Leaf dimorphism in Cupressus nootkatensis D. Don and Cupressus vietnamensis (Farjon & T.H. Nguyên) Silba (Cupressaceae) and its ecological and evolutionary significance

Key message We examined the leaf structure of two basal Cupressus species with a distinct leaf dimorphism. Some foliar features are regarded as ancestral and it is suggested that drought adaptation was one of the important ecological drivers in the evolution of the Cupressus genus. Leaf morphology and anatomy of two Cupressus species, C . nootkatensis and C . vietnamensis , were investigated with classical paraffin technique and scanning electron microscopy (SEM). Like all Cupressus species these two are characterised by a dramatic change in the foliage. Juveniles have needle leaves first before they change abruptly to the mature scale leaf type. In C . vietnamensis , needle-leaved shoots occur next to scale-leaved ones even on mature trees, which is unique among today´s Cupressus species. Adults of C . nootkatensis develop only scale leaves throughout. In both taxa, the scale leaves show a distinct dimorphism between lateral and facial leaves, which are arranged in a flat spray; the foliate shoots are two-dimensionally flattened. These scale leaves show several xeromorphic features; e.g. strongly reduced leaf size, stomata with high, collar-like Florin rings, the presence of a distinct hypodermis as a continuous layer and well-developed transfusion tissue. The needle leaf type is found in Cunninghamia which is the basal member of the Cupressaceae and so is regarded as the ancestral condition and scale leaves as a derived one. Scale leaves are found in all the members of the cupressoid clade even within the basal taxa from mesic habitats. However scale leaves are a preadaptation to survival under xeric conditions and they are likely an evolutionary driver of the radiation of Cupressus into arid environments, as has also been the case in genera such a Callitris.