| In cold climates, the expected global warming will lead to earlier cambial resumptions in spring, with a resultant lengthening of the growing season but unknown consequences on forest productivity. The phenological traits of cambium activity and xylem formation were analysed at a short time scale along a thermal gradient represented by an altilatitudinal range from the 48th to 53rd parallels and covering the whole closed blackspruce [Picea mariana (Mill.) BSP] forest in Quebec, Canada. A hypothesis was tested that warmer temperatures influence cambium phenology, allowing longer duration and higher intensity of growth, and resulting in proportionally increased xylem production. From April to October 2012, cell division in cambium and post-cambial differentiation of xylem were observed on anatomical sections obtained from microcores collected weekly from the stem of fifty trees. The southern and warmer site was characterized by the highest radial growth, which corresponded to both the highest rates and longest durations of cell production. The differences in terms of xylem phenology and growth were marginal between the other sites. Xylem growth was positively correlated with rate and duration of cell production, with the latter explaining most variability in growth. Within the range analysed, the relationship between temperature and most phenological phases of xylogenesis was linear. On the contrary, temperature was related with cell production according to an exponential pattern. Periods of xylogenesis of 14 days longer (+13.1%) corresponded to a massive increase in cell production (33 cells, +109%). This disproportionate change occurred at a May-September average temperature of c.a. 14 °C and a snow-free period of 210-235 days. At the lower boundary of the distribution of black spruce, small environmental changes allowing marginal lengthening of the period of cell division could potentially lead to disproportionate increases in xylem cell production, with substantial consequences for the productivity of this boreal species. Models are a mathematic description of a phenomenon. They are useful to simplify and describe natural relationships and can be used to determine the link between the tree development and the surrounding environment. The aim of this research was to represent the relation between xylogenesis and it's alti-latitudinal location. The resulting equations were then extrapolated into a map covering the entire region studied by permanent plots. Mcrocores were taken weekly from April to October 2012 for 50 black spruce [Picea mariana (Mill.) BSP] distributed into five sites. Stations were located along a latitudinal gradient extending from the 48th to the 53rd parallel north and covering the entire distribution of the closed black spruce forest. Cellular analysis of each microcores has enabled to distinguish the dates of beginning and ending and then the duration in days for cell enlargement, cell wall thickening and xylogenesis and the total annual cells production. Latitude was the factor with the greatest impact while the altitude had minor or no effect, especially for nonlinear relations. Most results were linear such as the beginning of enlargement, beginning, ending and duration of lignification and the whole duration of xylogenesis. With the increasing latitude, dates were proportionally delayed for the beginning and earlier for the ending, thus creating a shorter duration. The temperature was suggested as the main factor driving these patterns. Duration and ending of enlargement and the complete cells production were nonlinear, with a regression similar to a negative exponential curve. This type of relationship seemed to indicate the presence of a limiting factor around the 49th parallel. This pattern was more complex and probably resulting from a combination of different factors. |
