1.1. Problem and Background
 Terrestrial vegetation, which strongly influences the global hydrological cycle, has the capacity to modify water resources, ecosystem productivity, and vadose zone salinity from stand to regional scales. Transpiration, the principal component of evapotranspiration over most land surfaces, is strictly linked with carbon assimilation [Monteith, 1988]. Vegetation effects on water fluxes, and accompanying solutes, across the continuum of ecosystem-vadose zone-aquifer, also affect the direction and magnitude of salt exchange between ecosystems and groundwater [Schofield et al., 2001]. As a result of this influence on water fluxes, vegetation changes that alter evapotranspiration strongly, such as shifts between herbaceous and woody vegetation [Zhang et al., 2001] can leave an important imprint on salt distribution and accumulation patterns with potential feedbacks on ecosystem functioning. In this paper we characterize the regional patterns of soil salinization that accompany the establishment of tree plantations on native grasslands and their link with groundwater consumption along a broad climatic gradient in temperate South America.
 Grassland afforestation, mainly with fast growing species such as eucalypts and pines, has expanded rapidly in the last decades in South America, motivated in part by broad governmental incentives [Wright et al., 2000]. Numerous estimates of primary productivity in the region suggest higher values in the plantations compared to the native grasslands they replace [Deregibus et al., 1987; Frangi et al., 2000; Jobbágy et al., 2006; Piñeiro et al., 2006]. However, accompanying this higher carbon gain in the plantations is also a higher evaporative water loss, ultimately determined by the structure of trees and forests. Under wet conditions, the high aerodynamic conductance of trees lets them exchange water vapor with the atmosphere at rates up to 10 times higher than possible for shorter-statured vegetation [Calder, 1998] allowing a more exhaustive use of rainfall inputs. Under dry conditions, the deeper root distribution that trees often possess [Canadell et al., 1996] provides them with access to water sources (e.g., vadose zone moisture, groundwater) that are often inaccessible to herbaceous plants [Calder, 1998]. Evidence for these increased evaporative water losses that follow afforestation includes drier soils and vadose zones [Sapanov, 2000], reduced water yield [Jackson et al., 2005], and the elimination of groundwater recharge and the onset of discharge under afforested plots within herbaceous landscapes [Heuperman, 1999; Jobbágy and Jackson, 2004].
 Based on the higher water use of trees compared to herbaceous vegetation, afforestation has been proposed as a feasible management option for lowering the water table in regions with shallow groundwater [Schofield, 1992]. While tree establishment can provide increased productivity and beneficial “bio-drainage” where shallow saline water tables constrain crop production, continuous groundwater use by trees may also introduce a long-term, negative feedback on forest growth associated with salt accumulation, arising from salt exclusion during water uptake by roots [Heuperman, 1999; Jobbágy and Jackson, 2004, 2007]. Groundwater and soil salinization after afforestation has been reported in several regions around the world, for different types of trees (deciduous and evergreen, conifers and broadleaf) and in a broad range of climates [George et al., 1999; Heuperman, 1999; Jobbágy and Jackson, 2004; Nosetto et al., 2007; Sapanov, 2000; Vertessy et al., 2000]. However, the controls on this salinization and the context in which it is likely to occur remain poorly understood.
 Australia presents an interesting example of the opposite shift in vegetation, where the replacement of vast areas of woodlands by herbaceous species led to an increase in hydrological recharge [George et al., 1997]. This process raised the water table and moved deeply stored salts to the surface [Pierce et al., 1993]. Current estimates suggest that the reversion of this process would only by achieved by reforesting 70–80% of the watershed [George et al., 1999].
 In South America the opposite process (i.e., grassland afforestation) is taking place at high rates [Geary, 2001]. A better understanding is urgently needed for afforestation planning, especially because grassland afforestation is likely to grow across South America and other regions, partially motivated by the role of forests as carbon sinks [Wright et al., 2000].
1.2. The Context of Salinization: Mechanisms and Predictions
 We propose a hierarchy of climatic, hydrogeological, and biological factors that help predict the onset and rates of salt accumulation in afforested grasslands (Figure 1). On the basis of this hierarchy we identify mechanisms of soil and water salinization and predict patterns at the regional, landscape and stand levels that can be evaluated in the field. This theoretical framework can then be applied to other vegetation shifts that alter rates of evapotranspiration.
 At the regional scale, the annual climatic water balance (precipitation - potential evapotranspiration) strongly influences salinization (Figure 1). If this balance is positive, occasional contributions of groundwater to evapotranspiration (discharge) will be offset by precipitation inputs to groundwater (recharge). In this case a net downward water flux means that salinization will be unlikely, although temporary groundwater use and salt accumulation could occur during dry periods. On the other hand, if the water balance is negative and groundwater has the potential to offset this deficit, its contributions may exceed recharge, causing a net upward flux of water and accompanying solutes. Salinization will proceed faster where water balances are more negative and groundwater is saltier [Schofield et al., 2001].
 Hydrogeological factors also affect salinization from landscape to regional scales (Figure 1). Climate interacts with lithology and geomorphology to determine the presence and depth of phreatic groundwater and accompanying salts [Domenico and Schwartz, 1990]. Hilly landscapes with massive bedrock are less likely to provide widespread access to groundwater than flat sedimentary plains. Through its influence on the hydraulic conductivity of sediments, lithology also dictates the maximum rates at which groundwater flow to plants can be sustained. While the hydraulic resistance of clay sediments yields groundwater at rates that are likely to be orders of magnitude lower than vegetation demand, even with a water table gradient of several meters, sandy sediments can match this demand with only slight water table depressions [Jobbágy and Jackson, 2004; Sapanov, 2000]. Thus lithology and geomorphology act as filters on climate, restricting the extent of salinization to areas where groundwater can be accessed and used at significant rates by plants.
 Biological factors dictate the intensity of salinization and its location across the landscape, as well, by influencing maximum evapotranspiration and salinity tolerance, interacting with the large-scale factors mentioned above (Figure 1). Afforestation can shift the actual water balance from positive to negative, resulting in net groundwater discharge and salt accumulation [Jobbágy and Jackson, 2004]. Vegetation also dictates maximum salinity values in places where net groundwater discharge occurs. The combination of groundwater absorption and salt exclusion by roots eventually raises groundwater salinity to a concentration that hinders further groundwater uptake [Morris and Collopy, 1999]. Different salinity tolerances for tree species may determine how long a species can continue to use groundwater as salinity increases. Once the salinity tolerance is reached, further groundwater uptake is minimal for stands of that species.
 In this paper we characterize the process of salinization that accompanies grassland afforestation in the Río de la Plata Grasslands of South America and use that setting to evaluate the theoretical framework proposed above (Figure 1). In a previous study conducted in the region, Jobbágy and Jackson  evaluated the role of hydrogeological factors on salinization, showing that afforestation led to a stronger groundwater salinization on intermediate texture soils (silty) than in clayey and poorly conductive soils or in sand and highly permeable soils. In this work we explore the salinization process across a climatic gradient, integrating previous work on salinization controls into a more regional framework that accounts for climatic and biological influences.
 The Río de la Plata Grasslands offer vast areas with shallow water tables and highly conductive sediments [Soriano et al., 1991; Tricart, 1973], which provide groundwater access to vegetation. On the basis of our theoretical framework outlined above, we made two predictions: (1) As the water balance becomes more negative, soil salinity will show increasingly higher concentrations under plantations compared to adjacent grasslands. In humid areas salinization will be prevented by a positive water balance that cannot be reversed by trees; below a certain water balance threshold, however, plantations will be able to trigger this switch from recharge to discharge and salinization will proceed. (2) Tree species strongly influence soil and groundwater salinity driven by their respective salinity tolerances. Under homogeneous conditions, species with higher salinity tolerances will lead to stronger salinization than less salt-tolerant species.
 We tested both predictions across 32 field sites spanning a ∼700 mm a−1 range of annual water balance. The regional influence of climate on salinization was explored in pairs of adjacent grassland-eucalyptus stands, and the effects of tree species was evaluated in ten contiguous experimental plots at one of our study sites. Our measurements were complemented by an integrative estimate of water and carbon fluxes in grasslands and eucalyptus plantations based on a remotely derived vegetation index.