Raúl Giménez

Is aridity restricting deforestation and land uses in the South American Dry Chaco

ABSTRACT In this paper, we explored how aridity influences the regional deforestation and land-use patterns (i.e. crops/pastures) in South American Dry Chaco. To do this, we contrasted land use during last decade (2001–2012) with a spatially explicit aridity index, which we complemented with a crop water balance model. Land-use classifications were performed by considering the temporal variability of NDVI from MODIS satellites, showing that 40 and 60% of deforested land was assigned to crops and pastures, respectively. Results indicate that although the regional deforestation pattern was not associated with the aridity gradient, with drier areas similarly deforested as wetter areas, contrasting differences were observed in the use of this land, with crops mostly located (90%) in wetter areas and pastures evenly distributed across the whole aridity gradient. This research highlighted the strong effect of water limitations on the land-use option after deforestation and may help to set the basis for future land-use planning policies.

Litter is more effective than forest canopy reducing soil evaporation in Dry Chaco rangelands

Soil evaporation is a dominant water flux of flat dry ecosystems, reducing available water for plant transpiration. Vegetation plays a key role at controlling evaporation, especially by altering soil surface micro-meteorological conditions. Here we explored the vegetation cover effect on soil evaporation, differentiating the effects of canopy cover (shadow) and of surface cover (litter) in forests and pastures of Dry Chaco rangelands (San Luis, Argentina). We measured daily soil evaporation using irrigated micro-lysimeters installed at regularly spaced (2 m) patches along transects in native dry forests (n=54 patches) and pastures (n=27 patches). In each forest patch we established a pair of micro-lysimeters, one with litter (3 cm depth, representing high litter cover conditions of the site) and one with bare soil, while in pastures only one micro-lysimeter with bare soil was installed at each patch (representing the typical no litter cover conditions of pastures of the study site). We found that, when soil water was not limiting, litter cover had the strongest effect in reducing evaporation rates, with a 4- and 6.4-fold reduction respect to bare soil micro-lysimeters in the forest and pasture, respectively. Evaporation decreased sharply with declining incident radiation fraction in bare soil micro-lysimeters from 5.6 mm/day (full radiation) to 3.5 mm/day (full canopy shadow) (R2=0.50). Litter-covered micro-lysimeters showed lower and more stable evaporation rates, decreasing only from 1.35 to 1.03 mm/day under the same radiation conditions (R2=0.10). In accordance with J.T. Ritchie evaporation model, we identified a threshold of ~10.5 mm of cumulative evaporation at which evaporation switched from energy to water limitation in all situations, as revealed by declining evaporation rates and raising surface temperatures. Under typical wet-summer conditions, the pasture, the forest with bare soil and the forest with litter would need on average a drying cycle of 1.5, 2.5 and 9.5 days, respectively, to reach that threshold. Simulations showed that, considering the distribution of rainfall events at our study site, litter would maintain evaporation in the energy-limited mode most of the time (68.8% of summer days), strongly favoring transpiration. The ecohydrological key role of soil litter controlling evaporation highlights the importance of an accurate assessment of management practices controlling the evaporation/transpiration partition in dry ecosystems.

Ecohydrological transformation in the Dry Chaco and the risk of dryland salinity: Following Australia's footsteps?

During the last century the massive conversion of Australian dry forests to crops and pastures triggered the massive soil and groundwater degradation process known as dryland salinity. Currently, South American Chacos dry forests are undergoing a similar transformation, leading global deforestation rates. The goal of this study was to review existing ecohydrological information about natural and cultivated systems in the Chaco to assess the dryland salinity risks. We review deep soil water, salt stocks and groundwater recharge from agriculture/native dry forests stands located in a precipitation range of 450-1100 mm. We complement this with water table level records and geoelectric profiles together with personal observations. We use data from 15 Australian studies for comparison. Strong salt leaching, especially after 20 years of forest clearance, indicates the onset of deep drainage following forest conversion to agriculture in the Dry Chaco. Water stocks were more than double in the cleared stands compared to their dry forest pairs and recharge rates were up to two order magnitude higher in agricultural areas. While in the Dry Chaco lower atmospheric salt deposition, younger sediments, and relatively high water-consuming agricultural systems, attenuate salinization risks compared to Australia, the very flat topography and related shallow water table levels of the South American region could make groundwater recharge and salt mobilization processes more widespread and difficult to manage. The lack of awareness among the general public, farmers and decision makers about this issue amplifies the problem, making land management plans for the Argentine dry forest territories essential.