Traditional ploughing is critical to the conservation of threatened plants in Mediterranean olive groves

Arable plant diversity has been dramatically declining due to agriculture intensification, with several arable species currently included in national Red Lists. This is particularly relevant in the case of plant communities of the traditional Mediterranean agricultural systems. Despite the current knowledge about the factors affecting this diversity, it is not clear how these communities, which have likely evolved under the pressure of ploughing for millennia, depend on this regular soil disturbance. This is an important issue nowadays, because current management practices focused on the conservation of the ground vegetation cover, to protect soil and other biological groups, often exclude ploughing. In this paper, we test the hypothesis that traditional ploughing is favourable to more specialist arable plants, predictably more dependent on the regular soil disturbance. We address this issue in a Mediterranean agricultural system, the traditional olive groves of Southern Portugal, which is characterized by the annual ploughing of soil. A total of 90 plots containing 1350 sampling quadrats were sampled, and all plant species identified. We categorized plants in four target groups of conservation interest, and then used a joint species distribution model to model their occurrence in relation to three management practices: ploughing, cultivation and low-intensity grazing, using abandoned olive groves as reference level. Results suggest that ploughing is a key factor for the maintenance of arable plant diversity. Ploughing had a positive effect on the occurrence of rare arable plants, archaeophytes and on several Red listed species. In order to conserve these high value plant communities and endangered plant species, we recommend incentivizing ploughing using reduced tillage techniques (e.g. chisel ploughing) on these traditional Mediterranean agricultural systems.