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

MAGLIANO, PATRICIO N.; GIMÉNEZ, RAÚL; HOUSPANOSSIAN, JAVIER; PÁEZ, RICARDO A.; NOSETTO, MARCELO D.; FERNÁNDEZ, ROBERTO J.; JOBBÁGY, ESTEBAN G.

ECOHYDROLOGY

JOHN WILEY & SONS INC

Año: 2017

1936-0584

Soil evaporation is a dominant water flux of flat dry ecosystems, reducing available water forplant transpiration. Vegetation plays a key role at controlling evaporation, especially by alteringsoil surface micro‐meteorological conditions. Here, we explored the vegetation cover effect onsoil 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 soilevaporation 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 coverconditions of the site) and one with bare soil, but in pastures, only one micro‐lysimeter with baresoil was installed at each patch (representing the typical no litter cover conditions of pastures ofthe study site). We found that, when soil water was not limiting, litter cover had the strongesteffect in reducing evaporation rates, with a 4‐ and 6.4‐fold reduction respect to bare soilmicro‐lysimeters in the forest and pasture, respectively. Evaporation decreased sharply withdeclining 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 lowerand more stable evaporation rates, decreasing only from 1.35 to 1.03 mm/day under the sameradiation conditions (R2= 0.10). In accordance with J.T. Ritchie evaporation model, we identifieda threshold of ~10.5 mm of cumulative evaporation at which evaporation switched from energyto water limitation in all situations, as revealed by declining evaporation rates and raising surfacetemperatures. Under typical wet?summer conditions, the pasture, the forest with bare soil, andthe 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 atour study site, litter would maintain evaporation in the energy‐limited mode most of the time(68.8% of summer days), potentially favouring transpiration. The ecohydrological key role of soillitter controlling evaporation highlights the importance of an accurate assessment of management practices controlling the evaporation/transpiration partition in dry ecosystem.