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Sandro Pütz
 

Modelling long-term grazing impacts on vegetation dynamics in Patagonian Festuca grasslands

Sandro was the German PhD student in the joint research project with the Argentinean partners. He is now postoctoral researcher at the OESA and involved in the Mata Atlantica project on biodiversity conservation in fragmented landscapes at the Atlantic Plateau of São Paulo (Brazil) (BioCAPSP) .

Semi-arid and arid regions are endangered by desertification and degradation on a global scale. Most of the drylands are characterized by stochastic rainfall. The main focus of Sandros PhD  work was to investigate how grazing affects vegetation dynamics and its interactions with stochastic rainfall exemplified on a grass steppe in Patagonia. Although the problem has been known for decades, the details of the process of degradation are poorly understood.

Due to the highly variable rainfall, the strong dependence of vegetation on rainfall and the lack of empirical long-term studies on vegetation dynamics it is nearly impossible to perform an integrated study of semi-arid systems without a simulation model. To separate the natural and anthropogenic effects from each other Sandro built a small-scale grid and individual based simulation model. It operates at the scale where the grazing decisions take place. The originality of his approach consists of the presentation of a biologically plausible grazing model. Further, his approach includes the interaction of both stochastic rainfall with grazing onto the essential biological processes like seedling establishment, growth and mortality. This enables Sandro to estimate the effect of grazing mediated by stochastic climate in a biologically plausible manner. He demonstrate this with results for the dominant species of a temperate semi-arid steppe, Festuca pallescens in Northwest Patagonia (Argentine).
 

On the basis of a sophisticated calibrated simulation model, he showed how stochastic precipitation affects demographic processes of the perennial tussock grass F. pallescens. Furthermore he showed how grazing changes the demographic processes. Finally Sandro demonstrated how grazing and stochastic rainfall interact on the perennial tussock species.  The separation of the natural and the anthropogenic effects on vegetation greatly advanced the quantitative understanding of the grazing impact in drylands and provides robust arguments for the discussion of sustainable management of rangelands.
 

Competition and facilitation in the semiarid shrub-grass steppe in Patagonia

Grasses of the semiarid shrub-grass steppe of Patagonia form two types of patches: scattered tussocks in a matrix of bare soil, and clumped tussocks forming a ring around shrubs. Previous research explained this pattern by a cyclical successional model, which assumes that shrubs facilitate the establishment and growth of surrounding grasses. We explored an alternative hypothesis that relaxes the assumption of facilitation and proposes that the grass-ring-patch results from the geometry of shrub growth and the competition between shrubs and grasses in the ring.

Both this alternative hypothesis and the cyclic succession model were contrasted against the hypothesis of random distribution of both plant functional types. To this end we developed a spatially explicit cellular automaton model to investigate which of the hypotheses describe the observed vegetation patterns better. We show that relative grass cover in the grass-ring-patch, including the area under the shrubs, does not exceed the mean relative cover of grasses in the whole community. This result suggests that the local high density of grasses in the ring may not be the result of facilitation from the shrub, but simply arise from the geometry of the bare soil space generated under shrubs.