Our results show that conifers adapt to their environmental conditions through changes in their xylem structure. Tracheid diameter and tracheid wall thickness for latewood decreased with a rise in latitude, whereas tracheid wall thickness, but not diameter for earlywood, also showed a decreasing latitudinal trend. Climatic condition and soil properties contributed to this latitudinal pattern, which explains about half of the variance for tracheid diameter and wall thickness. Meanwhile, phylogeny was found to play an important role in variance of conifer xylem structure, with Pinaceae species showing strong divergence in contrast to the conservatism of species from Cupressaceae, Podocarpaceae and Taxaceae. Our comparative study of 79 conifer species provides evidence of xylem adaptation to environmental conditions when factoring for phylogeny.
Although many anatomical traits of woody angiosperms have shown a latitudinal trend (Wheeler et al. 2007; Zheng et al. 2019), studies on gymnosperm wood have been less abundant until recently (Rossi et al. 2016; Björklund et al. 2017). In our cross-species analyses, we revealed a decrease in tracheid wall thickness for both early- and latewood together with tracheid diameter in latewood along a latitudinal gradient in China (Fig. 3). In such a large latitudinal gradient under a monsoonal climate, the carbon investment for conifer xylem formation would decrease in the northern regions owing to a shortened photo-period during the growing season, resulting in a reduction in tracheid diameter and tracheid wall thickness in latewood. The latter finding is in good agreement with the fact that tree growth declines towards high latitudes. Besides, tracheid radial diameters of latewood were generally lower than 30 µm (Fig. 3C), which is consistent with the threshold diameter against freezing-thaw embolism (Pittermann and Sperry 2003). Therefore, the decreased stem hydraulic capacity in a cold region might be a consequence of the evolution of reduced vulnerability to freezing and drought-induced embolism, representing an ecological strategy for conifers distributed across colder regions (Creese et al. 2011). For instance, the wide distribution of Pinus species in the Northern Hemisphere was thought to be due to adaptation to cold temperature during the Eocene (Millar 1993).
Climatic conditions (i.e., MAT, MAP and TSEA) were correlated with different tracheid traits to varying degrees (Table S3), which was largely in line with findings that climate could strongly control wood anatomy and formation (Pandey 2021). Previous studies on boreal conifers showed that the period of wood formation lengthened linearly with MAT in a range of 14°C and contributed to increased tracheid dimensions for 10 conifer species in the Northern Hemisphere (Rossi et al. 2013). However, our results showed that temperature, precipitation and temperature seasonality all contributed to lumen diameter and tracheid wall dimensions for multiple conifer species across various climate conditions (i.e., temperate, subtropical, and tropical climate) after taking phylogeny into account. In addition, soil properties (i.e., PH, SILT and CLAY) could explain part of the variance in tracheid traits, especially for tracheid diameter in earlywood that did not demonstrate a latitudinal trend (Table 2). A possible explanation for the role of soil on tracheid dimensions was that a high soil fertility could increase the growth rate of conifers, resulting in a larger tracheid diameter and thinner tracheid wall thickness (Bergh et al. 1999). As there are only a few studies on the effect of soil condition on tracheid dimensions compared to growth rate or wood density, how the interaction of soil and climate influences wood anatomy requires further investigation.
There is a widely held idea that earlywood is composed of wide, thin-walled tracheids in contrast to the narrow thick-walled tracheids of latewood. Through a phylogenetic paired t-test, our results showed that tracheid diameter in earlywood was larger than in latewood (Fig. 2 BC, Table S4), which partly supported the latter idea. An explanation could be that the duration of latewood tracheid development was shorter than for earlywood, while a large proportion of carbohydrates produced in the current growing season had been consumed by earlywood cell wall thickening (Rathgeber et al. 2016). Since the principal function of tracheids in earlywood is water transport, in contrast to latewood, while functions more in mechanical support and water storage (Domec and Gartner 2002), the xylem structure presented by the tracheid diameter in early- and latewood were consistent with their functions. A large tracheid size in earlywood results in a large lumen diameter, enabling higher water conductivity but also increasing the risk of hydraulic failure associated with freezing-induced embolism. By contrast, a small tracheid diameter (< 30 µm) is assumed to protect against freezing-induced embolism for conifers at high latitudes or altitudes (Pittermann and Sperry 2003).
Contrary to tracheid diameter, the difference in tracheid wall thickness between earlywood and latewood was insignificant when taking phylogeny into account (Fig. 2 DE, Table S4). A possible explanation for our result was that the difference in tracheid wall thickness between early- and latewood was species-specific and may not always manifest the same pattern for numerous conifers. Although conifer species in boreal forests may have thicker cell walls in latewood compared to earlywood, conifer species in warm and wet regions, e.g., species of Podocarpaceae and Araucariaceae in the Southern Hemisphere, may not behave in the same way (Carlquist 2017). Previous studies also found that tracheid wall thickness in intra-ring cells was surprisingly constant for some Pinaceae species in temperate regions (Cuny et al. 2014). However, the majority of conifer species are distributed throughout subtropical climates in our study (Fig. 1), which may explain the insignificant differences in tracheid wall thickness between early- and latewood. From a functional point of view, mechanical strength enhancement of latewood tracheids was achieved not by cell wall thickness but due to the increased wall-to-range ratio from reduced tracheid diameter, especially in the radial direction (Pittermann et al. 2006).
Xylem structure is a canvas for the evolutionary strategies of opposing functions through the production of two kinds of tracheid cells each year (Carlquist, 2017). Previous studies on conifer species from Pinaceae (Martínez-Vilalta et al. 2004), Cupressaceae (Pittermann et al. 2012), Araucariaceae (Zimmer et al. 2015), and Podocarpaceae (Turner and Cernusak 2011), suggested that there are at least three evolutionary directions for conifer xylem adapting to different stresses in a geological time scale: cold adaptation, drought adaptation and shade adaptation (Fig. 6). During the evolution of conifers, selection acted to optimize xylem structure to fulfill both safety and efficiency of water transport (Hacke 2015). Conifers must adapt to diverse and often difficult environments by adjusting their xylem structure at different levels from micro-to-macro, e.g., the pit structure at the pit level (Bouche et al. 2014) through to tracheid dimensions at the cell level (Pittermann et al. 2006), and perhaps the amount of ray parenchyma at the tissue level (Olano et al. 2013). There are some hypotheses suggesting that coordinated traits combined - not one trait alone - determines the safety/efficiency requirements and ultimately the species distribution (Hacke 2015; Carlquist 2017). Recent physiological and anatomical studies on bordered pits provide new insights into the mechanisms and evolution of conifer’s drought resistance (Choat et al. 2008; Hacke 2015). However, the eco-physiological functions and evolution behind early- and latewood tracheids has been overlooked. We postulated that tracheid dimensional differences between early- and latewood have evolved to allow conifers to adapt to various environmental conditions, especially freezing conditions and drought.
Resistance to embolism is a crucial trait in trees to cope with cold and drought stresses, which further determines the geographic distribution of conifer species (Choat et al. 2012). Embolism resistance might be influenced by the structure and function of bordered pits and tracheids in xylem as well (Losso et al. 2018). A small alteration in xylem anatomy can lead to different performances in terms of water transport efficiency, embolism resistance and hydraulic capacitance (Hacke 2015). Our phylogenetic analyses provide some evidence for functional optimization of xylem structures for species from different clades (Fig. 4, Figure S3). However, due to a currently incomplete understanding of embolism in tracheids, it is unclear whether anatomical traits such as tracheid dimensions, pit structure and amount of axial parenchyma (Olano et al. 2013), evolved in parallel during conifer evolution, making it an interesting topic for further study.