Patagonian shrubs: drought or grazing tolerance?

Abstract

Drought is the main selective force in arid zones acting on plant populations. Features that confer plants tolerance to drought also provide tolerance to herbivory. Therefore, even the plants of arid zones with a short grazing history would be tolerant to herbivory. Evolutionary history of grazing in central and northwest Patagonian steppes has been considered brief and dominant spiny shrubs have been considered adapted to tolerate drought. Here, we present experimental evidences that question that conceptual model. (1) The three most conspicuous shrubs in the dominant community at south-western Chubut (Mulinum spinosum, Adesmia volckmanni and Senecio filaginoides) are not subjected to severe water stress conditions because their roots explore deep soil layers with relatively high water potentials almost all year. That is reflected in high leaf water potential, high isotopic discrimination against heavy carbon isotope and low response to rainfall. (2) Instead, the three shrubs show evasion herbivory strategies. M. spinosum and A. volckmanni have very aggressive thorns, while S. filaginoides has a high content of carbon-based secondary metabolites. These chemical compounds are believed to have an anti-herbivory role. Besides that, in two of these three species, levels of physical or chemical defences were increased under grazing conditions. (3) Finally, as defensive strategies do not preclude herbivore consumption in absolute terms, shrubs are important components of sheep diet. Sheeps eat leaves of non-chemically defended species and flowers of all of them. These evidences suggest that grazing pressure of native herbivores would have been high and persistent enough to promote natural selection processes that conducted to dominance of grazing resistant shrub genotypes.