Water status and development of tropical trees during seasonal drought

Summary

Bud break, shoot growth and flowering of trees involve cell expansion, known to be inhibited by moderate water deficits. In apparent contradiction to physiological theory, many trees flower or exchange leaves during the 6 month-long, severe dry season in the tropical dry forest of Guanacaste, Costa Rica. To explore this paradox, changes in tree water status during the dry season were monitored in numerous trees. Water potential of stem tissues (Ψ_stem) was obtained by a modification of the pressure chamber technique, in which xylem tension was released by cutting defoliated branch samples at both ends. During the early dry season twigs bearing old, senescent leaves generally had a low leaf water potential (Ψ_leaf), while Ψ_stem varied with water availability. At dry sites, Ψ_stem was very low in hardwood trees (<−4 MPa), but near saturation (>−0.2 MPa) in lightwood trees storing water with osmotic potentials between −0.8 and −2.1 MPa. At moist sites trees bearing old leaves rehydrated during drought; their Ψ_stem increased from low values (<−3 MPa) to near saturation, resulting in differences of 3–4 MPa between Ψ_stem and Ψ_leaf. Indirect evidence indicates that rehydration resulted from osmotic adjustment of stem tissues and improved water availability due to extension of roots into moist subsoil layers. In confirmation of physiological theory, elimination of xylem tension by leaf shedding and establishment of a high solute content and high Ψ_stem were prerequisites for flowering and bud break during drought.