1. a Agronomy Unit , INRA Bordeaux Aquitaine Research Center , BP 81, Villenave d'Ornon , F‐33883 , France
2. b Department SBG , Limburgs Universitair Centrum , Universitaire Campus, Diepenbeek , B‐3590 , Belgium
3. c Department of Ecology and Environmental Science , University of Kuopio , P.O. Box 1627, Kuopio , N‐70211 , Finland
4. d Universidade de Trás‐os‐Montes e Alto Douro , Apartado 202, Quinta de Prados PT‐5001, Vila Real , Portugal