Andrés Tálamo

Disentangling the effects of shrubs and herbivores on tree regeneration in a dry Chaco forest (Argentina)

Successful persistence of dry forests depends on tree regeneration, which depends on a balance of complex biotic interactions. In particular, the relative importance and interactive effects of shrubs and herbivores on tree regeneration are unclear. In a manipulative study, we investigated if thornless shrubs have a direct net effect, an indirect positive effect mediated by livestock, and/or an indirect negative effect mediated by small vertebrates on tree regeneration of two key species of Chaco forest (Argentina). In a spatial association study, we also explored the existence of net positive interactions from thorny and thornless shrubs. The number of Schinopsis lorentzii seedlings was highest under artificial shade with native herbivores and livestock excluded. Even excluding livestock, no seedlings were found with natural conditions (native herbivores present with natural shade or direct sunlight) at the end of the experiment. Surprisingly, seedling recruitment was not enhanced under thornless shrubs, because there was a complementary positive effect of shade and interference. Moreover, thornless shrubs had neither positive nor negative effects on regeneration of S. lorentzii. Regeneration of Aspidosperma quebracho-blanco was minimal in all treatments. In agreement with the experiment, spatial distributions of saplings of both tree species were independent of thornless shrubs, but positively associated with thorny shrubs. Our results suggest that in general thornless shrubs may have a negligible effect and thorny shrubs a net positive effect on tree regeneration in dry forests. These findings provide a conceptual framework for testing the impact of biotic interactions on seedling recruitment in other dry forests.

Composición, diversidad y estructura del ensamble de plantas leñosas en dos paleocauces con diferente intervención antrópica del Chaco semiárido, Argentina

Resumen es: Los ecosistemas del Chaco argentino vienen siendo perturbados por la actividad humana desde hace decadas, especialmente por la ganaderia extensiva y el f...

A woody plant community and tree-cacti associations change with distance to a water source in a dry Chaco forest of Argentina

In semiarid regions, livestock is concentrated around water sources generating a piosphere pattern (gradients of woody vegetation degradation with increasing proximity to water). Close to the water source, livestock may affect the composition, structure and regeneration strategies of woody vegetation. We used the proximity from a water source as a proxy of grazing pressure. Our objectives were (1) to compare woody vegetation attributes (richness, diversity, species composition, density and basal area) and ground cover between sites at two distances to a water source: near (higher grazing pressure) and far from the water source (lower grazing pressure), and (2) to quantify and compare cases of spatial association among the columnar cacti Stetsonia coryne (Salm-Dyck) Britton and Rose (Cactaceae), and the dominant tree Bulnesia sarmientoi Lorentz ex Griseb. (Zygophyllaceae). We used a paired design with eight pairs of rectangular plots distributed along a large and representative natural water source. We found lower total species richness, plant density and soil cover near than far from water source, and more cases of spatial associations between the two species studied. Our results show evidence of increased livestock impacts around water sources. However, we found no difference in terms of species composition or basal area at near versus far sites. We conclude that grazing pressure might be changing some attributes of the woody plant community, and that the association of young trees with thorny plants (grazing refuge) could be a regeneration mechanism in this semiarid forest with high grazing pressure.

Producción de semillas de quebracho blanco (Aspidosperma quebracho-blanco) en un sector del Chaco semiárido y su relación con el tamaño del fruto

Para manejar y conservar una poblacion biologica es necesario conocer las etapas de su ciclo de vida. El quebracho blanco es una especie clave en el ecosistema chaqueno y es necesario emprender estudios relacionados con su produccion de semillas. Se estudio el numero de semillas fertiles (con embrion) y vanas por fruto (sin embrion) y la relacion entre el numero de semillas fertiles con el tamano (largo y ancho en cm) del fruto del quebracho blanco en el sector sur-oeste del Parque Nacional Copo, Argentina. Se encontro un promedio de 23 semillas fertiles y dos semillas vanas por fruto. El ancho y el largo del fruto estuvieron positivamente asociados con el numero de semillas fertiles por fruto (bancho = 4,9, R2 = 0,18; blargo = 0,35, R2 = 0,33). Los modelos no serian una buena herramienta para predecir el numero de semillas fertiles por fruto debido a sus bajos valores de bondad de ajuste. Sin embargo, al momento de seleccionar frutos con el objetivo de recolectar semillas, se recomienda seleccionar frutos de mayor tamano. Se sugiere continuar el estudio mediante una estimacion del numero de frutos promedio por arbol para terminar de comprender esta etapa del ciclo de vida y estimar el potencial de regeneracion del quebracho blanco.