Data collection
China is the country with the highest number of conifer species in the
world among which more than 90 species are endemics (Li, Shen, Ying, and
Fang 2009). Meanwhile, the land mass of China covers a large latitudinal
gradient from 18° N to 54 ° N. Besides, although the precipitation is
generally higher in lower latitude regions, seasonal drought is common
in Southwest China, where a hotspot of conifer species is situated (Yang
2015; Sundaram et al. 2019). Such diverse species and environmental
conditions provide an opportunity for a cross-species study on the
correlation between xylem structure and climate. We compiled a tracheid
trait dataset for Chinese conifer species (Cheng Yang, and Liu 1993,
Yang Lu, Liu, and Wu 2001; Jiang Cheng, and Yin 2010). All taxonomic
names were verified using the Plant List (http://www.theplantlist.org/)
to correct for synonyms, and varieties and subspecies were removed since
the study is at the interspecific level. Finally, we obtained wood
anatomical data for 79 conifer species across China (Fig.1). Most tree
samples were taken from the Eastern Monsoonal climate zone in China,
where the temperate, subtropical, and marginal tropical natural forests
are distributed (Wu 1995). The maximum plant height for each species was
extracted from Fu (2012) to check their potential relationships with
conduit size.
Five cell dimensional traits from both earlywood and latewood for the 79
conifer species were selected, including tangential and radial tracheid
diameter, tangential and radial tracheid wall thickness, and tracheid
length (Fig. 2, Appendix 1, and Appendix 2). The anatomical data was
mainly collected from publications by The Chinese Academy of Forestry,
which hosts the largest wood collection in China, and the wood
anatomical traits were measured following their methods (Cheng et al.
1993; Zhou and Jiang 1994; Yang, Cheng, Yang, and Lu 2009). In brief,
most conifer tree species were sampled from mature trees over 60 years
old with a DBH of over 20 cm in natural forests, as previous studies
suggest that tracheid size is consistent in the sapwood of old trees
(Zhou and Jiang 1994). The sampled discs were collected at a height of
1.3m, avoiding compression wood. These discs were cut, from pith to
bark, into six equal parts according to the equidistance method and
anatomical samples were taken from the middle of the outermost part
(closest to the bark). These outermost samples were sectioned with a
microtome and the entire span of a randomly selected growth ring
analysed. Wood anatomical traits were measured using a light microscope,
including tracheid diameter, tracheid wall thickness, and tracheid
length for earlywood and latewood respectively. The early-and-latewood
subzones were chosen as close as possible to the ring boundaries, and
tracheids at the very end of latewood with irregular shapes were
avoided. For tracheid diameter and tracheid wall thickness, 100
tracheids were measured randomly in the earlywood and latewood
subsection of each sample. For tracheid length, 50 tracheids in
earlywood and latewood from a randomly selected growth ring were
measured using the maceration method after wood samples were treated
with glacial acetic acid and hydrogen peroxide for one to three days
(Yang et al. 2001).
As the sampling locations were not recorded for most of the species in
the database, geographical coordinates for the distribution of each
species in China were retrieved from Atlas of the Gymnosperms of China
(Ying, Chen, and Chang 2004) and combined with climatic data extracted
from WorldClim (https://www.worldclim.org/) and the aridity index from
the global aridity index database
(https://cgiarcsi.community/data/global-aridity-and-pet-database/) using
the R package ‘raster’ (Hijmans, 2015). The environmental variables
included: (1) Location indices: mid-point latitude (LAT, °), longitude
(LON, °), and altitude (ALT, m). (2) Annual climatic indices: mean
annual temperature (MAT, ℃), mean annual precipitation (MAP, mm), and
mean annual aridity index (MAI, mm) which is defined as the ratio of
mean annual precipitation and mean annual potential evapotranspiration.
Climatic variables retrieved from different locations were averaged for
each conifer species for further analysis.
Previous studies demonstrated that xylogenesis and the resulting xylem
structure were species-specific (Rathgeber et al. 2016; Chen et al.
2019). Therefore, the effect of phylogeny was taken into account using
various phylogenetic models in this cross-species study. We firstly
constructed a phylogenetic tree for the species using the largest dated
vascular plant phylogeny presently available, the GBOTB mega-tree of
Smith & Brown (2018), which includes 79,881 taxa and all families of
extant vascular plants based on combined molecular data from GenBank and
data from the Open Tree of Life project (Smith and Brown 2018). We then
pruned the supertree using the R package ‘V.PhyloMaker’ (Jin and Qian
2019) to generate a phylogeny for 79 conifer species.