and ecosystem functioning). The secondary succession on former arable land is dominated by opportunistic species (mainly arable weeds from the seed bank), which are major constraints in vegetation dynamics due to their delayed force role against the restoration of existing natural conditions prior to conversion. Our aim then was to change the initial vegetation succession stages by the direct sowing of later-successional plant species in two different seed mixture treatments: one contained 4 (low diversity, BD) and the other 15 (high diversity, AD) sown plant species. We compared the plant communities development between both seed mixture treatments and with those of unsown plots (CN) following a randomized block design.Only four of the fifteen sown species were dominant over all the experimental period, the other half were left as subordinate species while the remain ones completely disappeared. The most successful sown species to establish in sowing treatments was the exotic grass, Bromus inermis, which had a strong and negative effect on the other sown species success such as Plantago lanceolata, Sanguisorba minor and the legume Lotus corniculatus. However, these latter species gained certain dominance degree within the established plant communities in absence of the former sown grass species. In general terms, when sown species declined in both number and abundance over time, BD treatment tended to respond in similar way to AD treatment, in spite of being more related to unsown conditions (CN), during most part of the time, due to the lack of a dominant sown species that would able to suppress weeds.The sowing effect on the suppression of unsown species was more evident at the scale of subplots (1 sqm) rather than whole-plots (100 sqm), and was mainly caused by the decrease in the number of colonizing species in plots with sown species, which was most pronounced in AD plots. The lowest scale was associated with the formation of heterogeneous patches by Bromus inermis while at highest scale this effect was not clear because weeds had mayor chances of expression among the different established patches. Therefore, only a fraction of the weed species within a whole-plot will be found in the neighborhood of B. inermis and only a few of them were able to increase its abundance. This result was according to the Elton hypothesis in the sense that more diverse communities should be less susceptible to invasion by colonizing species. Consequently, B. inermis promotes a plant functional change because it excluded both sown and unsown grasses but facilitated other non-legume plant species.Despite of the important functional effect accounted, sowing did not affect the established plant community productivity. Moreover, the plant communities exhibited the same negative relationship between diversity and productivity in similar way as many authors have reported for various dehesa s zones. Therefore, other diversity components were more important than the number of species per se in explaining ecosystem functioning (including the productivity); for example, the species composition with high number of low-biomass species and low number of legumes. Finally, the diversity effect was highly species and context dependent and that the identity of B. inermis was the most significant because it changes functionally the established plant community but without increasing any species biomass in the process.

[es] Se estudi? la sucesi?n de las comunidades vegetales establecidas experimentalmente en tierras de cultivo abandonadas durante siete a?os de muestreo, bajo el desarrollo del proyecto CLUE (CHANGING LAND USAGE, Enhancement of biodiversity and ecosystem funtioning). En este tipo de tierras, las malas hierbas de cultivo que dominan la sucesi?n inicial, se convierten en una fuerza limitante de la din?mica de la vegetaci?n ya que ralentizan la vuelta a las condiciones naturales previas al cultivo. Para contrarrestar su impacto negativo, se manipul? la diversidad de especies mediante la siembra de plantas caracter?sticas de estadios m?s tard?os de la sucesi?n natural, en dos tratamientos de siembra: uno con 4 especies o baja diversidad (BD) y otro con 15 especies o alta diversidad (AD), que se compararon entre s? y con condiciones naturales (CN) siguiendo un dise?o de bloques completamente aleatorizados.S?lo cuatro de las quince especies sembradas fueron dominantes durante la mayor parte del tiempo examinado, la mitad de las restantes quedaron subordinadas y las otras desaparecieron completamente. La m?s exitosa fue la gram?nea ex?tica, Bromus inermis, que repercuti? negativamente sobre el desarrollo de Plantago lanceolata, Sanguisorba minor y la leguminosa Lotus corniculatus? especies que solamente alcanzaron un alto grado de dominancia en ausencia de la gram?nea anterior. En general, cuando las especies sembradas disminuyeron tanto en n?mero como en cobertura, a trav?s del tiempo, el tratamiento BD tendi? a parecerse a AD, aunque en todo momento estuvo m?s fuertemente relacionado a CN, al no existir una especie dominante sembrada que influyera considerablemente sobre la dominancia de las malas hierbas de cultivo.El efecto de la siembra sobre la supresi?n de las malas hierbas se hizo m?s perceptible cuando se analiz? a una escala espacial peque?a (1m2), asociada a la formaci?n de parches heterog?neos por Bromus inermis; mientras que a una escala espacial mayor (100 m2), este efecto qued? ocluido debido a que las malas hierbas presentaron mayores posibilidades de expresi?n entre los diferentes parches. Estos parches no dejaron espacio disponible para nuevas colonizaciones pero al mismo tiempo permitieron que algunas malas hierbas ya presentes aumentaran relativamente su cobertura. Este resultado sigui? la hip?tesis de Elton que predice en comunidades m?s organizadas una mayor resistencia a nuevas colonizaciones. Consecuentemente, B. inermis promueve un reemplazo funcional al excluir por competencia interespec?fica otras gram?neas, mientras que permite el desarrollo, hasta cierto l?mite de espacio, de especies pertenecientes a otras familias distintas a las leguminosas.A pesar del importante efecto funcional encontrado, la siembra no reverti? la baja productividad de las comunidades establecidas. Es m?s, siguieron la misma relaci?n negativa entre la productividad y diversidad que muchos autores han reportado para diversas zonas de dehesa. Por lo tanto, otros componentes distintos a la riqueza de especies fueron responsables de la baja productividad del ecosistema, por ejemplo, la alta presencia de especies de bajo porte pero baja de leguminosas. Se confirm? tambi?n que el efecto de la diversidad fue muy dependiente del efecto de identidad de B. inermis, ya que su forma de crecimiento clonal, facilit? unas especies pero elimin? otras, cambiando funcionalmente la comunidad de plantas pero sin ning?n aumento de la biomasa de las especies involucradas en el proceso.