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Responses of Tropical Understory Plants to a Severe Drought: Tolerance and Avoidance of Water Stress 1

Shade‐tolerant understory shrubs and subcanopy trees constitute most of the woody species in Neotropical moist forest, but studies demonstrating physiological differences among these species are few. Shade‐tolerant species that coexist in the forest understory exhibit differences in leaf life span that have been associated with variation in physiological traits. We hypothesized that water relations of understory species with widely divergent leaf life spans differ in response to drought. Although severe drought is infrequent in Neotropical moist forest, we studied the water relations of shade‐tolerant understory species with short or long leaf life spans during the severe 1991‐1992 dry season on Barro Colorado Island, Panama. The predawn leaf water potential declined to ‐2.8 and ‐3.6 MPa during the dry season in Hybanthus prunifolius and Psychotria horizontalis, respectively, two species with short 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, Alms blackiana, decreased early in the dry season, a period during which all three had 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 observed little change in predawn water potential for S. simplex and O. lucens. We conclude that the variable and potentially severe dry season in Neotropical moist forest can be sufficiently intense to severely limit soil moisture availability for understory plants. H. prunifolius and P. horizontalis tolerated dehydration, whereas S. simplex and O. lucens postponed dehydration.