Tree improvement programs have two important components: (1) induction of desirable types of genetic variation by breeding, and (2) selection and maintenance of the traits by sexual or asexual propagation methods. By combining tissue culture systems and morden DNA technology with conventional breeding methods, both aspects of tree improvement will benfit. New tissue culture systems are also important to the development of commercial applications of improved trees;Therefore, several new approaches to the in vitro culture of hybrid aspen (Populus alba L. x P. grandidentata Michx. 'Crandon') were studied. To investigate multiple shoot regeneration capacity of different cells, tissues, and organs of this hybrid aspen, ex vitro and in vitro derived stem nodes, root, and callus derived from each tissues were examined. Although high levels of cytokinin produced greatly increased numbers of proliferated shoots, the survival (in vitro) of the shoots were relatively low compared with the shoots obtained by low levels of cytokinin. In a subsequent study, techniques were developed to rescue and use large number of shoots induced by high cytokinin treatments. After development of these rapid propagation systems, the variation of tissue-culture-derived plants was investigated at the phenotypic, chromosomal, and protein levels;To obtain large numbers of syncronized micropropagules for transplanting to nursery beds or as a tool for germplasm conservation, in vitro cold-storage systems were developed and evoluated. To achieve high survival rates of tissue culture plants in field conditions, different types of acclimatization methods were tested using thousands of tissue culture derived plants. An integrated system using polyterra peat plugs, intermittent misting, and 30% shade in greenhouse conditions gave almost 100% shoot survival.