Do tree stems shrink and swell with the tides

young Norway spruce (Picea abies L. Karst.) trees grown in containers at a controlled temperature in continuous darkness fluctuated with the tide. They attributed these changes to a rhythmic reversible flow from symplast to apoplast that is “influenced by the Moon.” This claim is remarkable, as one would expect stem diameter to vary with only the tissue osmotic potential or the tension in the xylem and phloem, the latter being closely linked to the transpiration rate and thus varying diurnally (Kozlowski 1976). To explore the periodicity of stem fluctuations in relation to the gravitational influence of the Moon, we applied Fourier analysis to data on variations in stem and xylem widths of Scots pine trees (Pinus sylvestris L.) collected over periods of 1 to 4 months. No indication of tidal influence on stem diameters was found. Diameter variations were measured at the SMEAR II station (southern Finland, 61°51′ N, 24°17′ E) (Vesala et al. 1998) and in Devilla forest (central Scotland, 56°2′ N, 3°43′ W) (Irvine et al. 1998). In Finland, the trees were 12 m high and mean diameter at breast height was 13 cm. In Scotland, the corresponding values were 15 m and 20 cm. In both Finland and Scotland, a rigid steel frame, housing a sensitive displacement transducer (Sylvac Inc., Switzerland, and an LVDT, Schlumberger Industries, U.K., respectively) was mounted around the circumference of each tree. In Finland, measurements were taken both above and below the bark. For whole-stem measurements, the loose bark was removed, and for xylem measurements, tiny screws were attached directly to the xylem. In Scotland, small windows in the bark, cambium and phloem were made so that the LVDT rested on the xylem. Corrections were made for the thermal expansion of the frame (coefficient of linear expansion used was 10 × 10 K and 20 × 10 K in Finland and Scotland, respectively) and the change in linear dimensions of fresh wood (–4 × 10 K). Data were collected from May to September (1996–1998) and April to September (1995) in Finland and Scotland, respectively. The calculated gravimetric curves (variations in gravity detectable as variations in acceleration due to it) for the locations of the stands were supplied by the Finnish Geodetic Institute. Figure 1 presents an illustrative sample of data sets for gravimetric tide and diameter variations. Tree Physiology 20, 633–635 © 2000 Heron Publishing—Victoria, Canada