Reply to Elmendorf and Ettinger: Photoperiod playsa dominantand irreplaceable role in triggering secondary growth resumption

In their Letter, Elmendorf and Ettinger (1) question the dominant role of photoperiod in driving secondary growth resumption (hereafter referred to as xylem formation onset) of the Northern Hemisphere conifers, recently reported by Huang et al. (2). Their opinions are grounded on the following three aspects, including 1) the seasonality of the photoperiod, 2) the dependence of the predictor variables (e.g., photoperiod, forcing, and chilling) on the response variable (the date of onset of xylem formation, day of the year [DOY]), and 3) the limited value of the obtained models for interannual forecasting. We think they bring up an interesting issue that deserves further discussion and clarification. Photoperiod is acknowledged to regulate spring bud swelling while wood formation starts (3, 4). Although photoperiod seasonality occurs at each site, its influence is marginal in our study given that the analysis involved comparisons among sites across the Northern Hemisphere. Our conclusion that photoperiod plays a dominant role was built upon the combination of several coherent pieces of evidence, rather than “the crux of Huang et al….” as they pointed out. First, we clearly stated that model 2, which modeled DOY as a function of the mean annual temperature of the site (MAT), forcing, chilling, and soil moisture, was considered the best model in terms of parsimony according to minimum Akaike information criterion and Bayesian information criterion, rather than R2 as referred to in their Letter. Second, photoperiod interacted with MAT and can explain 61.7% of the variance of MAT alone (2). Therefore, we concluded that secondary growth resumption was driven primarily by MAT and photoperiod or by their interaction, which is challenging to be disentangled without experimental data, we agree. In terms of biological functioning, they play an ..

Linking intra-tree-ring wood density variations and tracheid anatomical characteristics in Douglas fir (Pseudotsuga menziesii (Mirb.) Franco)

In this study, a geometric approach was used in order to model the relationships between intra-tree-ring wood density variations and tracheid anatomical characteristics. One Douglas fir was studied in detail. Anatomical slides of 256 tree-rings were compared with the corresponding density profiles. Radial and tangential tracheid diameters decline from earlywood to latewood by 50% and 15%, respectively. At the same time, radial and tangential cell-wall thicknesses increase by 110% and 132%, respectively. Wood density exhibits an S-shape profile with a slight decrease of 10% at the beginning of a ring and an increase of 212% thereafter. Model simulations showed that wood density increase is mainly due to cell-wall thickening in tangential and radial directions as well as cell size reduction in radial direction. Simulations also showed that the knowledge of tracheid anatomical characteristics is not sufficient to fully explain wood density decrease at the beginning of earlywood. © INRA, EDP Sciences, 2006.SCOPUS: ar.jinfo:eu-repo/semantics/publishe