Variation of xylem traits reveals evidence of adaptation to climatic conditions in conifers along a latitudinal gradient across China

Extant conifer species are adapted to a range of climate conditions, which would be reflected in their xylem structure, especially in tracheid characteristics of early-and-latewood. With an anatomical dataset of 79 conifer species native to China, an interspecific study was conducted within a phylogenetic context to find latitudinal patterns in tracheid cell dimensions in response to climate. The analyses showed that there is a significant difference in tracheid length and diameter between early-and-latewood, but no significant difference in cell wall thickness. An opposite divergence pattern was found based on the PC1 axis of tracheid traits, with species of Pinaceae showing strong divergence in their xylem structure in contrast to a conservative xylem structure for species of the other families studied. Except for tracheid diameter in earlywood, tracheid characteristics decreased as latitude increased. Mean annual temperature, precipitation, and their interaction could explain 24.7% to 49.2% of the variance of the tracheid features measured, while phylogeny accounted for 12.5 to 50.5%, suggesting that both temperature and precipitation play a major role in shaping conifer xylem structure. These results provide valuable insight into the effect of climate on the xylem structure of conifers, helping to further our understanding of trees’ adaptation to climate.