Hydraulic properties of European elms: xylem safety-efficiency tradeoff and species distribution in the Iberian Peninsula

Ulmus minor and U. glabra show a trade-off between safety and efficiency in water transport, and U. laevis shows adaptations to waterlogged environments. Three native elm species grow in Europe: Ulmus minor Mill., U. glabra Huds. and U. laevis Pall., and within the Iberian Peninsula their habitats mainly differ in water availability. We evaluated firstly whether vulnerability to xylem embolism caused by water-stress has been a determinant factor affecting their distribution; secondly, if their xylem anatomy differs due to water availability dissimilarities; and thirdly, if these species present a trade-off between water transport safety and efficiency. Plants of the three species were grown in a common-garden in Madrid, Central Spain. The centrifuge method was used for constructing the vulnerability curves, and anatomical measurements were carried out with an optical microscope. We found clear differences in conductivity and cavitation vulnerability between the three species. Although all three elms were highly vulnerable to cavitation, U. minor was significantly more resistant to water stress cavitation. This species reached 50 % loss in conductivity at -1.1 MPa, compared to U. glabra that did so at -0.5 MPa, and U. laevis at -0.4 MPa. Maximum xylem specific conductivity and maximum leaf specific conductivity were two to three times higher in U. glabra when compared to U. minor. A clear trade-off between safety against losses of conductivity and water transport efficiency was observed considering both U. minor and U. glabra samples. Ulmus minor's hydraulic configuration was better adapted to overcome drought episodes. The expected aridification of the Iberian Peninsula could compromise Ulmus populations due to their high vulnerability to drought stress.