Rain exclusion affects cambial activity in adult maritime pines

- Rain exclusion decreased the rate of xylem cell production resulting in fewer tracheids.
- Latewood tracheids from rain exclusion trees presented narrower lumen diameter.
- Cell wall thickness was not affected by rain exclusion.
- The position of latewood intra-annual density fluctuations is determined by the timing of cambial reactivation in autumn.

Drought frequency and intensity are expected to increase in the Mediterranean basin with unforeseen impacts on forest ecosystems. In this study, we investigated the effect of a rain exclusion experiment on the growth dynamics of adult maritime pine trees. The rain exclusion experiment took place from March to October 2014. Cambial activity and xylem development was weekly monitored in five treated and five control trees during the 2014 growing season. Although growth started simultaneously in all trees, the number of differentiating xylem cells and the rates of cell production were reduced in the rain exclusion trees. Rain exclusion reduced the number and the lumen diameter of latewood tracheids. Cell wall thickness was not affected by the treatment. After the summer, both groups of trees resumed cambial activity, with rain exclusion trees resuming growth two weeks later. The second period of cambial activity observed in autumn, produced earlywood-like tracheids after latewood formation, the so called intra-annual density fluctuations (IADF). The position of the IADF within latewood differed between groups, being located within and at the end of latewood in control and rain exclusion trees, respectively. These findings demonstrate that the position of IADFs is determined by the timings of cambial reactivation in autumn.