The possible effects of climate change on the advance of the tree line are considered. As temperature, elevated CO(2) and nitrogen deposition co-vary, it is impossible to disentangle their impacts without performing experiments. However, it does seem very unlikely that photosynthesis per se and, by implication, factors that directly influence photosynthesis, such as elevated CO(2), will be as important as those factors which influence the capacity of the tree to use the products of photosynthesis, such as temperature. Moreover, temperature limits growth more severely than it limits photosynthesis over the temperature range 5-20 degrees C. If it is assumed that growth and reproduction are controlled by temperature, a rapid advance of the tree line would be predicted. Indeed, some authors have provided photographic evidence and remotely sensed data that suggest this is, in fact, occurring. In regions inhabited by grazing animals, the advance of the tree line will be curtailed, although growth of trees below the tree line will of course increase substantially.