Comparative analysis of leaf angle and sclerophylly of Aspidosperma quebracho-blanco on a water deficit gradient

Aspidosperma quebracho-blanco is found throughout the Chaco (17 degrees-33 degrees S) in Argentina, and it is the dominant tree species in the arid Chaco. Under the hypothesis that morpho-physiological features of A. quebracho-blanco change as a function of its geographical position on a water deficit gradient, it was predicted that with increasing water stress, leaf angles (specifically horizontal) would be greater and mean values of the leaf mass per area would increase. These leaf characteristics were compared at three points on a water deficit gradient extending from the humid Chaco through semi-arid Chaco to the arid Chaco of Argentina (south-west to north-east rainfall gradient, from 350 to 1200 mm annual mean precipitation). Twig and leaf positions were modified and water potentials were measured at the highest heating hour of the day at a site of the arid Chaco. Daily and seasonal water potential variations of untreated twigs were also observed. Leaf angle modification towards horizontal produced more negative twig water potentials with respect to those of leaves in non-horizontal positions. The comparison of the three sites along the gradient showed contrasting patterns of leaf-angle frequency distribution of adults. In Chancani (mean annual temperature: 18-24 degrees C, mean annual precipitation: 450 mm, arid) there was a higher frequency of angles near 90 degrees for non-pendulous and about 270 degrees for pendulous trees. Leaf angles in Copo (semi-arid) and Chaco National Park (mean annual temperature: 20-23 degrees C, mean annual precipitation: 1300 mm humid) were widely distributed with higher frequency towards the angles near 0 degrees and 180 degrees. This sclerophyllous tree species showed plasticity in its leaf traits along the precipitation gradient. Plasticity in leaf mass per area and leaf position enables plants to develop efficiently in contrasting environmental conditions of humidity and aridity

Effect of shrubs and seasonal variability of rainfall on the establishment of Aspidosperma quebracho-blanco in two edaphically contrasting environments

Three experiments were conducted to verify if an increase in environmental stress level would affect the interactions between two species of nurse shrubs and seedlings of Aspidosperma quebracho-blanco. This is a mesic species with a generalist distribution over an extensive environments gradient. The relationship between Larrea divaricata and seedlings of A. quebracho-blanco was studied in two contrasting soils, a silty loam soil with higher surface clay content and a sandy loam soil. The effect of seasonal variability of rainfall on the initial establishment of seedlings under the shade of L. divaricata was evaluated in three consecutive years. The effect of nurse plant shade was tested comparing two shrub species with different types of leaf life span (sclerophyllous-evergreen and leguminous-deciduous). The natural establishment of A. quebracho-blanco depended on shaded microsites, but not on the type of shade provided by different nurse shrubs. Emergence and initial establishment depended on interactions of soil type and seasonal rainfall variation with nurse plants. The importance of facilitation increased with clay soil (CS). Sandy soil was 'less humid' than CS under shrub shade. However, establishment success depends on opportune even rainfall distribution in interaction with nurse plant presence

Generalized linear models to study spatial distribution of tree species in Argentinean and Chaco

This work adapts some generalized linear models in order to study the spatial pattern of an important tree species. The classical multivariate Ising model, which incorporates the dependence on neighbour individuals in a regular lattice, was adapted by setting a Poisson regression with an extra variation parameter to fit over-dispersion. Because the spatial pattern is only evident to a special reference scale, plots were sampled at two different scales. Two individual presence-absence matrices were analysed for each case through over-dispersion Poisson regression and log-linear models, including binary indicators for a neighbour in the four directions in the linear predictor. The results showed that the species, in the adult stage, has a spatial distribution in patches having no more than two adult individuals.</p

Generalized linear models to study spatial distribution of tree species in Argentinean and Chaco

This work adapts some generalized linear models in order to study the spatial pattern of an important tree species. The classical multivariate Ising model, which incorporates the dependence on neighbour individuals in a regular lattice, was adapted by setting a Poisson regression with an extra variation parameter to fit over-dispersion. Because the spatial pattern is only evident to a special reference scale, plots were sampled at two different scales. Two individual presence-absence matrices were analysed for each case through over-dispersion Poisson regression and log-linear models, including binary indicators for a neighbour in the four directions in the linear predictor. The results showed that the species, in the adult stage, has a spatial distribution in patches having no more than two adult individuals.</p

Subtropical Dry Forests: The Main Forest Ecoregion of Argentina

The Chaco is a sparsely populated, wooded grassland natural region of the Río de la Plata basin, where four physiognomic regions can be identified: Humid Chaco, Semi-arid Chaco, Arid Chaco, and Chaco Serrano. In this introduction to the section of subtropical dry forests, the most relevant tree genera and species are presented. A severe degradation and deforestation process characterize this region in the current century. The main productive systems are described, together with their effects on environmental, economic, and social aspects: extraction of firewood, production of charcoal, and extensive livestock. The recent advance of the agricultural frontier threatens the native communities living within and from the forest in an ancient equilibrium: changes in land use, technological advances of agriculture, increase of logging in the natural forest are occurring. A new tree-centered production paradigm is needed. “Quebracho” species represent a historical paradigm of overexploitation but a promise of possible sustainable management. Key species for the reconversion of the productive systems in the Chaco (not treated in the following chapters) are presented, taking into account their potential for breeding and relevance for restoration, within the environmental, economic, and social context of the region.Instituto de Fisiología y Recursos Genéticos VegetalesFil: Verga, Anibal. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria La Rioja. Agencia De Extensión Rural La Rioja; ArgentinaFil: Lopez Lauenstein, Diego. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Fisiología y Recursos Genéticos Vegetales; ArgentinaFil: Lopez Lauenstein, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Estudios Agropecuarios (UDEA); Argentin