Among the various plant nutrients, calcium appears to occupy a unique position, acting as an important regulator in many processes related to both growth and responses to environmental stresses. This applies to stomatal function, cell division, cell wall synthesis, signalling functions in plant defence, repair of damage from biotic and abiotic stress and to the structural chemistry and function of woody tissues. The calcium content in the cambium of poplar was shown to rise transiently by as much as 40% in spring, indicating the significant role that calcium plays in the onset of cambial reactivation. Moreover, during bud flush and the beginning of cell division, calcium was reported to increase significantly in the apical meristem. A reduction in calcium supplies also proved to strongly affect wood formation, as evidenced in the pronounced reduction in wood increment, vessel size and fibre length, as well as in reduced carbonyl and methoxy groups from S-lignin. Induced wounding revealed that calcium acts as an intracellular signal and, furthermore, proved its involvement in long-distance electrical signalling. Environmental stimuli such as cold shock or wounding showed that poplar grown under calcium-starved conditions was incapable of responding to this type of stress. The above evidence highlights the important role of calcium in tree functions, both as a signal in minute physiologically active pools within the cytoplasm, and in higher concentrations for its impact on the structural integrity of cell walls and woody tissues.