Vessel features of broadleaf trees may contain information about both spatial and temporal variations of environmental conditions. We report quantitative data about annual vessel characteristics for Oriental beech (Fagus orientalis Lipsky) in the Caspian forest in the Alborz Mountains, northern Iran. Time series of tree-ring width and of various vessel variables were constructed over a 50-year period for three sites along an altitudinal gradient from 1260 to 2200 m a.s.l. We evaluated the dependencies between ring width and vessel variables and estimated the influence of altitude and climate on these variables. Mean ring width and average vessel-lumen area decreased towards the high-elevation site whereas total vessel-lumen area was independent from altitude, and vessel density increased. Above-average warm summer and autumn seasons in the year prior to growth were negatively correlated with ring width and average vessel-lumen area whereas precipitation in the current June showed a positive association; vessel density responded exactly opposite to these climatic variables. Total vessel-lumen area was the only variable that reacted differently from site to site, mainly positively to temperature at 1200 m a.s.l. and negatively at 2000 and 2200 m a.s.l., whereas a consistent response to precipitation was lacking. The results indicate that vessel variables are meaningful indicators of changes in dynamics of wood formation in relation to climate along an altitudinal gradient in the Caspian mountains. As for climate reconstruction, we have to conclude that for our study area, vessel variables do not add much to information derived from ring width.

Trees can control their shape and resist gravity thanks to their ability to produce wood under tensile stress. This stress is known to be produced during the maturation of wood fibres but the mechanism of its generation remains unclear. This study focuses on the formation of the secondary wall in tension wood produced in artificially tilted poplar saplings. Thickness of secondary wall layer (SL) and gelatinous layer (GL) were measured from cambium to mature wood in several trees sampled at different times after tilting. Measurements on wood fibres produced before tilting show the progressive increase of secondary wall thickness during the growing season. After the tilting date, SL thickness decreased markedly from normal wood to tension wood while the total thickness increased compared to normal wood, with the development of a thick GL. However, even after GL formation, SL thickness continues to increase during the growing season. GL thickening was observed to be faster than SL thickening. The development of the unlignified GL is proposed to be a low cost, efficient strategy for a fast generation of tensile stress in broadleaved trees.