This review on current biotechnological methods in forestry for in vitro tissue cultures to define the effect of stress conditions on trees, concentrates on somatic embryogenesis. Callus tissue, the key product of somatic embryogenesis, grows over a tree wound under ex vitro conditions. Callus tissue can be used in research in areas such as pathogenic susceptibility at the embryonic level, effect of heavy metals, influence of low temperatures (cryopreservation), production of secondary metabolites and transformation of plants. Callus of arborescent plants can be induced in vitro by fungal elicitors to produce secondary metabolites for pharmaceutical and cosmetic industries and are strongly repellant to herbivores and can thus act to protect forests. Analyses of dual cultures demonstrated that callus tissue exposed to a pathogenic fungus responds by synthesizing low-molecular-mass pro-teins belonging to an immune protein class. Cryopreser-vation of embryonic callus tissue also has broad applications, e.g., for valuable plant genotypes in gene banks. Without strategies to protect forests against stress factors, forest ecosystems will degrade to the detriment of all life, including humans. In vitro biotechnological research using callus tissue contributes to progress in for-estry and the disciplines of ecology, physiology, phy-topathology, culture and selection of plants.