Vessel diameter polymorphism determines vulnerability-to-embolism curve shape

Sherwin Carlquist proposed several functional interpretations for vessel diameter, including that narrow vessels were hydraulically “safe” while wide diameter vessels were more vulnerable to embolism. He applied these ideas both across species and biomes, and within xylem tissue, where function of growth ring types was inferred from vessel diameter polymorphisms (occurrence of differing vessel diameter classes within a tissue). Following on Carlquist’s ideas, we were interested in evaluating if vessel diameter polymorphism could be linked to vulnerability-to-embolism curve shape. Vulnerability curves were fit with a model that included a continuous shape term (alpha). We predicted that high variation in vessel diameter would result in lower alpha (more r-shaped) curves whereas low variation would produce higher alpha (more s-shaped) curves, with potential for intermediate curve types. Our findings support Carlquist’s functional interpretations of vessel diameter. Vessel diameter polymorphism was strongly correlated to vulnerability curve shape. Homogeneous vessel diameters were associated with more s-shaped curves. As vessel diameter polymorphism increased, alpha declined and curves became more linear. High polymorphism was associated with r-shaped curves. There was no relationship between vessel length and curve shape. Vessel diameter was strongly correlated with a common estimate of embolism resistance (P50), with wider diameter vessels associated with increased vulnerability. Vulnerability curves show enormous variety in shape and scale, and both parameters are likely critical in understanding and predicting plant function. Carlquist’s ecological anatomy data, predictions, and functional inferences will continue to be valuable as we expand our understanding of structure–function links in plant anatomy.