Downy willow (Salix lapponum L.) is a shrub that grows widely in the subarctic and boreal peat bogs of western Siberia and north-eastern Europe. In Poland, this glacial relic, occurring at the limits of its natural range, has the status of a critically endangered species. Due to the progressive decline in the downy willow population in Poland, a programme for active protection of this species was created, with the primary objective of reintroducing it and replenishing its existing populations. At each stage of reintroduction (from the laboratory to the acclimatization station to the natural habitats), the biometric and physiological parameters of plants were monitored to identify critical periods during the process. Parameters of chlorophyll a fluorescence, the concentrations of photosynthetic pigments, and foliar accumulation of reactive oxygen species (ROS) were used as an integral criterion for determining the functional and physiological status of S. lapponum during its reintroduction. Analysis of these criteria during successive stages of plant growth and morphogenesis revealed that at several points along the way from the laboratory to the peat bogs, the specimens created during microreproduction are exposed to the risk of impairment of their physiological condition. This is mainly associated with changes in location and growth conditions, which for plants, as sedentary organisms, may be a significant stress factor inducing photosynthesis disorders and thus oxidative stress.