2. Evolutionary outcome of xylem structure
Previous studies of conifer species from Pinaceae, Cupressaceae, and Podocarpaceae suggest there are at least three evolutionary directions for conifers adapting to different stresses at a geological time scale: cold adaptation, drought adaptation, and shade adaptation (Appendix 5). Species of Pinaceae possessed the most evolutionary advanced structural type of pit membrane (Bauch, Liese, and Schultze 1972; Zhou, Jiang, and Zhang 1990), and were generally drought vulnerable but cold tolerant (Martínez-Vilalta, Sala, and Piñol 2004). Cupressaceae spp. possessed drought tolerant strategies (Pittermanna et al. 2015), whereas Podocarpaceae species were thought to have evolved in response to light competition, not drought or cold (Brodibb 2011). Considering the distribution of conifers, boreal forests mainly consist of Pinaceae species that must cope with freeze-thaw cycles, while some species of Cupressaceae are found in very arid regions or tree lines with extreme drought. Podocarpaceae are generally growing in humid tropical/subtropical regions with many angiosperm counterparts. Taken together, the xylem structure of Pinaceae species could have evolved mainly in response to freezing temperature, while species of other families were more conserved in this respect but might be more diverged in other xylem properties, i.e., pit structure or parenchyma (Braun 1984).
Small alterations in xylem anatomy can lead to a different performance in terms of water transportation, embolism resistance and capacitance (Hacke 2015). Our phylogenetic analyses provide some evidence for such functional changes between species (Fig. 4). However, owing to a currently incomplete understanding of embolism formation in tracheids, it is unclear whether tracheid characteristics, pit dimension and pit membrane morphology, and other morphological traits such as leaf shape, evolved in parallel during conifer evolution, but it is certainly an interesting topic to study further.