Maximum and minimum stomatal conductance, as well as stomatal size and rate of response, are known to vary widely across plant species, but the functional relationship between these static and dynamic stomatal properties is unknown. The objective of this study was to test three hypotheses: (i) operating stomatal conductance under standard conditions (g op) correlates with minimum stomatal conductance prior to morning light [g min(dawn)]; (ii) stomatal size (S) is negatively correlated with g op and the maximum rate of stomatal opening in response to light, (dg/dt)max; and (iii) g op correlates negatively with instantaneous water-use efficiency (WUE) despite positive correlations with maximum rate of carboxylation (Vc max) and light-saturated rate of electron transport (J max). Using five closely related species of the genus Banksia, the above variables were measured, and it was found that all three hypotheses were supported by the results. Overall, this indicates that leaves built for higher rates of gas exchange have smaller stomata and faster dynamic characteristics. With the aid of a stomatal control model, it is demonstrated that higher g op can potentially expose plants to larger tissue water potential gradients, and that faster stomatal response times can help offset this risk.

In Mediterranean ecosystems vegetation overlying shallow, transient aquifers is often dominated by woody phreatophytes, trees and shrubs that have been shown to be dependent on groundwater for their water requirements. Natural and anthropogenic alterations of groundwater tables (abstraction) are of clear importance to phreatophytic vegetation as reduction of water tables may sever these plants from their natural water sources. Seasonal water sources were determined for species growing on a coastal dune system that overlies a shallow sandy aquifer in south-western Australia. The plants studied grew over groundwater that ranged in depth from 2.5 to 30 m. The naturally occurring stable isotope of hydrogen (deuterium, δH) was used to distinguish potential water sources. Isotopic ratios from vascular water of the dominant species of the study area (Banksia ilicifolia R. Br. and Banksia attenuata R. Br. trees) were compared with those of potential sources of precipitation, soil moisture and groundwater. A relatively shallow-rooted perennial shrub, Hibbertia hypericoides Benth., was also included as an isotopic reference. The results suggest that both B. attenuata and B. ilicifolia are phreatophytic as they derived some of their water from groundwater throughout the dry-wet cycle, with the exception of B. attenuata at the site of greatest depth to groundwater (30 m) which did not use groundwater. A high proportion (>50%) of groundwater use was not maintained throughout all seasons. With the onset of the hot Mediterranean summer, progressive drying of the surface soils resulted in increased use of groundwater and deep soil moisture. During the wet winter plants used proportionately more water from the upper layers of the soil profile. The degree to which groundwater was utilised by the study species was dependent on the proximity of groundwater, availability of moisture in shallower horizons of the soil profile, root system distribution and maximum root depth.

Isohydric and anisohydric regulations of plant water status have been observed over several decades of field, glasshouse and laboratory studies, yet the functional significance and mechanism of both remain obscure. We studied the seasonal trends in plant water status and hydraulic properties in a natural stand of Eucalyptus gomphocephala through cycles of varying environmental moisture (rainfall, groundwater depth, evaporative demand) in order to test for isohydry and to provide physiological information for the mechanistic interpretation of seasonal trends in plant water status. Over a 16 month period of monitoring, spanning two summers, midday leaf water potential ( Y leaf ) correlated with predawn Y leaf , which was correlated with water table depth below ground level, which in turn was correlated with total monthly rainfall. Eucalyptus gomphocephala was therefore not seasonally isohydric. Despite strong stomatal down-regulation of transpiration rate in response to increasing evaporative demand, this was insufficient to prevent midday Y leaf from falling to levels below -2 MPa in the driest month, well into the region likely to induce xylem air embolisms, based on xylem vulnerability curves obtained in the study. However, even though midday Y leaf varied by over 1.2 MPa across seasons, the hydrodynamic (transpiration-induced) water potential gradient from roots to shoots ( D Y plant ), measured as the difference between predawn and midday Y leaf , was relatively constant across seasons, averaging 0.67 MPa. This unusual pattern of hydraulic regulation, referred to here as isohydrodynamic, is explained by a hydromechanical stomatal control model where plant hydraulic conductance is dependent on transpiration rate.

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In the past, the phrase ‘environmental allocations of water’ has most often been taken to mean allocation of water to rivers. However, it is now accepted that groundwater-dependent ecosystems are an important feature of Australian landscapes and require an allocation of water to maintain their persistence in the landscape. However, moving from this theoretical realisation to the provision and implementation of a field-based management regime is extremely difficult. The following four fundamental questions are identified as being central to the effective management of groundwater-dependent ecosystems (GDEs): (1) How do we identify GDEs in the field; put another way, which species or species assemblages or habitats are reliant on a supply of groundwater for their persistence in the landscape; (2) what groundwater regime is required to ensure the persistence of a GDE; (3) how can managers of natural resources (principally water and habitats), with limited time, money and other resources, successfully manage GDEs; and (4) what measures of ecosystem function can be monitored to ensure that management is effective? This paper explicitly addresses these questions and provides a step-by-step theoretical and practical framework for providing answers. In particular, this paper provides an introduction to some of the relevant literature and from this, presents a synthesis, presented in the form of a functional methodology for managing groundwater dependent ecosystems.