Decreased growth of pedunculate oak (Quercus robur L.) during the 20th century has been documented in Europe and is considered as one of the symptoms of oak decline. However, there is insufficient information on wood formation of oak in Latvia (Baltic countries). Tree-ring width is a commonly used proxy for evaluation of growth, but other wood proxies, such as earlywood vessel size, can provide additional information on mechanisms affecting growth. In this study, we investigated changes in tree-ring width and cross-section area of earlywood vessels for the periods of 1899–2009. Since the 1980s, abrupt changes in wood formation were observed, particularly in the more continental eastern Latvia. Tree-ring width showed significant decrease, while lumen cross-section area of earlywood vessels significantly increased in the eastern part of Latvia. These changes might have been triggered by an extremely cold December in 1978.

Key words: earlywood vessel, tree-ring width, growth changes, climatic extreme, Quercus robur