Tracheid morphology in earlywood and latewood
Tracheid diameters in early-and-latewood differed significantly (Table 1), partly supporting the widely recognized premise that earlywood is composed of wide, thin-walled tracheids in contrast to the narrow thick-walled tracheids in latewood. An explanation for the latter could be that the duration of latewood tracheid development is shorter than earlywood tracheid development, while a large proportion of non-structural carbohydrate produced in the current growing season is consumed by earlywood cell wall thickening (Rathgeber et al. 2016). Since the principal function of tracheids in earlywood is water transport, while tracheids in latewood are primarily involved in mechanical support and possible water storage (Domec and Gartner 2002), the xylem structure presented by the cell diameter in early-and-latewood of our study is in line with their functions. A large tracheid size in earlywood results in a large lumen diameter, enabling higher water conductivity (Pittermann and Sperry 2003). Moreover, a small tracheid diameter (<30 µm) in latewood is assumed to protect against freezing-induced embolism in conifers at high latitudes and altitudes (Davis Sperry, and Hacke 1999; Pittermann and Sperry 2006).
The difference in tracheid wall thickness between early- and latewood was insignificant when taking phylogeny into account, which was unexpected. However, Cuny et al. (2014) found that the area of tracheid walls was quite constant along growth rings except for the last quarter of the ring, which may be explained by the strong negative correlation between the duration and rate of secondary wall deposition for the three studied members of Pinaceae, Abies alba , Picea abies andPinus sylvestris (Cuny, Rathgeber, Frank, Fonti, and Fournier 2014). Wilkinson et al. (2015) also found that tracheid wall thickness in intra-ring cells was surprisingly constant for Pinus pinaster(Wilkinson, Ogée, Domec, Rayment, and Wingate 2015). A possible explanation for our result is that the difference in tracheid wall thickness between early- and latewood was species-specific and may not always manifest the same pattern for various conifers (Kim, Funada, and Singh 2016). Although some boreal conifer species have thicker cell walls in latewood compared to earlywood, the distinctiveness between the two regions may not apply to all conifer species, especially those conifer families of warm regions, e.g., Podocarpaceae and Araucariaceae. For instance, lumen diameter and cell size in the wood ofPodocarpus neriifolius (an evergreen conifer species growing in tropical China), did not differ noticeably between earlywood and latewood (Jiang et al. 2010). The majority of conifer species in this study are distributed throughout subtropical climates (Fig.1), which could explain the insignificant difference in tracheid wall thickness between early- and latewood.
During the evolution of conifers, selection has acted on optimizing xylem structure to fulfill both safety and transport requirements (Hacke 2015). Conifers have adapted to diverse and often difficult environments by adjustment of their xylem structure at different levels, such as the amount of ray parenchyma at the tissue level (Olano et al. 2013), tracheid size, wall composition and structure at the cell level (Lachenbruch and McCulloh 2014), and pit membrane structure at the pit level (Lachenbruch and McCulloh 2014; Hacke 2015). There is an assumption that coordinated traits combined, not just a single trait, determine the safety/efficiency trade-off and further determine species distribution (Martínez-Vilalta et al. 2004; Pittermann et al. 2010; Bouche et al. 2014; Pittermanna Stuart, Dawson, and Moreau 2015). Intraspecific studies over broad latitudinal and elevation gradients also suggest that anatomical traits might not be closely related to efficiency and safety in contrast to leaf to sapwood area ratio and plant height for Pinus sylvestris and Juniperus communis(Martínez-Vilalta et al. 2009; Unterholzner et al. 2020). Besides the importance of xylem adjustment at the pit level and other mechanisms, we here postulate that tracheid dimensional contrasts between early- and latewood may be an ecological strategy for various conifers when adapting to different environmental conditions along latitude, especially cold temperature at high latitude.