Importance of very fine roots in deep soil layers for the survival of rainfed olive trees

Abstract

In Portugal, traditional olive stands are usually rainfed, in spite of the hot and dry summers. Aiming to quantify water use versus water status and understanding survival strategies, in relation to summer water scarcity, an experiment was installed on a 20-year-old rainfed olive orchard (Olea europaea 'Cobrancosa'), planted in a square grid of 7 m, in a calcic Vertisol soil (profile ApBtCk) in southern Portugal (climate Csb, according to Koppen-Geiger classification). From mid-2010 to 2013, the convection fluxes of heat and water vapour, sap flow in stem and roots and water status in plants and soil were measured. This period included the driest winter since 1931 (2012), followed by the usual dry summer, allowing the analysis of plant behaviour under such extreme conditions. Trenches were opened down to 2.5 m, starting from below a tree, to analyse root distribution within the soil layers. We concluded that roots colonize the soil horizontally, with a homogenous distribution in solum horizons (Ap and Bt horizons), with abundant large, medium and fine roots. Large roots were absent below the solum, Ck horizons (0.7-3 m), presenting only fine (<1 mm) and very fine roots. We report here measurements on volumetric water content in soil around 12 tubes (neutron scattering method), located in variable positions in relation to the trees. No water table existed down to 5 m; capillary rise could be considered null. In conducting these experiments, the temporal and spatial distribution of water uptake from the soil volume was analysed, confirming the direct observations above and providing insights into the great importance of very fine roots, and, in general, the “hidden half”, for plant survival.