Understanding the process of wood formation, xylogenesis, is fundamental for the ecological management and conservation of forests due to the implications of tree responses to the impact of climate change, especially in harsh environments. Such is the case with the coastal forests of Pinus pinea L. in the Llobregat Delta (Barcelona, Spain), classified as habitats of special interest and conservation under European regulations.

In this study, we selected four stands of these coastal forests, which differ in tree density and phreatic water parameters, determining the xylem formation process in 20 adult Pinus pinea L. trees (5 from each stand of similar age and diameter). Concretely, we show results from two of these stands, both differing just in tree density.

Procedures. Every twenty days from November 2020 to December 2023, we extracted 1-1.5 cm long minicores, from which we obtained histological sections (6-8 µm). These sections were photographed (40x) to determine the number of cells and the ring width. Tracheids were classified in terms of blue cells (elongation and thickening cells) and early- and late-wood cells. The trees selected were dominant, healthy individuals, representative of each stand, located at a mean of 15 meters from each other. The parameters of brackish groundwater are located at a depth of 1.3 meters.

Results. The results indicate that Pinus pinea L. exhibits greater phenotypic plasticity than anticipated, suggesting that competition among trees exerts a significant influence on phenology. We found that the growing period for the less dense stand is quite long—March to July and September to December—while that ofthe densest is from March to May and July to November. Moreover, the less dense stand can triplicate the number of cells compared to the dense one. The same happens with the ring length width. These results were obtained by studying the number of early- and late-wood cells and features such as width.