Water status and development of tropical trees during seasonal drought

Abstract

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 (tlJstem) 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 (Wleaf), while tIJstem varied with water availability. At dry sites, Wstem 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 Wstem increased from low values (<-3 MPa) to near saturation, resulting in differences of 3-4 MPa between Wstem and ~dleaf. 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 ~Fstem were prerequisites for flowering and bud break during drought.

Environmental drivers of leaf phenology in trees of the tropical species Ficus obtusifolia

“…pattern of leaf and flower/fruit phenology: The individuals of Ficus obtusifolia showed a brevi-deciduous behaviour, one of the four functional ecotypes identified in tropical forests based on vegetative phenology (details in Borchert, 1994a). This behaviour was observed in the entire study area, leaf fall being generally massive, and flushing almost immediate and synchronous in each tree, in spite of possible differences in soil water availability (season, watertable height and irrigation) and, in the case of the individuals at IBE, at a high leaf water status.…”

“…Vegetative as well as reproductive phenology of tropical trees frequently occurs synchronously at the end of the dry season (Borchert, 1994a). Leaf age, water deficit and changes in day length (DL) have been related to the control of vegetative phenology and partially explain leaf abscission in tropical trees (Borchert et al, 2002;Munné-Bosch and Alegre, 2004).…”

“…The long-term observation of many forest species in Costa Rica (ca. 10º N) indicates that leaf fall and bud emergence take place during the driest and warmest part of the year, coinciding with the spring equinox (Borchert, 1994a;Borchert and Rivera, 2001). …”

Deciduousness in a seasonal tropical forest in western Thailand: interannual and intraspecific variation in timing, duration and environmental cues

“…Kramer and Kozlowski 1979;Valentine 1983;Lechowicz 1984). In tropical forests, climatic seasonality is primarily a function of intra-annual variation in water availability (Borchert 1994a(Borchert , 1994b(Borchert , 1994cReich et al 2004). In the seasonal or monsoonal tropical forests, where rainfall seasonality is most pronounced, this has led to a great diversity of leaf exchange patterns-trees may produce new leaves during the wet season, the dry season, or the transition between seasons; trees may lose all of their leaves, some of their leaves, or none of their leaves; and these patterns may vary within and among individuals and species and from year to year.…”

“…Given sufficient reserves, the timing of leaf expansion may then depend upon the appropriate environmental cue, such as rainfall (Walter 1971;Lieberman 1982;Reich and Borchert 1984;Borchert 1994b), temperature (Walter 1971;Morellato et al 2000) or photoperiod Borchert et al 2005;Elliott et al 2006). Many phenological studies of seasonal tropical forests have focused on rainfall as a proximate cue for leaf flushing because of the rate-limiting threshold imposed by water availability (Reich and Borchert 1984;Borchert 1994aBorchert , 1994bBorchert , 1994c. However, the results have been equivocal.…”