Trees are the fundamental element of forest ecosystems, made possible by their mechanical qualities and their highly sophisticated conductive tissues. The evolution of trees, and thereby the evolution of forests, were ecologically transformative and affected climate and biogeochemical cycles fundamentally. Trees also offer a substantial amount of ecological niches for other organisms, such as epiphytes, creating a vast amount of habitats. During land plant evolution, a variety of different tree constructions evolved and their constructional principles are a subject of ongoing research. Understanding the “natural construction” of trees benefits strongly from methods and approaches from physics and engineering. Plant water transport is a good example for the ongoing demand for interdisciplinary efforts to unravel form-function relationships on vastly differing scales. Identification of the unique mechanism of water long-distance transport requires a solid basis of interfacial physics and thermodynamics. Studying tree functions by using theoretical approaches is, however, not a one-sided affair: The complex interrelationships between traits, functionality, trade-offs and phylogeny inspire engineers, physicists and architects until today.