How wood evolves: a new synthesis

Recent advances in wood physiology, molecular phylogeny, and ultrastructure (chiefly scanning electron microscopy, SEM), as well as important new knowledge in traditional fields, provide the basis for a new vision of how wood evolves. Woody angiosperms have, in the main, shifted from conductive safety to conductive efficiency (with many variations and modifications) and from ability to resist cavitation (low vulnerability) to ability to refill vessels. The invention of the vessel was a kind of dimorphism (vessel elements plus tracheids) that permitted division of labor and many kinds of wood repatterning that suit conductive safety–efficiency trade-offs. Angiosperms were primarily adapted to mesic habitats but were not failures or “unstable.” They have survived to the present in such habitats well, along with older structural adaptations (e.g., the scalariform perforation plate) that are still suited to such habitats. These “primitive” features are evident in earlier branchings of phylogenetic trees based on multiple genes. Older features may still be functional and thus persist, although newer formulations are overriding in effect. There are, however, numerous instances of “breakouts” in a number of clades (ecological iterations and bursts of speciation and diversification related to new ways of dealing with water economy), whereas in other branchings, other clades show ecological stasis over long periods of time. Newer physiological and anatomical mechanisms have permitted entry into habitats with marked fluctuation in moisture availability. Wood evolves progressively, and literal character state reversal may be unusual: genomic and developmental information holds answers to these changes. Wood is a complex tissue, and each of the histological components shows polymorphism as an evolutionary mechanism. Cell types within wood evolve collaboratively. Shifts in wood features (e.g., simplification of the scalariform perforation plate) are commonly homoplastic. Manifold changes in habit and in leaf physiology, morphology, and anatomy accompany wood evolution, and wood should be studied with relationship to real-world ecology, information that cannot be gleaned from literature or other secondary sources. Heterochrony (protracted juvenilism, accelerated adulthood) characterizes angiosperm xylem extensively, far more so than in other vascular plants, and these mechanisms have resulted in many remarkable changes (e.g., monocots have permanently juvenile xylem, woody trees represent accelerated adulthood). Understanding the many successful features of angiosperm wood evolution must ultimately rest on syntheses.