Timing restriction and variability in tree-ring formation of European and North American conifers.
Deslauriers, Annie*,1, Rossi, Sergio1, Anfodillo, Tommaso1, Morin, Hubert2, 1 Università degli Studi di Padova, Padova, Italy2 Université du Québec à Chicoutimi, Chicoutimi, Québec, Canada
ABSTRACT- The timing and variability of tree-ring development was studied between 1996 and 2004 in the main European and North American conifers. Intra-annual dynamics of tree-ring formation was studied in Canada and Italy by means of repeated xylem cell analyses (XCA) and automatic dendrometer measurements (ADM), assessing cell production and stem radial growth. All the annual patterns of tree-ring development were modeled with a Gompertz function defining parameters A (upper asymptote), (x-axis placement or onset) and (changing rate of the curve). Timing of wood formation was assessed by comparing the behavior of parameters and and evaluating their common features. A linear relationship was found between growth onset and rate for both XCA (R2=0.96) and ADM (R2=0.93). We mathematically demonstrated that the regression slopes (164.5 for XCA and 170.8 for ADM) correspond to the time when the maximum growth rate culminates and that all inflection points converge at the same moment of the year, showing a clear development timing among all species, sites and years. Two fundamental aspects of annual wood formation were therefore identified. First, the culmination of maximum growth rate during the growing season is constrained by a definite time limit, corresponding to the photoperiod maximum. Because of high temperature variation or unusual temperatures during the growing season, the photoperiod represents a more constant signal. Synchronizing the maximum growth rate with temperature culmination (end of July) as proposed before, represents a risk for the plant as the last tracheids formed remain in differentiation until late autumn. The second important aspect concerns a wide variability observed in onset time, rate and duration, optimizing the wood production timing with temperature and photoperiod each year in all environments. Growing season timing and variability are fundamental aspects for the understanding of global change because they represent a time window where environmental factors directly affect wood formation. Any temperature changes would have an effect on the growing season timing only on the variable parts of the growth curves (timing of positive and negative exponential part), leaving the timing of maximum growth rate culmination unchanged.
Key words: Growing season, Annual timing, Photoperiod, Temperature
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