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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.

The Late Glacial and Early Holocene environmental history of shal low lakes in Es to nia, re vealed from subfossil ostracod data

Sohar K., Meidla T.

Geological Quarterly

2009

Vol. 53, No 2

209-218

The Late Glacial and Early Holocene shallow lake history in Estonia is documented from the fresh water ostracod subfossil record. Three cores studied consist of Late Glacial and Holocoene sediments: gyttja, calcareous mud and peat, with ostracod subfossils being well preserved in the calcareous mud. 18 fresh water ostracod species were recorded in the cores: the most common species are Metacypris cordata, Limnocythere inopinata, Cyclocypris ovum, Cypridopsis vidua, and Candona candida. Changes in the ostracod succession of the lakes can be interpreted in the context of environmental changes that were not contemporaneous but were related to the evolution of particular water bodies. Darwinula stevensoni and Metacypris cordata do not appear together in the Late Glacial and Early Holocene lacustrine sediment records of Estonia. M. cordata appears as the water body evolves or by lowering of the water level. The appearance of Scottia pseudobrowniana in the sediments refers to the stage of an overgrown lacustrine system. M. cordata appeared in south eastern Estonia at the end of the Late Glacial (~12 800 cal. BP), when the calcareous mud started to form. The ea li est subfossil r cord of M. cordata from south wes ern Estonia is from the Early Holocene.