WATER AS A DETERMINANT OF THE DISTRIBUTION OF TREES AT THE BOUNDARY OF THE MEDITERRANEAN ZONE

1 At the boundary of the Mediterranean zone in southern France different woodland communities occur on north-facing and south-facing slopes. 2 The regional climate is transitional between that of the Mediterranean zone with a well-marked summer drought and that of the cooler and moister mountains. 3 Summer-green deciduous woodland with many species characteristic of central Europe (e.g. Fraxinus excelsior and Tilia cordata) occurs on steep north-facing slopes and in ravines. 4 Woodland dominated by the evergreen Quercus ilex with many Mediterranean species (e.g. Cistus salvifolius and Lavandula stoechas) occurs on south-facing slopes. 5 Preliminary studies in England show that Tilia cordata wilts, sheds leaves and has reduced growth when water potential of the shoots falls to - 2.1 MPa. Quercus ilex shows no injury when water potential of the shoots falls to - 3.5 MPa. 6 In the Haut Vallespir conditions on north-facing slopes result in the water potential of shoots of T. cordata not falling below a mean value of - 1.5 MPa during the day and recovering to - 0.4 MPa by dawn. There are no signs of drought injury. 7 In contrast the water potential of shoots of Q. ilex on south-facing slopes falls to - 3.0 MPa during the day and the extent of recovery by dawn is variable. 8 Stomatal resistance rises during the afternoon in T. cordata but remains low (5.0 s cm-1) in Q. ilex. 9 Growth in height and stem diameter of T. cordata is severely reduced by drought. 10 The results demonstrate the primary importance of water supply in controlling both the small- and large-scale pattern of distribution of T. cordata near its southern limit in Europe