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.