The seasonal variation in leaf xylem pressure potential at dawn
(ψdawn), 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, ψdawn 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
(gmax) was maximal in the wet and
decreased during the dry season. Diurnally,
(gmax 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
(gmax (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
gmax (> 400 mmol
m-2 s-1) throughout the dry
season, suggesting it had access to groundwater. For each species,
gmax declined with decreasing dawn
water potential in a log-linear manner; the slope of this relationship tended
to increase with increasing degree of deciduousness.