Seasonal and daily cycles of stem radial variation of Pinus pinaster in a drought-prone environment

High resolution measurements of stem diameter variation can provide valuable information on the growth process as well as the tree water status. This study investigates the hourly variations in stem radial increment of maritime pine growing on the west coast of Portugal, under a Mediterranean climate with a pronounced summer drought. We tested the hypotheses that (1) stem radius variation in maritime pine has a daily and seasonal pattern that reflects the availability of water and (2) that once the internal water storage is depleted the tree enters a quiescent state. During 2010, stem radius variations were monitored in four similar trees with automatic dendrometers. Tree water deficit was extracted from the stem radial variation. The stem cycle approach was used to divide the daily cycles into contraction, recovery and increment phases. The seasonal cycle was divided into five periods: winter dormancy, spring growth, pre-summer contraction, summer quiescence and autumn re-hydration. Amplitude and duration of each phase were calculated for each cycle and correlated with precipitation, maximum and minimum temperature. Continuous positive radial increment started in spring and reached its maximum by the end of June. A shrinking period was observed in summer, with amplitudes of contraction and recovery 10 times higher than in the other periods. The inability of the trees to recover from the water lost due to transpiration was responsible for stem shrinking and quiescence observed during summer. In autumn, a period of re-hydration and rapid expansion was observed after precipitation, restoring the tree to a physiologically active state. Daily variations in stem radius of maritime pine were mainly determined by the course of transpiration and thus dependent on temperature and tree water status.