Physiological responses of Fagus sylvatica L. seedlings under Pinus sylvestris L. and Quercus pyrenaica Willd. overstories

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 (psi(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 (Psipi(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,,) 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 mumol m(-2) s(-1) for A(max) and 263 +/- 12 and 185 +/- 6 mmol m(-2) s(-1) for Differences in gas exchange between sites were more evident in 1997, as soil moisture and Psi(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. (C) 2002 Elsevier Science B.V. All rights reserved.