“Free air” ozone canopy fumigation in an old-growth mixed forest: concept and observations in beech. - Nunn,A..J..; Reiter,I..M..; Häberle,K.-H..; Werner,H..; Langebartels,C..; Sandermann,H..; Heerdt,C..; Fabian,P..; Matyssek,R.. (Phyton, 2002)