Questions: Woody encroachment is a complex ecological phenomenon, which modifies the composition and diversity of species, productivity, carbon storage and cycling, water dynamics, among others. In Argentina’s semiarid grasslands, an increase in the range of distribution and the density of the woody species Geoffroea decorticans has been observed since the last decades. The aims of this research were: a) to classify the woody thickets dominated by Geoffroea decorticans, b) to relate the floristic and functional composition of the herbaceous layer to environmental characteristics, and c) to compare the functional composition between thickets and grassland vegetation. Study area: The study was conducted in the grasslands of the semiarid western ex-treme of the Pampas phytogeographical region. Methods: Seventy three woody thickets were surveyed recording the cover of all herbaceous species, light interception under tree canopies and the environmental context surrounding (i.e. natural grassland, pastures and annual crops). A cluster analysis was carried out to classify the thickets, and the composition of major functional groups was compared among cluster groups. To order the sampling sites along a gradient, and to evaluate the environmental context as constraint external variables a Canonical Correspondence Analysis was carried out. Finally, richness, diversity, exotic and annual species´ richness, and shared species was compared between woody thickets and grassland vegetation based on biblio-graphic records. Results: The cluster analysis classified woody thickets into four groups. The environmental context surrounding the woody thickets and light interception explain 83.7% of the observed floristic composition variability. Palatable species had a significantly higher cover than non-palatable ones. Woody thickets had more richness and diversity, mainly by a higher number of annual and exotic species. Conclusions: The increase in woody cover would increase the heterogeneity, richness and diversity of natural vegetation. But, at the same time, it would favour the entrance of exotics and annuals.

Fil: Rauber, Ruth Bibiana. Instituto Nacional de Tecnología Agropecuaria. Centro Regional la Pampa-san Luis. Estación Experimental Agropecuaria San Luis. Agencia de Extensión Rural Villa Mercedes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis; Argentina

Fil: Cipriotti, Pablo Ariel. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Métodos Cuantitativos y Sistemas de Información; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

Fil: Demaría, Manuel. Instituto Nacional de Tecnología Agropecuaria. Centro Regional la Pampa-san Luis. Estación Experimental Agropecuaria San Luis. Agencia de Extensión Rural Villa Mercedes; Argentina

Fil: Arroyo, Daniel. Instituto Nacional de Tecnología Agropecuaria. Centro Regional la Pampa-san Luis. Estación Experimental Agropecuaria San Luis. Agencia de Extensión Rural Villa Mercedes; Argentina