Raymond Froend

Smaller, faster stomata: scaling of stomatal size, rate of response, and stomatal conductance

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.

A functional methodology for determining the groundwater regime needed to maintain the health of groundwater-dependent vegetation

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.

Loss and degradation of wetlands in southwestern Australia: underlying causes, consequences and solutions

Many wetlands (estimated to be about 70%) have been lost in the coastal plain region of southwestern Australia since British settlement (in 1829), primarily as a result of infilling or drainage to create land for agricultural use or urban development. While further loss is almost universally acknowledged as undesirable, wetland degradation continues with little overt public recognition of the causes or consequences. Obvious and direct causes include nutrient enrichment, salinization, pollution with pesticides and heavy metals, the invasion of exotic flora and flora, loss of fringing vegetation and altered hydrological regimes occurring as a result of urbanization and agricultural practices. Underlying causes include a lack of understanding of wetland hydrology and ecology on behalf of both planning agencies and the private sector, and poor coordination of the many different agencies responsible for wetland management. Public and political awareness of wetland values continues to increase, but sectoral organization and responsibilities for wetland management lag behind. Sufficient scientific information now exists for improved management, protection and restoration of wetlands in southwestern Australia. However, this improvement cannot occur without the necessary political will and corresponding sectoral responses needed to implement coordinated wetland management policies and actions.

Groundwater-dependent ecosystems : the where, what and why of GDEs

Until the early 1970s, the management of water resources in Australia was predominantly concerned with the assessment, development and harnessing of new water resources for irrigation, urban and industrial, stock and domestic water supply. The consequences of excessive and unsympathetic groundwater abstraction on groundwaterdependent (phreatophytic) vegetation, such as tree decline and mortality, have been observed throughout Australia (Arrowsmith 1996; Hatton and Evans 1998; Clifton and Evans 2001). With increasing demand for water and a changing climate regime, the need to mitigate the environmental impacts of groundwater development is increasing. Current borefield operation in Australia is largely responsive to consumption demand and often in conflict with environmental needs for groundwater, resulting in drought stress and sometimes death of phreatophytic vegetation and other impacts on GDEs. Groundwater resource managers commonly ask how much water can be taken from the aquifer while still maintaining a low level of risk to GDEs. This requires quantified information on the relationship between the health of a GDE and groundwater depth (or other parameter; see Eamus et al. 2006a). Recommendations are generally made by defining the acceptable level to which groundwater can be allowed to fall, while maintaining important environmental values (see Murray et al. 2006). The Council of Australian Governments (COAG) endorsed reforms in 1994 to achieve a sustainable water industry that included allocations for the environment

Availability of seed for recruitment of riparian vegetation: a comparison of a tropical and a temperate river ecosystem in Australia

Processes that are important for the recruitment of plants include aspects of the reproductive phenology, development and release of propagules, dispersal of propagules and the storage of mature seed ready for germination when conditions are suitable. This paper explores the relative importance of these mechanisms by examining the contents of the seedbank in the soil, the reproductive phenology of particular overstorey species, the importance of dispersal by water and the survival and longevity of seed on two contrasting rivers in Western Australia. Examination of the soil seedbank showed that regeneration of vegetation from this source is probably important for annual species of herbs and grasses but of only minor significance for perennial species. This is most likely due to high levels of disturbance and the unstable soils in the riparian zone. Reproductive phenology of the four overstorey species monitored in this study appears to be well-adapted to the hydrological regimes on the respective rivers. For the seed of riparian overstorey species examined, seed longevity was poor and seed predation rates were high. The occurrence of seed in floodwater debris indicated the importance of secondary dispersal of seed by water, particularly for the Ord River. For the two overstorey riparian species examined on the Ord River in the subtropical north of Australia, there is little storage of seed and plants are reliant on favourable conditions prevailing at the time of seed fall. The likelihood of seed finding a safe site for successful germination is enhanced by secondary dispersal in high river flows. For overstorey species on the Blackwood River in the temperate zone of south-western Australia there is some storage of seed in the canopy but dispersal of seed to safe sites is also enhanced by river flow. For riparian vegetation on these rivers, regeneration from seed can occur through several processes. The relative importance of these different mechanism varies for each river, reflecting their vastly different hydrology and climate.

What happens when you add salt: predicting impacts of secondary salinisation on shallow aquatic ecosystems by using an alternative-states model

Alternative-states theory commonly applied, for aquatic systems, to shallow lakes that may be dominated alternately by macrophytes and phytoplankton, under clear-water and enriched conditions, respectively, has been used in this study as a basis to define different states that may occur with changes in wetland salinity. Many wetlands of the south-west of Western Australia are threatened by rapidly increasing levels of salinity as well as greater water depths and permanency of water regime. We identified contrasting aquatic vegetation states that were closely associated with different salinities. Salinisation results in the loss of freshwater species of submerged macrophytes and the dominance of a small number of more salt-tolerant species. With increasing salinity, these systems may undergo further change to microbial mat-dominated systems composed mostly of cyanobacteria and halophilic bacteria. The effect of other environmental influences in mediating switches of vegetation was also examined. Colour and turbidity may play important roles at low to intermediate salinities [concentration of total dissolved solids (TDS) 10 000 mg L–1 TDS). The role of nutrients remains largely unquantified in saline systems. We propose that alternative-states theory provides the basis of a conceptual framework for predicting impacts on wetlands affected by secondary salinisation. The ability to recognise and predict a change in state with changes in salinity adds a further tool to decision-making processes. A change in state represents a fundamental change in ecosystem function and may be difficult to reverse. This information is also important for the development of restoration strategies. Further work is required to better understand the influence of temporal variation in salinity on vegetation states and probable hysteresis effects.

Water stress vulnerability of four Banksia species in contrasting ecohydrological habitats on the Gnangara Mound, Western Australia.

This study investigated the interspecific differences in vulnerability to xylem embolism of four phreatophytes - two facultative phreatophytes (Banksia attenuata and B. menziesii) and two obligate phreatophytes (B. ilicifolia and B. littoralis). Species differences at the same position along an ecohydrological gradient on the Gnangara Groundwater Mound, Western Australia were determined in addition to intraspecific differences to water stress between populations in contrasting ecohydrological habitats. Stem- and leaf-specific hydraulic conductivity, as well as Huber values (ratio of stem to leaf area), were also determined to support these findings. We found that where water is readily accessible, there were no interspecific differences in vulnerability to water stress. In contrast both facultative phreatophyte species were more resistant to xylem embolism at the more xeric dune crest site than at the wetter bottom slope site. B. ilicifolia did not differ in vulnerability to embolism, supporting its classification as an obligate phreatophyte. Other measured hydraulic traits (K(S), K(L) and Huber value) showed no adaptive responses, although there was a tendency for plants at the wetter site to have higher K(S) and K(L). This study highlights the influence site hydrological attributes can have on plant hydraulic architecture across species and environmental gradients.

Variability in flood disturbance and the impact on riparian tree recruitment in two contrasting river systems

The vegetation within the riparian zone performs animportant ecological function for in-stream processes.In Australia, riparian zones are regarded as the mostdegraded natural resource zone due to disturbancessuch as river regulation and livestock grazing. Thisstudy looks at factors influencing vegetation dynamicsof riparian tree species on two contrasting riversystems in Western Australia. The Blackwood River insouth-western Australia is influenced by aMediterranean type climate with regular seasonalwinter flows. The Ord River in north-western Australiais characterized by low winter base flows andepisodic, extreme flows influenced by monsoon rains inthe summer. For both rivers, reproductive phenology ofstudied overstory species is timed to coincide withseasonal river hydrology and rainfall. An evendistribution of size classes of trees on the BlackwoodRiver indicated recruitment into the population iscontinual and related to the regular predictableseasonal river flows and rainfall. In contrast, on theOrd River tree size class distribution was clustered,indicating episodic recruitment. On both rivers treeestablishment is also influenced by elevation abovethe river, microtopography, moisture status and soiltype. In terms of vegetation dynamics riparianvegetation on the Ord River consists of long periodsof transition with short lived stable states incontrast to the Blackwood river where tree populationstructure is characterized by long periods of stablestates with short transitions.

Defining phreatophyte response to reduced water availability: preliminary investigations on the use of xylem cavitation vulnerability in Banksia woodland species

The consideration of phreatophyte response to changes in water availability is important in identifying ecological water requirements in water-resource planning. Although much is known about water-source partitioning and intra- and interspecific variability in groundwater use by Banksia woodland species, little is known about the response of these species to groundwater draw-down. This paper describes a preliminary study into the use of xylem cavitation vulnerability as a measure of species response to reduced water availability. A response function and critical range in percentage loss of conductance is identified for four Banksia woodland overstorey species. Similarity in the vulnerability curves of B. attenuata R.Br. and B. menziesii R.Br. at low tensions supports the notion that they occupy a similar ecohydrological niche, as defined by their broad distributions relative to depth to groundwater. B. ilicifolia R.Br., however, as an obligate phreatophyte, has a range restricted to environments of higher water availability and shallower depth to groundwater and this is reflected in greater vulnerability to cavitation (relative to other Banksia) at lower tensions. The wetland tree Melaleuca preissiana Schauer generally expressed a greater vulnerability at any given xylem water potential (Ψx). This paper identifies the range in Ψx within which there is an elevated risk of tree mortality, and represents a first step towards quantifying the critical thresholds in the response of Banksia woodland species to reduced water availability.

Dynamics of phreatophyte root growth relative to a seasonally fluctuating water table in a Mediterranean-type environment

While seasonal redistribution of fine root biomass in response to fluctuations in groundwater level is often inferred in phreatophytic plants, few studies have observed the in situ growth dynamics of deep roots relative to those near the surface. We investigated the root growth dynamics of two Banksia species accessing a seasonally dynamic water table and hypothesized that root growth phenology varied with depth, i.e. root growth closest to the water table would be influenced by water table dynamics rather than surface micro-climate. Root in-growth bags were used to observe the dynamics of root growth at different soil depths and above-ground growth was also assessed to identify whole-plant growth phenology. Root growth at shallow depths was found to be in synchrony with above-ground growth phenophases, following increases in ambient temperature and soil water content. In contrast, root growth at depth was either constant or suppressed by saturation. Root growth above the water table and within the capillary fringe occurred in all seasons, corresponding with consistent water availability and aerobic conditions. However, at the water table, a seasonal cycle of root elongation with drawdown in summer followed by trimming in response to water table rise and saturation in winter, was observed. The ability to grow roots year-round at the capillary fringe and redistribute fine root biomass in response to groundwater drawdown is considered critical in allowing phreatophytes, in seasonally water-limited environments, to maintain access to groundwater throughout the year.