Supervisors: prof Jaak Jaagus, dotsent Alar Läänelaid
Opponent: prof Tomasz Wazny (Nicolaus Copernicuse Ülikool, Poola; Arizona Ülikool, USA)
Trees record environmental conditions in their annual growth layers and provide key clues to past climate and to possible future variations. Hence, the thesis investigates the weather impact on radial increment of pedunculate oak (Quercus robur L.) at its northern distributional limit in Finland and Estonia, and explains its sapwood variability in the eastern Baltic region (i.e. in Finland and the Baltic States). The research resulted in the first countrywide oak tree-ring chronology (1646-2011) for Estonia as well as in respective Finnish, Latvian, Lithuanian chronologies. The similarity of oak growth pattern within the studied region is strong and extends up to northern Poland. Oak in the eastern Baltic region has 6-19 sapwood rings (within 95% confidence limits). This estimation increases the precision of oak tree-ring dating in the region, since the sapwood is often missing in the datable objects, but the last sapwood ring in the trunk indicates the felling date. At its northern distributional limit, oak's radial growth is limited by summer weather. Accordingly, oaks growing on shallow soil in Finland and western Estonia are positively influenced by summer precipitation. Moreover, sequential summer droughts are a likely factor inducing oak decline in the studied Finnish forest. On the other hand, the oaks on the deeper drought resistant soil in northeastern Estonia are favoured by June temperature, and the oaks in the southeastern part of the country depend on both summer precipitation and temperature. In addition, the thesis presents the first tree-ring based palaeoclimatic reconstruction for Estonia. The pointer years identified in the retrodiction fairly agree with the summer precipitation extremes reported in historic sources. Thus the reconstruction model has adequate capacity of detecting past rainfall extremes.