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Oak decline in a southern Finnish forest as affected by a drought sequence

Sohar K., Helama S., Läänelaid A., Raisio J., Tuomenvirta H.

Geochronometria

2014

Vol. 41, no. 1

92--103

We investigated the decline of a pedunculate oak (Quercus robur L.) forest growing on shallow soil at the northern distributional limit of the species in southern Finland, using the dendro-climatic approach. About 200-year-old trees in three vigour classes – healthy, declining and dead – were sampled in 2008. Annual tree-ring, earlywood and latewood widths were measured and chro-nologies were established. The tree-ring data were correlated with monthly and seasonal climate data. Radial increment of oaks was positively related to the June and July precipitations. This was ex-pressed especially in total ring width and latewood width, whereas the earlywood was more influ-enced by the warmer winter and spring. Furthermore, the correlation between the current year ear-lywood width and the preceding year latewood width was higher than between the earlywood and latewood of the same year. The analyses showed that the dead oaks and part of the declining oaks had ceased growing during 2005-2007 after a decade-long summer drought series. This indicates a time lag in the oak dieback. The radial growth of the declining and the dead oaks had dropped already since the 1990s, while the healthy oaks had better long-term growth and higher adaptive capacity to climate variation.