Onset and Seasonal Kinetics of Xylogenesis in Pinus sylvestris L. on the Southern Fringes of Its Distribution Depend on Early Spring Air and Soil Temperature

Climatic variation is inherently linked with tree phenology; however, phenological triggers depend on species and habitat. We analyzed key climatic factors for the onset of secondary growth for Scots pine (Pinus sylvestris L.) at the southern limit of its distribution in Siberia. From direct observations of developing tree rings, seasonal curves of the number of cells in the cambial zone, in the cell-expansion zone, and the total number of xylem tracheids were developed over seven years with a wide variety in the phenological dates. We found that later and shorter intervals of these stages of xylogenesis were compensated by higher maximums of kinetics curves, probably due to higher temperatures and daylengths during the respective phenophases. Air temperature and soil temperature at a depth of 20 cm converged to values (mean ± SE) 6.6 ± 0.9 °C (air) and 3.7 ± 0.4 °C (soil) for a 15-day interval prior to cambial activity onset. Date of Tsoil ≥ 3.5 °C was most closely related to cambial activity onset (r = 0.99) and preceded it by 9.6 ± 1.1 days. Cumulative temperature sums were less reliable. Apparently, both air and soil temperature thresholds have to be reached for cambial division to start in this species-habitat combination. Late abundant snowfall can yield divergence between air and soil temperatures and delay the onset of xylogenesis.