Responses of tropical understory plants to a severe drought: Tolerance andavoidance of water stress

Shade-tolerant understory shrubs and subcanopy trees constitute most of the woody species in Neotropical moist forest, bur studies demonstrating physi ological differences among these species are few. Shade-tolerant species th at coexist in the forest understory exhibit differences in leaf life span t hat have been associated with variation in physiological traits. We hypothe sized that water relations of understory species with widely divergent leaf life spans differ in response to drought. Although severe drought is infre quent in Neotropical moist Forest, we studied the water relations of shade- tolerant understory species with short or long leaf life spans during the s evere 1981-1992 dry season on Barro Colorado Island, Panama. The predawn le af water potential declined to -2.8 and -3.6 MPa during the dry season in H ybanthus prunifolius and Psychotria horizontalis, respectively two species with shore leaf life spans, but remained above -1.3 MPa in two species with long leaf life spans, Swartzia simplex and Ouratea lucens. The midday leaf water potential dropped as low as -3.4 and -4.5 MPa for H. prunifolius and P. horizontalis, respectively. The osmotic potential of H. prunifolius and P. horizontalis and another species with short leaf life span, Alseis blac kiana, decreased early in the dry season, a period during which all three h ad substantially negative predawn water potential. In contrast, the osmotic potential of S. Simplex, O. Lucens, and Licania platypus, a third species with long leaf life span, declined late in the dry season, even though we o bserved little change in predawn water potential for S, simplex and O. luce ns. We conclude that the variable and potentially severe dry season in Neot ropical moist forest can be sufficiently intense to severely limit soil moi sture availability for understory planes. H. prunifolius and P horizontalis tolerated dehydration, whereas S. simplex and O. lucens postponed dehydrat ion.