Desertification and climate change challenge grazing management in semi-arid Patagonian rangelands

CIPRIOTTI, P.A.; PARUELO, J.M.; WIEGAND, T.; AGUIAR, M.R.

Bonn

Congreso; 9no Foro de Ganadores del premio von Humboldt ?Fronteras en Biogeografía, Ecología, Antropología y Evolución. Humboldt y el ?Cosmos? revisitado en el siglo XXI; 2019

Alexander von Humboldt Stiftung

Drylands occupy a vast area of terrestrial ecosystems (ca. 30%) and provide many ecosystem goods and services, particularly as rangelands for animal husbandry. However, many dryland ecosystems have been degraded during the last century, mostly due to non-sustainable grazing management in combination with high rainfall variability and complex rangeland dynamics. Additionally, the drier climate predicted by climate change scenarios may amplify these effects and even lead to catastrophic vegetation shifts. As a result, there is an urgent need to better understand how dryland vegetation responds to grazing and climate change, and to develop sustainable grazing management for improved ecosystem restoration techniques and strategies. Given the long histories of grazing by domestic livestock and the associated decline in the ability of dryland ecosystems to maintain their biodiversity and functioning, a major question that remains is whether or not grazing management can recover degraded rangelands? We assess the long-term effects of grazing and rainfall on various aspects of vegetation structure including the grass-shrub balance, the maintenance of spatial vegetation patterns, and the decline or recovery of palatable grasses. Specifically, we address the following questions: (i) How do changes in the long-term rainfall regime and in the stocking rate of continuous grazing impact grass-shrub coexistence and spatial vegetation structure? (ii) If degradation has already occurred, what is the effect of rainfall, stocking rate, and rotational grazing management (i.e., alternating years of grazing and rest) on the recovery of palatable grass species?One approach to assess the potentially complex fine-scale responses of dryland vegetation to grazing and climatic variation is to use simulation modelling techniques. We used the eco-hydrological and individual-based simulation model DINVEG for this purpose, which describes the spatiotemporal dynamics of grass-shrub steppes based on six decades of field research in western Patagonia. DINVEG has been validated against multiple field datasets (e.g. life form and species-specific cover and densities, total aboveground net primary production (ANPP), spatial patterns, and size structure).Rainfall and grazing affected the simulated vegetation structure in different ways. Total plant cover was mostly influenced by rainfall, but the cover of palatable grasses was mostly influenced by stocking rate. Dry conditions and low stocking rates (122 mm/yr and < 0.2 sheep·ha-1) favoured grasses over shrubs, whereas shrub encroachment occurred only in the high rainfall scenario combined with high stocking rates (181 mm/yr and > 0.2 sheep·ha-1). High stocking rates and/or drier conditions caused only gradual shifts in spatial vegetation patterns, but maintained the observed positive association for grasses around shrubs. In contrast, shrub encroachment was associated with repulsion between grasses and shrubs and the formation of shrub clusters into a matrix of scattered less palatable grasses. Plant compositional changes occurred through grass species replacement (i.e. unpalatable by palatable species) and the associated hysteresis effect of palatable grass species: model simulations suggest that few decades of heavy and continuous grazing can drive palatable grasses to close to extinction, whereas natural recovery of degraded steppes may take 100 years or longer.