Leaf phenology of woody species in a North Australian tropical savanna

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Journal Article
Ecology, 1997, 78 (8), pp. 2542 - 2558
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Leaf phenology was monitored for 49 woody species (trees and tall shrubs) each month over a 2.5-year period in a humid, wet-dry tropical eucalypt savanna at Solar Village, near Darwin, Australia. In the 10 most common species, which spanned the range of phenological types, phenology was monitored every two weeks. To investigate the relationships between leaf phenology and plant water status, pre-dawn leaf water potential was monitored in eight common species every 4-6 weeks. Four main phenological types were described: (1) evergreen species, which retained full canopy throughout the year; (2) brevi- or partly deciduous species, in which the amount of canopy fell significantly, but briefly, during at least one dry season during the study period, but to levels not 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 dry season, and remained leafless for at least one month. Of these 49 species, 24% were evergreen, 20% were brevideciduous, 29% were semideciduous, and 27% were fully deciduous. Leaf fall occurred 1-2 months earlier in the dry season for the fully deciduous species than for the semideciduous species. Leaf fall in all species was coincident with the attainment of seasonal minima in leaf water potential, which were, on average, about -1.5 to -2.0 MPa in the evergreen and semideciduous species, compared with about -0.5 to -1.0 MPa in the fully deciduous species. Leaf flushing occurred throughout the dry season in the two evergreen species, with a major peak in the late dry season. In five semideciduous species and one of the fully deciduous species, leaf flushing commenced in the late dry season prior to the occurrence of any rain. In two fully deciduous species, leaf flushing occurred only after the first 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-dawn leaf water potential. Soil moisture at 1 m did not fall below permanent wilting point during the dry season; hence, reserves of soil water at the end of the dry season were sufficient to support the whole-plant rehydration that preceded leaf flushing in the absence of rain. These results are consistent with hypotheses, developed in the neotropics, that leaf phenology in trees from the wet-dry tropics is largely controlled by endogenous mechanisms.
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