Afforestation and groundwater use in South American grasslands: impacts on productivity and salinity across a rainfall gradient

NOSETTO, M D; JOBBÁGY, E G; TÓTH, T; JACKSON, R; OYARZABAL, M

Berlin, Alemania

Conferencia; International Conference: Impact Assessment of Land Use Changes; 2008

Terrestrial vegetation is a main control of the hydrological cycle and has the capacity to modify the direction and intensity of water fluxes and salt exchange between ecosystems and groundwater. Afforestation in the native grasslands of temperate South America often triggers an intense groundwater consumption that can have cascading consequences on net primary productivity (NPP), through increased carbon and water exchange with the atmosphere, and on salt distribution in soils and groundwater, through a switch in the hydrological regime from net recharge to net discharge. We explored the role of climate and tree species shaping these effects across 50 pairs of contiguous grassland-plantation stands along a broad precipitation gradient in Argentina and Uruguay. Satellite EVI data was used to estimate NPP and geoelectric surveying and groundwater sampling to assess salinization. Tree plantations displayed higher NPP than grasslands (p<0.01, n=32), with contrasts becoming larger in more arid sites and smaller or nil in areas without access to groundwater. At the regional scale, geoelectric surveying revealed that the salinization of plantation soils depended strongly on climate, occurring only where the annual water balance was <100 mm yr-1 (p<0.01, n=24). At the local scale, we observed that groundwater salinities observed under ~50-year old plantations of different species were associated with their tolerance to salinity (p<0.01, n=10) (higher tolerance leading to higher salinity). As grassland afforestation continues its expansion, a better recognition of its positive and negative impacts on soil and water resources is needed. Groundwater consumption appears as a key process explaining carbon uptake shifts and salt accumulation following this land use change. Our results helped identify the climatic zones and tree species that would be least vulnerable to the later process.