Both temperature and precipitation play a role in shaping xylem structure
Climatic conditions (i.e., mean annual temperature; MAT and mean annual precipitation; MAP) and their interaction were correlated with different cell traits to various degrees (Table 2). Also, the shifting patterns of cell traits along MAT and MAP gradients had a similar increasing tendency as latitude (not shown). Responses of most latewood cell traits to MAT and MAP gradients were stronger than those of corresponding earlywood cell traits, except that no significant correlation was detected between tangential/radial cell diameter in earlywood and the two climatic variables. Although Rossi et al. (2016) found that the period of wood formation increased linearly with MAT in a range of 14 ℃ for 10 conifer species in cold biomes of the Northern Hemisphere (Rossi et al. 2013; Rossi et al. 2016), our results suggest that both temperature and precipitation contribute to tracheid size and tracheid wall dimensions for multiple conifer species across various climate conditions, i.e., temperate, subtropical, and tropical climate, while taking phylogeny into account. The similar impacts of MAT and MAP on xylem structure possibly lies in the fact that temperature and precipitation are largely synchronic under a monsoonal climate in the eastern part of China (Ding 2013), which is supported by a close correlation between MAT and MAP (r=0.81, p<0.001) (Appendix 4).
We did not find clear evidence for the association between drought and xylem structure using mean annual climatic variables, as MAI was not included in any top models with MAT and MAP as climatic variables. Although seasonal drought does exist in subtropical regions in Southwest China, the annual rainfall does not differ much among these regions (Ding 2013). Furthermore, it was the length of the drought season rather than other climatic variables that determined a plant’s response to climate in these regions (Lian et al. 2020), which could not be presented by the mean annual aridity index used in this study. Finer temporal resolution of climate data, such as weekly or daily data of air temperature and soil water deficit, together with long-term chronological anatomical data would help to address these questions. Given the fast development of image processing techniques and automatic climate and soil monitoring methods, we believe that a combination of information from radial growth, wood density and xylem anatomy will open up a new research area for tree-ring science and allow for the detection of large structural, biogeographical and environmental patterns and their interpretation.