The relationship between environmental heterogeneity and the vegetation of the levee neo-ecosystems in the Delta of the Paraná River was studied. These habitat types were considered plant communities of recent origin related to local productive activities. Vegetation coverage was evaluated in 97 plots of three different environmental units (A, B and C), using classification analysis, indirect and direct ordinations. The differential vegetation in different environmental units could be related to a greater fluvial influence of the Paraná River on unit A and a greater tidal influence of the de la Plata River on units B and C. The Lower Delta hydrological regimes only affect a few of edaphic variables particularly pH, organic matter percentage and clay content. To understand the spatial pattern of neo-ecosystems vegetation, it is also necessary to consider the invasion of alien species that has been taking place for over 50 years.

Th e native forests once occupying the coastal levees of the Lower Delta islands of the Paraná River (in Argentina) have been replaced by commercial forests. Many of those forests have been abandoned, resulting in secondary forest formation that is subject to numerous invasive exotic species. A priori observations suggest that successional trends do not lead to recovery of the original forest. Th e aim of the present study was to analyze the role of invasive exotic species in these environments and the likelihood of recovery of the original forest cover. Th erefore, we examined the composition of the seed bank and of the standing vegetation, as well as the population structure of tree species. Secondary forests are described as being in an intermediate successional stage, with few exotic species (mainly trees) but with a high abundance of standing vegetation and seed banks. Th ese exotic species will likely continue to successfully predominate in diff erent forest strata over the next stages. Because of the low density of native tree species, it is diffi cult to predict their future persistence. In conclusion, ecological restoration strategies will be needed in order to increase native tree species richness and biodiversity of the forests in the Lower Delta of the Paraná River. Acta Botanica Brasilica 27(1): 40-49. 2013.

Composición y estructura de los bosques de albardón del delta frontal del río Paraná, ArgentinaComposition and structure of levee forests in the frontal Paraná River Delta, Argentina

ABSTRACT. The distribution and abundance of exotic and native species in levee neoecosystems were analyzed. No invasive species were found in unit A; their absence could be explained by the fluvial action of the Paraná river, extraordinary flood episodes and anthropic disturbances. Invasive species associated with the tidal regimen of the de la Plata river were present in units B and C, particularly Chinese privet (L. sinense), green ash (F. pennsylvanica), honey locust (G. triacanthos), Japanese honeysuckle (L. Japonica), blackberry (Rubus spp.), box elder (A. Negundo) and glossy privet (L. Lucidum). Native species showed low recovery values, both at a regional level and within each unit, with the exception of A. The neo-ecosystems with the greater degree of abandonment of units B and C exhibited dominance of exotic tree species and, to a lesser extent, recovery of native species of the original gallery forest (seibo, Erythrina crista galli; laurel, Nectandra falcifolia; canelón, Rapanea spp. and arrayán, Blepharocalyx tweediei).Key words: wetlands, lower delta, neo-ecosystems, invasive species, "Monte Blanco" species.RESUMO. A distribuição de espécies exóticas e nativas em diques marginais do baixo delta do rio Paraná, Argentina. Analisam-se a distribuição e a abundância das espécies exóticas e nativas nos neo-ecossistemas de diques marginais. A unidade A não apresentou espécies invasoras. A ação fluvial do rio Paraná, acontecimentos como inundações extraordinárias e distúrbios antrópicos poderiam explicar a sua ausência. B e C apresentaram espécies invasoras associadas ao regime de marés do rio de la Plata. Entre as espécies destacam-se: alfeneiro-dachina, ligustro-chinês (L. sinense), freixo vermelho (F. pennsylvanica), acácia meleira (G. triacanthos), madrissilva do Japão (L. japonica), amoreira-preta (Rubus spp.), bordo (A. negundo) e ligustro (L. lucidum). As espécies nativas apresentaram baixos valores de recuperação em nível regional e em cada unidade. A exceção foi a unidade A. Os neo-ecossistemas com maior abandono nas unidades B e C apresentaram dominância de espécies arbóreas exóticas e em menor escala a recuperação de espécies da selva em mata ciliar original (corticeira, Erythrina crista gall; louro, Nectandra falcifolia; capororoca, Rapanea spp. e guapuriti, Blepharocalyx tweediei).

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This study adds information on the dynamics of pioneer forest trees in the face of frequent fire. In particular, effects of fire on the regeneration dynamics and ecological attributes of a Tessaria integrifolia forest and a Cortaderia selloana grassland, located in the Costanera Sur Ecological Reserve, Buenos Aires, are analyzed. Based on the study of ten transects along a forest-grassland gradient, a considerable incidence of fire and resilient response to it were observed on grasslands. On the other hand, the forest proved resistant to fire, not showing evidence of its action. Despite its invasive tendencies, C. selloana was unable to establish itself in the forest or in the forest-grassland ecotones. Conversely, following the action of fire, T. integrifolia could be found in areas previously dominated by grassland, leading to an expansion of the forest. Replacement of grasslands with a T. integrifolia forest indicates a reduction in the incidence of and surface area susceptible to fire, as well as an increase in biodiversity. The post-fire colonization observed for T. integrifolia has not been previously described for this widely distributed South American species.

The common bean (Phaseolus vulgaris L.) is a food of worldwide vital importance. Like any crop, the conservation of its wild relatives is essential to maintain the genetic variability needed for plant breeding. In situ conservation of such population has the additional benefit of generating new variations through the evolution of its natural environment. In 2004, wild populations of P. vulgaris var. aborigineus were found in the province of Co ´rdoba (Argentina), being the southernmost specimen of its current distribution. In order to study the state and characterize the environment of such populations, vegetation censuses were conducted along a stretch of the Tanti stream-arroyo Tanti-(Punilla, Co ´rdoba). Results showed that the wild bean has an intermediate to high constancy throughout the watercourse and its presence decreases upstream and away from the stream. Most cover took place in plots with scarce slope, high incidence of light and intense magnitude of floods. Vegetation develops in an environment surrounded by forests, meadows and intermediate physiognomy.Besides adventitious species, the studied area still has native species of high conservation value. Based on the analysis of this work, and considering the biogeographical significance of this population of wild bean, we conclude that this stretch of the studied stream is a priority area for the in situ conservation of P. vulgaris var. aborigineus.

The River Plate coastal plain is a 160-km-long, 3-10-km-wide strip, located to o5 m a.s.l. on the right bank of the estuary. It is formed by sediments and landforms generated by the littoral transport and marine ingressions and regressions during the Holocene. The coastal plain faces heavy pressures from Buenos Aires Metropolitan Area, with about 13 million inhabitants, industrial, mining and rural activities. Here, we analyse the relations existing between landforms, soils and vegetation. Despite the anthropogenic influence, the natural landscape is more or less preserved in some areas, such as patches of the southernmost gallery forest in the world in natural levees, the xerophytic forests located in well-drained soils of beach ridges and other units such as Inland and Coastal Mudflats with wetland soils and vegetation, Tidal Flats with contrasting soils and plant communities among others.