| Summary: | Xylem cell formation and stem radial fluctuations for jack pine in Yellowknife, NT were assessed during the 2017 season. The characterization of the timing of the onset of xylogenesis reactivation, period(s) of rapid cell development, and termination of seasonal stem growth were recorded over the estimated growing season. Two methods, microcores and automatic point dendrometers were used to better understand what the characteristics of a growth season looks like for jack pine at its northern limit within the boreal forest. It was demonstrated that onset of cell development at the study site “Treeville” occurred once temperatures had exceeded the 4-5ºC temperature threshold, the period of rapid growth was observed around June 21 for all phases of xylogenesis, aligning with the timing of annual longest photoperiod during summer solstice. It is demonstrated that jack pines in this region are much slower growing, with cell development at ~0.28 cells/day during the period of rapid growth, and site wide average of ~13 cells. Precipitation was highly correlated with the observation of the SRI phase throughout the season, most notably in the period prior to cell wall-thickening. A multi-year cell development assessment indicated a relationship with June precipitation, demonstrating lower cell counts in years with low June precipitation. Cell growth was recorded to end in late August, although some trees demonstrated cells in developing phases into early September. Overall, results suggest (1) strong relationship between the onset of jack pine wood formation when temperatures exceed established thresholds for growth, (2) demonstrated that the period of rapid growth aligns with longest daylength June, and (3) annual cell count is controlled by June precipitation.
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