Carlquist’s indices for vulnerability and mesomorphy of wood: are they relevant today?

Sherwin Carlquist established xylem vessel indices and parameters to quantify the degree of mesomorphy or xeromorphy exhibited by plants. These indices were developed as part of efforts to establish a quantitative approach to plant anatomy and the developing fields of functional and ecological anatomy. In this paper, we discuss the origin of such parameters and their merits and demerits in light of current theory and practice. Vessel diameter, vessel element length, and vessel density (vessels/mm2) remain relevant anatomical characters that describe and quantify plant function and ecology. From a functional perspective, mean diameter can be replaced by hydraulic mean diameter (dh), inspired by the Hagen–Poiseuille Law. Vessel density is presumably linked to hydraulic safety through redundancy and embolism resistance and is an essential feature of xeromorphic woods that tend to have many narrow vessels. Although vessel element length strongly correlates to xeromorphy, the reasons for the link between element length and xeromorphy are unclear. The use of anatomical indices, such as those developed and proposed by Carlquist, helped shape our understanding of plant hydraulic strategies and will continue to be important as we connect plant anatomy to plant function.