Survival and ecophysiology of tree seedlings during El Nino drought in a tropical moist forest in Panama

In tropical forests, severe droughts caused by El Nino events may strongly influence the water relations of tree seedlings and thereby increase their mortality. Data on known-aged seedlings of three common shade-tolerant canopy tree species (Trichilia tuberculata, Tetragastris panamensis and Quararibea asterolepis) in a Panamanian moist forest are presented. Seedling survival during a severe El Nino dry season (1997-98) was compared with prior long-term survival data, and levels of drought stress were assessed by measuring plant water potentials and gas exchange characteristics. Contrary to prediction, dry-season seedling survival was not dramatically reduced in any species compared with that expected in 'normal' years. In Trichilia and Quararibea, pre-dawn water potentials averaged -2 MPa and midday water potentials about -3 MPa. Stomatal conductances were very low, averaging 26 mmol m(-2) s(-1) for Tetragastris and 11-13 mmol m(-2) s(-1) for Trichilia and Quararibea. Photosynthetic rates also were very low but consistently positive, averaging 0.8-1.1 mumol m(-2) s(-1). The findings suggest that, once established, seedlings of common tree species in this semi-deciduous forest may be tolerant of drought events.