Low resistance to cavitation and xylem anatomy partly explain the decrease in the endemic Rhamnus ludovici-salvatoris

The low recruitment success of some endemic species under Mediterranean environmental conditions is primarily the result of a low ability to create an efficient (highly conductive) and safe (able to maintain an intact water column under negative pressure) hydraulic system. Consequently, their lower resistance to drought-induced cavitation may induce mortality in seedlings and, during summer droughts, in mature individuals. In this study, the hydraulic safety (water potential at 50% loss of conductivity, P50), hydraulic efficiency (specific conductivity, Ks) and xylem anatomy were compared between Rhamnus ludovicisalvatoris, an endemic species of the Balearic Islands whose distribution area is being reduced, and two populations of Rhamnus alaternus, which is widely distributed along the Mediterranean basin. R. ludovicisalvatoris was found to be more susceptible to drought-induced cavitation and less efficient at conducting water in comparison with R. alaternus. Moreover, R. ludovici-salvatoris demonstrated a lower vessel area, wood density and inter-vessel wall strength than R. alaternus. These results are in accordance with the lower ability of R. ludovici-salvatoris to recruit seedlings under Mediterranean conditions in comparison with R. alaternus; this may partially explain the reduction in the distribution area of R. ludovici-salvatoris. (C) 2017 Elsevier GmbH. All rights reserved.