Differences in plant and soil water relations in and around a forest gap in West Africa during the dry season may influence seedling establishment and survival.

Soil and plant water relations were studied in a canopy opening and surrounding forest shade in a moist tropical forest in Ghana, using seedlings of two tree species with different regeneration strategies - the pioneer Terminalia superba and the non-pioneer light demander Entandrophragma utile. During two consecutive dry seasons, soil matric potential varied from -30 kPa at the end of the rainy season, to below -2.5 MPa in the middle of the dry season. During part of the first dry season soil matric potential was higher in the gap than in the surrounding forest. In the second dry season no differences were observed. Leaf water potential varied from about -750 kPa at dawn for unstressed seedlings of both species to <-2.5 MPa when seedlings were permanently wilted. For both species, the maximum stomatal conductance of unstressed seedlings varied from 300 to 450 mmol m-2 s-1 for those in the gap centre to 150 mmol m-2 s-1 for those under forest shade. Maximum conductance decreased to 10-30 mmol m-2 s-1 in drought-stressed seedlings during the middle of the dry season. The two species responded similarly to a drying soil profile but the decrease in leaf water potential and stomatal conductance was more rapid under the forest canopy than in the centre of the gap. Only 7% of the seedlings of T. superba and 4% of E. utile survived the dry season under forest shade. In contrast, 85 and 78%, respectively, survived of those growing in the centre of the forest gap. At all positions, seedlings of T. superba had larger relative height growth rates during the wet season than E. utile (29.6, 21.4 and 8.4% month-1 compared with 5.2, 20.0 and 1.6% month-1, for gap centre, margin and forest shade, respectively). The observed changes in leaf water potential and stomatal conductance indicated that despite the reduced irradiance, shaded seedlings experienced more drought stress during the dry season than seedlings growing in the forest gap. The reasons for this and possible consequences for trees which differ in leaf phenology and regeneration strategy are discussed.