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Tree water use in two contiguous communities (eucalypt open-forest and Melaleuca paperbark forest) was measured in tropical Australia, over a 2-year period. The aims of the study were to ( 1) quantify daily and seasonal patterns of water use in each community, ( 2) compare patterns of water use among the communities and ( 3) compare relationships among tree size, sapwood area and water use within the two contrasting vegetation communities. Access to deep soil water stores and the effect of run-on from the eucalypt forest resulted in a relatively high pre-dawn water potential throughout the year, particularly for Melaleuca forest. There were no differences in daily rates of water use, expressed on a sapwood area (Q(s)) basis, between the two eucalypt species examined (Eucalyptus miniata Cunn. Ex Schauer and E. tetrodonta F.Muell) at any time in the eucalypt forest. For both the eucalypt and Melaleuca forests, there was less seasonal variation in water use expressed on a leaf area (Q(l)) basis than on a Q(s) basis, and neither year nor season were significant factors in Q(1). In the mono-specific Melaleuca forest, Q(s) was not significantly different between years or seasons. Water use on a Q(l) basis was similarly not significantly different between years or seasons in the Melaleuca forest. Leaf area index (LAI) of the eucalypt forest was about half of that of the Melaleuca forest throughout the year but sapwood area per hectare was 33% larger in the eucalypt than the Melaleuca forest, despite the basal area of the Melaleuca forest being almost double that of the eucalypt forest. There was no significant difference in stand water use (mm day(-1)) between eucalypt and Melaleuca forests during 1998; however, in 1999 Melaleuca stand water use was larger than that of the eucalypt forest. Because of the enhanced dry-season availability of water in the Melaleuca forest and its larger LAI, average annual water use of the Melaleuca forest was almost 60% larger than that of t

This book is not a textbook on Australian ecology. The purpose of this text is to reveal and discuss the links between vegetation function and water in landscapes - that is, to discuss ecohydrology.We focus primarily (but not exclusively) on the interactions among the woody components of vegetation, rainfall and changes in groundwater availability.Woody vegetation is the focus because of the centrality of changes in woody vegetation cover to the ecohydrology of Australia over the past 100 to 200 years. Furthermore we focus on vegetation function (ecophysiology) rather than structure, because it is the functioning of vegetation that influences hydrology inthe first instance. Although this book uses Australian examples,the principles, philosophy and methodological approach are applicable worldwide.

Deciduous trees of Australia's northern savannas typically have less-negative leaf water potentials than evergreen species and their stomata are more sensitive to soil drought than those of evergreen species. This paper presents the first investigation of the role of xylem sap pH and abscisic acid content in explaining stomatal behaviour of Australian trees in the field. We measured stomatal conductance, leaf-to-air vapour pressure difference (D) and leaf water potential, xylem abscisic acid (ABA) concentration and xylem sap pH of evergreen, semi-deciduous and fully deciduous tree species in the field over a 15-month period. Measurements were made during both the wet and the dry seasons. Stomata closed in response to increasing D in both evergreen and deciduous species and were equally sensitive to increasing D or declining leaf water potential. Xylem ABA concentration increased with declining leaf water potential in evergreen and semi-deciduous species, but not deciduous species. Similarly, there was an inverse correlation between stomatal conductance and xylem ABA concentration. Xylem sap pH increased as leaf water potential declined from wet to dry season for evergreen and semi-deciduous species but not for deciduous species. Deciduous species had less-negative water potentials and lower xylem ABA concentrations than evergreen species or semi-deciduous species. We conclude that changes in xylem sap pH and ABA content do occur seasonally in the wet-dry tropics of Australia and that these changes influence stomatal conductance, but only in evergreen and semi-deciduous species. Deciduous species do not appear to modulate either of these chemical signals.

Seasonal trends in pre-dawn leaf water potential and morning and afternoon rates of light-saturated assimilation and stomatal conductance were studied in saplings of the deciduous tree Terminalia ferdinandiana Excell. Mean daily maximum assimilation rates ranged from 11 µmol m-2 s-1 in the wet season to 8 µmol m-2 s-1 during the transition from the wet to the dry season. Saplings were without leaves from June to October inclusive (dry season). There was a log–linear relationship between stomatal conductance and pre-dawn leaf water potential (r = 0.76, n = 325), and a weak linear relationship between daily maximum assimilation and pre-dawn leaf water potential (r = 0.39, n = 184). Assimilation rates were higher in the morning than in the afternoon in April and May, but were similar throughout the day from December to March. Seasonal trends in assimilation could be attributed primarily to stomatal closure, but diurnal differences could not. High leaf temperatures may have been responsible for observed lower assimilation rates in the afternoon in April and May. Assimilation and stomatal conductance decreased when leaf temperatures rose above 38˚C and/or the leaf-to-air vapour pressure difference exceeded 4–4.5 kPa. Pre-dawn leaf water potentials decreased more quickly, and stomatal conductance was more sensitive to this decrease, in T. ferdinandiana saplings than in saplings of Eucalyptus tetrodonta F.Muell. a co-occurring evergreen tree. Specific leaf area and assimilation per unit dry weight were higher in T. ferdinandiana than in E. tetrodonta which is consistent with other studies of costs and benefits of deciduousness.

Seasonal and diurnal trends in carbon assimilation, stomatal conductance and leaf water potential were studied using 1–3 m tall saplings of Eucalyptus tetrodonta (F.Muell.). The study site was in an unburnt savanna near Darwin, where rainfall is strongly seasonal. Mean daily maximum assimilation rates ranged from 14.5 µmol m-2 s-1 in May to 4.8 µmol m-2 s-1 in October. There was a linear relationship between daily maximum assimilation rates and pre-dawn leaf water potential (r = 0.62, n = 508) and a log–log linear relationship between daily maximum stomatal conductance and pre-dawn leaf water potential (r = 0.68, n = 508). Assimilation rates and stomatal conductance were always higher in the morning than in the afternoon, irrespective of season. Stomatal conductance responded more strongly to leaf-to-air vapour pressure difference when pre-dawn leaf water potentials were moderately low (–0.5 to –1.5 MPa) than when they were very low (< –1.5 MPa) or high (> –0.5 MPa). Assimilation decreased sharply when temperature exceeded 35˚C. Seasonal trends in assimilation rate could be attributed primarily to stomatal closure, but diurnal trends could not. High leaf temperatures were a major cause of lower assimilation rates in the afternoon. Approximately 90% of leaves were lost by the end of the dry season, and above-ground growth was very slow. It is hypothesised that E. tetrodonta saplings allocate most photosynthate to root and lignotuber growth in order to tolerate seasonal drought and the high frequency of fire in northern Australian savannas.

The seasonal variation in leaf xylem pressure potential at dawn (cdawn), 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, cdawn 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, p0 = –2.15 MPa) than the fully deciduous species (p0 = –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 potent

The wet–dry tropics of northern Australia are characterised by extreme seasonal variation in rainfall and atmospheric vapour pressure deficit, although temperatures are relatively constant throughout the year.This seasonal variation is associated with marked changes in tree canopy cover, although the exact determinants of these changes are complex. This paper reports variation in microclimate (temperature, vapour pressure deficit (VPD)), rainfall, soil moisture, understorey light environment (total daily irradiance), and pre-dawn leaf water potential of eight dominant tree species in an area of savanna near Darwin, Northern Territory, Australia. Patterns of canopy cover are strongly influenced by both soil moisture and VPD. Increases in canopy cover coincide with decreases in VPD, and occur prior to increases in soil moisture that occur with the onset of wet season rains. Decreases in canopy cover coincide with decreases in soil moisture following the cessation of wet season rains and associated increases in VPD. Patterns of pre-dawn water potential vary significantly between species and between leaf phenological guilds. Pre-dawn water potential increases with decreasing VPD towards the end of the dry season prior to any increases in soil moisture. Decline in pre-dawn water potential coincides with both decreasing soil moisture and increasing VPD at the end of the dry season. This study emphasises the importance of the annual transition between the dry season and the wet season, a period of 1–2 months of relatively low VPD but little or no effective rainfall, preceded by a 4–6 month dry season of no rainfall and high VPD. This period is accompanied by markedly increased canopy cover, and significant increases in pre-dawn water potential, which are demonstrably independent of rainfall. This finding emphasises the importance of VPD as a determinant of physiological and phenological processes in Australian savannas.

Allosyncarpia ternata S.T.Blake (Myrtaceae) is an evergreen tree, restricted largely to rocky habitats on the Arnhem Land Plateau in the wet–dry tropics of northern Australia. Allosyncarpia ternata grows in a wide range of habitats, including sites near permanent springs, where it forms a distinctive closed-canopy forest with an understorey of rainforest plants, and sites on exposed cliffs and hilltops, where it occurs in open forest and woodland. Leaf water relations differ markedly between these contrasting sites. During the dry season, trees at open sites show strong diurnal hysteresis in stomatal conductance (gs); afternoon depressions in gs coincide with regular afternoon increases in vapour pressure deficit. Pressure–volume analyses indicate that A. ternata maintains turgor down to leaf water potential values of about –2.8 MPa, close to the minimum experienced by hilltop leaves late in the dry season. By contrast, trees on the ravine floor, with year-round access to water, exhibit much smaller diurnal and seasonal variation in stomatal conductance and little seasonal variation in leaf water potential. It is concluded that this flexible response in leaf water relations to seasonally dry conditions is partly responsible for the ability of A. ternata to occupy and dominate the vegetation in such a wide variety of habitats. The near confinement of the species to the Arnhem Land Plateau is in part due to the water-holding capacity of the bedrock.

The Symposium ‘Plant Ecophysiology: Linking Pattern and Process’ was held as part of the 1995 meeting of the Ecological Society of Australia (ESA). The aim of the Symposium was to highlight work that examined mechanisms underlying ecological patterns and linked them to ecological and/or evolutionary processes. Another aim was to expose ecologists to the methods available to examine the mechanistic and functional basis of the organisms and systems under study. Much early ecological research has been concerned with the description and classification of vegetation types, with relatively little effort devoted to understanding the underlying processes that determined distribution. A more quantitative approach based on knowledge of the underlying mechanisms can further improve understanding of systems. This was amply demonstrated in a Symposium on the effects of elevated atmospheric CO2 on vegetation dynamics, also held in conjunction with an ESA meeting (see papers in Australian Journal of Botany, Volume 40(2)). Recent technological advances have stimulated rapid progress in the field of ecophysiology and hence an increasing process-based understanding is developing. The 1995 Symposium was seen as an opportunity to highlight more recent work in what is a relatively new field in Australia (albeit a well-established field in Europe and America), especially in situ studies and research from relatively little studied areas like northern Australia. The response to the Symposium was encouraging, with 25 spoken papers and poster-papers presented. In this paper, some of the unifying aspects of the papers presented in the symposium are drawn together, and placed in the context of likely future developments in ecophysiology in Australia. Based on this analysis, future directions and gaps are identified.