1 Centro de Investigação e de Tecnologias Agroambientais e Biológicas, Universidade de Trás-os-Montes e Alto Douro, Quinta de Prados, 5000-801 Vila Real, Portugal. Electronic address: pfern@utad.pt.
2 Department of Agricultural and Forest Engineering, University of Lleida, Lleida, 25198, Spain; Department of Geography and Land Management, University of Zaragoza, GEOFOREST Group, 50009, Spain. Electronic address: marcos.rodrigues@udl.cat.