Involved in the synthesis of labdane-related hydrocarbons by catalyzing the conversion of geranylgeranyl diphosphate (GGDP) to (+)-copalyl diphosphate ((+)-CDP). (2E,6E,10E)-geranylgeranyl diphosphate = (+)-copalyl diphosphate Binds 3 Mg(2+) ions per subunit. The DXDD and DEXXE motifs are important for the catalytic activity. Belongs to the terpene synthase family.