Stem water potential and leaf conductance:: A comparison of Sorbus aucuparia and Sambucus nigra

Sorbus aucuparia is a pioneer wood which can naturally be found on extreme habitats, e.g. rocky slopes and mountain forests. Sambucus nigra is a species of floodplain forests. The different strategies of survival of these two species were studied on planted shrubs which were exposed to the same microclimatic conditions in the outdoors of the Botanical Garden of Dusseldorf University. This situation - both species in direct neighbourhood - also exists where Sorbus aucuparia and Sambucus nigra are brought together by man in common habitats like hegerows. Measurements of stem water potential and leaf conductance were carried out on these planted shrubs during the growing season of 1995 under natural conditions. A hot dry period caused a decrease of stem water potential of Sorbus aucuparia below -4 MPa, while Sambucus nigra merely reached a minimum of -1.64 MPa. Sambucus had a lower maximum of leaf conductance than Sorbus and avoided a decrease of stem water potential by reducing leaf conductances early after the beginning of drought. By contrast Sorbus aucuparia showed high leaf conductances during one month of drought until water potentials of the shrub had been fallen below -4 MPa. These contrasting patterns of seasonal variations of water potential and stomatal control of transpiration confirm the classification of Sorbus aucuparia as a hydrolabile and Sambucus nigra as a hydrostabile species. (C) 1998 Elsevier Science Ltd. All rights reserved.