Frost-induced reversible shrinkage of bark of mature subalpine conifers

Temporal and spatial patterns of stem and root radius changes were continuously measured on mature, subalpine Norway spruce (Picea abies (L.) Karst,) over 2 years. In addition, freezing experiments with stem segments of saplings were carried out in a climate chamber. The dynamics of the radius fluctuations were analyzed in relation to temperature profiles of air, bark, and soil. We found that bark thickness sharply decreases by 1 mm and more within 2-3 days when the air temperature falls below -5 degrees C, In contrast to the elastic tissues of the bark, the rigid xylem remains largely unaffected. This frost-induced shrinkage of the bark is up to 10 times larger than the measured amplitude of diurnal stem radius fluctuations in summer. During periods of rising air temperatures (>-12 degrees C), the radius suddenly returns to its original size. We conclude, that the large stem and root radius changes in winter are related to changing bark water content, as is also the case during size fluctuations in summer For a mature tree with a height of 25 m, the shrinkage of the entire stem bark is equivalent to approximately 201 of water. A hypothesis is discussed which suggests a frost-induced transport of this water between bark and wood. It is based on the initial freezing of water in the xylem resulting in a water potential gradient between bark (solution) and wood (ice). The hypothesis suggests that the water transport between bark and wood is mainly determined by physical changes and that no biochemical transport energy or physiological control mechanisms are involved. As long as ice is initially formed in the xylem and not in the bark, this mechanism of bark dehydration comes into play and protects the living cells from frost damage. (C) 2000 Elsevier Science B.V. All rights reserved.