Seasonal Variation in Water Relations of Trees of Differing Leaf Phenology in a Wet–Dry Tropical Savanna near Darwin, Northern Australia

Abstract

The seasonal variation in leaf xylem pressure potential at dawn (ψ_dawn), leaf tissue water characteristics and daily maximum leaf conductance was measured in eight woody species in a wet–dry tropical savanna near Darwin, northern Australia, between October 1992 and October 1993. The species were Eucalyptus miniata, E. tetrodonta, E. clavigera, Xanthostemon paradoxus, Erythrophleum chlorostachys, Planchonia careya, Terminalia ferdinandiana and Cochlospermum fraseri. The species represented the major leaf phenological types, evergreen, semi-deciduous and fully deciduous. The climate of the region is characterised by annual drought during the winter months, when virtually no rain falls and vapour pressure deficit (VPD) in the afternoon reaches 3 kPa for 5 consecutive months each year. Despite this drought, ψ_dawn remained high (–1.3 to –1.5 MPa in evergreen species and –0.5 to –1.5 MPa in deciduous species) relative to those trees that experience summer drought in temperate and arid Australia. There was a tendency for evergreen and semi-deciduous species to maintain positive turgor to lower xylem pressure potentials (mean osmotic potential at incipient plasmolysis, π₀ = –2.15 MPa) than the fully deciduous species (π₀ = –2.03 MPa). For all species, the daily maximum leaf conductance (g_max) was maximal 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 during the dry season and the ‘buildup’, the transitional period between the dry and wet seasons. There was substantial decrease in (g_max (from 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 species Eucalyptus miniata, by contrast, had high g_max (> 400 mmol m^-2 s^-1) throughout the dry season, suggesting it had access to groundwater. For each species, g_max declined with decreasing dawn water potential in a log-linear manner; the slope of this relationship tended to increase with increasing degree of deciduousness.