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Abstract
The effects of experimentally delayed soil thawing were studied on a 32-year-old stand of Norway spruce [Picea abies (L.) Karst.] located in the eastern part of Finland. While the soil temperature at depths of 10 and 50 cm rose above 0°C at the beginning of May in the control treatment (CTRL), it stayed close to 0°C until mid-July in the delayed soil-thawing treatment (FROST). Budburst was delayed by a few days by the FROST treatment. Treatment affected the physiology of previous- and current-year needles, i.e. their electrical impedance, potential efficiency of photosystem II (F v/F m), and chlorophyll a and b content. At the end of the growing season, in November, both the area of the central cylinder and all of the measured dimensions inside the cylinder in the current needles and number of sieve cells in the previous year's needles were smaller in the FROST than the CTRL trees. At the same time, the fully developed dormant buds showed differences only in their side shoots, the width of the buds being smaller in the FROST than the CTRL trees. No effect was found on the onset of wood formation. In contrast, the radial increment tended to be faster in the delayed soil-thawing treatment during late summer.
We thank Lasse Loven and Kalle Eerikäinen for their support with the field study (Life to Koli project), Ismo Hyttinen, Eija Koljonen, Seija Repo, Urho Kettunen, Martti Malinen and Raino Lievonen for their technical assistance, Leena Karvinen for drawing the figures, and John A. Stotesbury for revision of the English used in the manuscript. This study was funded by the Finnish Forest Research Institute (project 3394).