SEASONAL-VARIATION IN WATER RELATIONS OF TREES OF DIFFERING LEAF PHENOLOGY IN A WET-DRY TROPICAL SAVANNA NEAR DARWIN, NORTHERN AUSTRALIA

The seasonal variation in leaf xylem pressure potential at dawn (psi(d awn)), leaf tissue water characteristics and daily maximum leaf conduc tance was measured in eight woody species in a wet-dry tropical savann a near Darwin, northern Australia, between October 1992 and October 19 93. The species were Eucalyptus miniata, E. tetrodonta, E. clavigera, Xanthostemon paradoxus, Erythrophleum chlorostachys, Planchonia careya , Terminalia ferdinandiana and Cochlospermum fraseri. The species repr esented the major leaf phenological types, evergreen, semi-deciduous a nd fully deciduous. The climate of the region is characterised by annu al drought during the winter months, when virtually no rain falls and vapour pressure deficit (VPD) in the afternoon reaches 3 kPa for 5 con secutive months each year. Despite this drought, psi(dawn) remained hi gh (-1.3 to -1.5 MPa in evergreen species and -0.5 to -1.5 MPa in deci duous species) relative to those trees that experience summer draught in temperate and arid Australia. There was a tendency for evergreen an d semi-deciduous species to maintain positive turgor to lower xylem pr essure potentials (mean osmotic potential at incipient plasmolysis, pi (0) = -2.15 MPa) than the fully deciduous species (pi(0) = -2.03 MPa). For all species, the daily maximum leaf conductance (g(max)) was maxi mal in the wet and decreased during the dry season. Diurnally, g(max) occurred near midday in the wet season, but at about 0800-1000 hours d uring the dry season and the 'buildup', the transitional period betwee n the dry and wet seasons. There was substantial decrease in g(max) (f rom 650-1000 mmol m(-2) s(-1) in March to 200 mmol m(-2) s(-1) in May) early in the dry season in two of the three fully deciduous species ( Planchonia careya and Cochlospermum fraseri). The dominant evergreen s pecies Eucalyptus miniata, by contrast, had high g(max) (> 400 mmol m( -2) s(-1)) throughout the dry season, suggesting it had access to grou ndwater. For each species, g(max) declined with decreasing dawn water potential in a log-linear manner; the slope of this relationship tende d to increase with increasing degree of deciduousness.