Using the Trephor tool, we collected microcores from the stems of Norway spruce (Picea abies) and coast Douglas-fir (Pseudotsuga menziesii) trees at a selected site in Jablje, Slovenia, in 2024. Sampling of microcores was carried out every two weeks from April (DOY 96) to October (DOY 299) and permanent microscope slides containing phloem, cambium and xylem were prepared. The samples were analysed using a light microscope and software. We monitored the width of the cambial zone, postcambial cells, the width of the tissue in the stage of secondary wall formation, mature cells and non-collapsed phloem. In Douglas-fir, cambial activity started earlier (DOY 110) than in spruce (DOY 127) and lasted longer (Douglas-fir: DOY 285, spruce: DOY 264). The average width of the cambial zone was greater in Douglas-fir (~172 µm) than in spruce (~76 µm). At the end of the growing season, Douglas-fir also showed a higher increment (2317 µm) compared to spruce (1395 µm) and a larger proportion of earlywood. No intra-annual density fluctuations (IADFs) were observed in the analysed species for the year 2024, indicating stable growth conditions. Such fluctuations were found only in the spruce tree ring for the year 2023. However, IADFs and double rings were present in spruce in 2023, reflecting higher sensitivity to climatic extremes. Douglas-fir showed a more uniform wood structure and higher density, confirming its greater plasticity, growth stability, and resistance to climatic extremes, which are key traits for future forest adaptation.
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