LEAF PHENOLOGY OF WOODY SPECIES IN A NORTH AUSTRALIAN TROPICAL SAVANNA

Leaf phenology was monitored for 49 woody species (trees and tall shru bs) each month over a 2.5-year period in a humid, wet-dry tropical euc alypt savanna at Solar Village, near Darwin, Australia. In the 10 most common species, which spanned the range of phenological types, phenol ogy was monitored every two weeks. To investigate the relationships be tween leaf phenology and plant water status, pre-dawn leaf water poten tial was monitored in eight common species every 4-6 weeks, Four main phenological types were described: (1) evergreen species, which retain ed full canopy throughout the year; (2) brevi- or partly deciduous spe cies, in which the amount of canopy fell significantly, but briefly, d uring at least one dry season during the study period, but to levels n ot below 50% of full canopy; (3) semideciduous species in which canopy fell to below 50% of full canopy in each of the dry seasons; and (4) fully deciduous species, which lost all leaves during the early-mid dr y season, and remained leafless for at least one month. Of these 49 sp ecies, 24% were evergreen, 20% were brevideciduous, 29% were semidecid uous, and 27% were fully deciduous. Leaf fall occurred 1-2 months earl ier in the dry season for the fully deciduous species than for the sem ideciduous species. Leaf fall in all species was coincident with the a ttainment of seasonal minima in leaf water potential, which were, on a verage, about -1.5 to -2.0 MPa in the evergreen and semideciduous spec ies, compared with about -0.5 to -1.0 MPa in the fully deciduous speci es. Leaf flushing occurred throughout the dry season in the two evergr een species, with a major peak in the late dry season. In five semidec iduous species and one of the fully deciduous species, leaf flushing c ommenced in the late dry season prior to the occurrence of any rain. I n two fully deciduous species, leaf flushing occurred only after the f irst storms of the early wet season. There was variation in the timing of flushing, both between species within years and between years for some species. However, all species commenced leaf flushing after water potentials rose, following the attainment of seasonal minima in pre-d awn leaf water potential. Soil moisture at 1 m did not fall below perm anent wilting point during the dry season; hence, reserves of soil wat er at the end of the dry season were sufficient to support the whole-p lant rehydration that preceded leaf flushing in the absence of rain. T hese results are consistent with hypotheses, developed in the neotropi cs, that leaf phenology in trees from the wet-dry tropics is largely c ontrolled by endogenous mechanisms.