The latitudinal pattern of xylem structure of conifer species
Although many anatomical traits of woody angiosperms demonstrate a latitudinal trend in xylogenesis (Lens, Luteyn, Smets, and Jansen 2004; Wheeler, Baas, and Rodgers 2007; Zheng, Zhao, Morris, and Jansen 2019), gymnosperm wood has been less studied until recently (St-Germain and Krause 2008; Rossi et al. 2016; Björklund et al. 2017; Huang et al. 2020). In our cross-species study, most tracheid traits showed a clear latitudinal pattern, especially in traits related to cell wall dimensions in latewood (Fig. 3). We also reveal a decrease in tracheid length and tracheid wall thickness for both early-and-latewood together with tracheid diameter in latewood along the latitudinal gradient. Regarding such a large latitudinal gradient under a monsoonal climate in China, the carbon investment for xylem formation would decrease in the northern regions due to a shortened photoperiod during the growing season, which may lead to a reduction in tracheid diameter and tracheid wall thickness in latewood. The latter finding is in good agreement with the fact that tree growth rate declines towards high latitudes. Besides, radial diameters of latewood tracheids were generally lower than 30 μm, which is consistent with the threshold diameter for frost-induced embolism (Pittermann and Sperry 2003). Therefore, the decreased stem hydraulic capacity in a cold climate might be a consequence of the evolution of reduced vulnerability of xylem to freezing-induced embolism (Creese, Benscoter, and Maherali 2011), which represents an ecological strategy for conifers distributed in colder climates. For instance, the wide distribution of Pinus species in the Northern Hemisphere was thought to be due to adaptation to cold temperatures during the Eocene (Millar 1993).