Physiological responses of Fagus sylvatica L. seedlings under Pinus sylvestris L. and Quercus pyrenaica Willd. overstories Ismael Aranda, Luis Gil, Jose Alberto Pardos * Unidad de Anatomı ´a, Fisiologı ´a y Gene ´tica Forestal, Escuela Te ´cnica Superior, de Ingenieros de Montes, Universidad Polite ´cnica de Madrid, Ciudad Universitaria s/n, 28040, Madrid, Spain Received 23 May 2000; received in revised form 15 December 2000; accepted 15 January 2001 Abstract Water status and gas exchange of beech seedlings, planted in a scots pine stand (Pinus sylvestris L.) and a melojo-oak stand (Quercus pyrenaica Willd.), were monitored during two growing seasons. In 1996, a very dry year, minimum predawn water potential (C pd  S:E:) of beech seedlings was, respectively, 1:88  0:13 and 1:11  0:07 MPa under the melojo oak and pine canopies; these values revealed higher water stress in the oakwood. Also a lower osmotic potential at full turgor (Cp full ) was measured under this canopy (1:61  0:1 MPa versus 1:37  0:05 MPa in the pinewood) at the end of summer in both years. The higher osmotic adjustment that beech seedlings showed below the oak canopy was not enough to overcome the water stress they suffered at the end of summer 1996. The net photosynthesis (A)—photosynthetic photon flux density (PPFD) relationship reflected the acclimatization of beech seedlings to a brighter environment under the oak canopy. It brought about higher A and water vapor stomatal conductance (g wv ) for a given PPFD level, but this response was only observed in dates with high water availability. Maximum values in the oak and pine stands were, respectively, 7:76  0:17 and 4:62  0:27 mmol m 2 s 1 for A max and 263  12 and 185  6 mmol m 2 s 1 for g wvmax . Differences in gas exchange between sites were more evident in 1997, as soil moisture and C pd measurements were higher than in 1996 because a more rainy summer. This fact allowed the maintenance of higher gas exchange in beech seedlings growing under oakwood linked to their higher photosynthetic capacity. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Seedlings; Fagus sylvatica L.; Water potential; Gas exchange 1. Introduction In the Mediterranean basin, natural beech (Fagus sylvatica L.) stands are confined to the mountains, where altitude tempers the usual scarce summer rainfall (Thie ´baut, 1984; Peters, 1997). Even so, in the southern border of the tree’s range, the dry conditions of some locations at the end of summer strongly influence its physiological behavior (Aranda, 1998) and endanger seedlings survival (Aguirre et al., 1991). Although it is well known that light plays an important role for beech seedling survival and growth (Thie ´baut et al., 1992; Minotta and Simone, 1996; Welander and Ottosson, 1998a,b), water availability Forest Ecology and Management 162 (2002) 153–164 * Corresponding author. Tel.: þ34-1-3367113; fax: þ34-1-5439557. E-mail addresses: iaranda@montes.upm.es (I. Aranda), jpardos@montes.upm.es (J.A. Pardos). 0378-1127/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0378-1127(01)00502-3