High-altitude and high-latitude tree growth in relation to past, present and future global climate change

Recent dendroecological studies indicate that the growth rates of trees at some high-latitude and high-altitude sites are increasing. The date of the onset of increased growth is variable ranging from AD 1850 to 1940, with more recent increases occurring in less climatically-stressed areas. The growth data are supported by studies of tree-line dynamics which indicate that many northern and alpine tree- lines have advanced within the last 100 years. Although the causes of the growth increases are uncertain, climatic change appears to be the most likely explanation. As neither growth rates nor the northern extent of forests matches levels reached in the mid-Holocene, when the climate was considerably warmer, the mid-Holocene may represent a useful analogue for studies to predict the effects of global warming. However, in some areas, fertilization by atmospheric nitrogen is likely to be involved, and the possible impact of CO 2 fertilization cannot be discounted. Unravelling the effects of these different factors represents a major challenge to dendroecologists.