Impact of evergreen oaks on soil fertility and crop production in intercropped dehesas

Dehesa, open oak woodland, is the most extended agroforestry system in Europe, with more than 3 million hectares existing in the south-western Iberian peninsula. However, today, thousands of trees are still being lost every year and soils that are more fertile have already been completely deforested. A better understanding of the effects of oak trees on crops could help in designing management practices to ensure sustainability of this traditional agroforestry system. The aim of this study was to examine the effect of Holm-oak (Quercus ilex L.) on soil nutrient concentration and its consequent effect on the production of understorey crops (oats: Avena sativa L.) in intercropped dehesas of central-western Spain. Crop production, crop plant characteristics (weight and size), the nutritional status of crop plants (N, P, K and Mg) and soil nutrient content in the topsoil layer (0–30 cm: texture, pH, soil organic matter (SOM), total and mineral N, available P, cation exchange capacity (CEC) and base cations) were determined. Samples were taken around 6 trees per plot at distances ranging from 2 to 30 m from the tree trunk, in 16 intercropped plots differing in soil fertility and fertilisation rate. Soil fertility increased near the trees, with a significant increase of SOM, total N, available P, CEC and exchangeable Ca2+ and K+. By contrast, pH and soil particle size did not vary significantly with the distance from the tree. In unfertilised plots, trees had a positive effect on crop production, with higher crop production occurring beneath trees than beyond tree canopy projection. Fertilisers outweighed the positive effect of trees in fertilised plots, where crop production was reduced near to the trees. This could be explained mainly by management practices (biased distribution of seeds and fertilisers by broadcast spreaders), whereas light reduction probably played a minimal role (explained only 2% of the crop yield variability).