It has been proposed that aridity and grazing are convergent selective forces: each one selects for traits conferring resistance to both. However, this conceptual model has not yet been experimentally validated. The aim of this work was to experimentally evaluate the effect of aridity and grazing, as selective forces, on drought and grazing resistance of populations of Trichloris crinita, a native perennial forage grass of the Argentinean Arid Chaco region. We collected seeds in sites with four different combinations of aridity and grazing history (semiarid/ subhumid x heavily grazed/lightly grazed), established them in pots in a common garden, and subjected the resulting plants to different combinations of drought and defoliation. Our results agreed with the convergence model. Aridity has selected T. crinita genotypes that respond better to drought and defoliation in terms of sexual reproduction and leaf growth, and that can evade grazing due to a lower shoot: root ratio and a higher resource allocation to reserves (starch) in stem bases. Similarly, grazing has selected genotypes that respond better to drought and defoliation in terms of sexual reproduction and that can evade grazing due to a lower digestibility of leaf blades. These results allow us to extend concepts of previous models in plant adaptation to herbivory to models on plant adaptation to drought. The only variable in which we obtained a result opposite to predictions was plant height, as plants from semiarid sites were taller (and with more erect tillers) than plants from subhumid sites; we hypothesize that this result might have been a consequence of the selection exerted by the high solar radiation and soil temperatures of semiarid sites. In addition, our work allows for the prediction of the effects of dry or wet growing seasons on the performance of T. crinita plants. Our results suggest that we can rely on dry environments for selecting grazing-resistant genotypes and on high grazing pressure history environments for selecting drought-resistant ones.

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T-4464 Buffelgrass (Cenchrus ciliaris L.) is an introduced grass used extensively for restoring forage capacity of denuded shrublands in the arid Chaco region in central western Argentina. Currently, the technique of roller chopping and simultaneous buffelgrass seeding is widespread, but information is lacking about its effects on forage production and other vegetation attributes at the regional scale. The purpose of our study was to compare grass yield; vegetation cover and density; and species richness, diversity, and evenness, between roller chopping and simultaneous buffelgrass seeding with nontreated native degraded shrublands. Eight sites, including matched pairs of treated and nontreated areas, were studied. Grass yield, total perennial grass cover, and total perennial grass density were higher (P , 0.05) on treated areas. Increase in grass yield on treated areas compared with nontreated areas was related to the increase in total perennial grass cover due to added buffelgrass cover (r 2 ¼ 0.79). Shrub cover was lower (P , 0.05) on treated areas, but shrub density was not different between treatments (P. 0.05). Nonsignificant differences (P. 0.05) between treated and nontreated areas were detected in tree cover and density, species number, diversity index, and evenness. The results of this study indicate that, in the short term, roller chopping and simultaneous buffelgrass seeding produces rapid increases in grass cover and grass yield, and a drastic reduction in shrub cover without affecting species diversity on degraded shrublands of the arid Chaco region of Argentina. Resumen El buffel T-4464 (Cenchrus ciliaris L.) es un pasto introducido, utilizado para restaurar la capacidad forrajera de matorrales degradados en el sector centro-occidental de la regió n del Chaco á rido, Argentina. En la actualidad el rolado y siembra simultá nea de buffelgrass está ampliamente difundido en la regió n. Sin embargo, se carece de informació n a nivel regional, relacionada con los efectos de la aplicació n de esta técnica sobre la producció n de gramíneas y otros atributos de la vegetació n. El objetivo de nuestro estudio fue comparar diferentes atributos de la vegetació n entre á reas roladas y sembradas con pasto buffel y matorrales degradados (testigo). Para ello se seleccionaron ocho á reas roladas y sembradas, y ocho áreas testigo adyacentes. Se evaluó la producció n de gramíneas, la cobertura y densidad de la vegetació n, y el nú mero de especies, diversidad y equitatividad. La producció n de gramíneas, la cobertura de gramíneas perennes totales, y la densidad de gramíneas perennes totales fue mayor (P , 0.05) en las á reas sometidas a rolado y siembra. La mayor producció n de gramíneas en las áreas roladas y sembradas respecto a las testigo, se relacionó positivamente con la cobertura de pasto buffel (r 2 ¼ 0.79). La cobertura de arbustos fue menor (P , 0.05), en á reas roladas y sembradas. Sin embargo, no se detectaron diferencias (P. 0.05) entre áreas en la densidad de arbustos. Tampoco se detectaron diferencias ...

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