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.