Changes in climatic signals of English oak tree-ring width and cross-section area of earlywood vessels in Latvia during the period 1900–2009

We investigated changes in response of wood formation in English oak (Quercus robur L.) to climatic factors since 1900. It was hypothesised that the effect of winter and spring temperatures has weakened, while summer precipitation has become limiting. Increment cores were taken from 40 sites across Latvia. Tree-ring width and cross-section area of earlywood vessels were measured and cross-dated. Regional chronologies were built by pooling time series of trees within two regions of Latvia (western and eastern region), which differed in continentality. Climatic signals differed between the proxies (tree-ring width and earlywood vessel cross-section area) and between regions. Mean cross-section area of earlywood vessels was mainly influenced by temperature during winter and spring. However, the temperature-sensitivity of this proxy to spring to winter temperature weakened towards the end of the previous century, while a positive effect of August precipitation and negative effect of July temperature in the previous year on tree ring width appeared. Thus, climate change has led to lower sensitivity of wood formation of oak to winter temperature and growing season length. However, growth can be limited by summer droughts, which are becoming more common, particularly in the eastern region of Latvia.

► Earlywood vessel size contains stronger climatic signals than tree-ring width.
► Earlywood vessel size is limited by temperature in winter and spring.
► Tree-ring width in oak in Latvia is limited by summer temperature in the previous year.
► Under global warming, winter temperature is losing effect while effect of summer precipitation is strengthening.