▪ Abstract The use of stable isotope techniques in plant ecological research has grown steadily during the past two decades. This trend will continue as investigators realize that stable isotopes can serve as valuable nonradioactive tracers and nondestructive integrators of how plants today and in the past have interacted with and responded to their abiotic and biotic environments. At the center of nearly all plant ecological research which has made use of stable isotope methods are the notions of interactions and the resources that mediate or influence them. Our review, therefore, highlights recent advances in plant ecology that have embraced these notions, particularly at different spatial and temporal scales. Specifically, we review how isotope measurements associated with the critical plant resources carbon, water, and nitrogen have helped deepen our understanding of plant-resource acquisition, plant interactions with other organisms, and the role of plants in ecosystem studies. Where possible we also...

Stable Isotopes in Plant Ecology

Salinization, a widespread threat to the structure and ecological functioning of inland and coastal wetlands, is currently occurring at an unprecedented rate and geographic scale. The causes of salinization are diverse and include alterations to freshwater flows, land-clearance, irrigation, disposal of wastewater effluent, sea level rise, storm surges, and applications of de-icing salts. Climate change and anthropogenic modifications to the hydrologic cycle are expected to further increase the extent and severity of wetland salinization. Salinization alters the fundamental physicochemical nature of the soil-water environment, increasing ionic concentrations and altering chemical equilibria and mineral solubility. Increased concentrations of solutes, especially sulfate, alter the biogeochemical cycling of major elements including carbon, nitrogen, phosphorus, sulfur, iron, and silica. The effects of salinization on wetland biogeochemistry typically include decreased inorganic nitrogen removal (with implica...

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A global perspective on wetland salinization: ecological consequences of a growing threat to freshwater wetlands

https://espace.library.uq.edu.au/view/UQ:7862/R103_Forrester_p.pdf

Mixed-species plantations of Eucalyptus with nitrogen-fixing trees: A review

Planted forests and water in perspective

Physiological regulation of productivity and water use in Eucalyptus: a review

In arid/semi-arid environments, where rainfall is seasonal, highly variable and significantly less than the evaporation rate, groundwater discharge can be a major component of the water and salt balance of a wetland, and hence a major determinant of wetland ecology. Under natural conditions, wetlands in arid/semi-arid zones occasionally experience periods of higher salinity as a consequence of the high evaporative conditions and the variability of inflows which provide dilution and flushing of the stored salt. However, due to the impacts of human population pressure and the associated changes in land use, surface water regulation, and water resource depletion, wetlands in arid/semi-arid environments are now often experiencing extended periods of high salinity. This article reviews the current knowledge of the role that groundwater–surface water (GW–SW) interactions play in the ecology of arid/semi-arid wetlands. The key findings of the review are as follows: 

1.GW–SW interactions in wetlands are highly dynamic, both temporally and spatially. Groundwater that is low in salinity has a beneficial impact on wetland ecology which can be diminished in dry periods when groundwater levels, and hence, inflows to wetlands are reduced or even cease. Conversely, if groundwater is saline, and inflows increase due to raised groundwater levels caused by factors such as land use change and river regulation, then this may have a detrimental impact on the ecology of a wetland and its surrounding areas.
2.GW–SW interactions in wetlands are mostly controlled by factors such as differences in head between the wetland surface water and groundwater, the local geomorphology of the wetland (in particular, the texture and chemistry of the wetland bed and banks), and the wetland and groundwater flow geometry. The GW–SW regime can be broadly classified into three types of flow regimes: (i) recharge—wetland loses surface water to the underlying aquifer; (ii) discharge—wetland gains water from the underlying aquifer; or (iii) flow-through—wetland gains water from the groundwater in some locations and loses it in others. However, it is important to note that individual wetlands may temporally change from one type to another depending on how the surface water levels in the wetland and the underlying groundwater levels change over time in response to climate, land use, and management.
3.The salinity in wetlands of arid/semi-arid environments will vary naturally due to high evaporative conditions, sporadic rainfall, groundwater inflows, and freshening after rains or floods. However, wetlands are often at particular risk of secondary salinity because their generally lower elevation in the landscape exposes them to increased saline groundwater inflows caused by rising water tables. Terminal wetlands are potentially at higher risk than flow-through systems as there is no salt removal mechanism.
4.Secondary salinity can impact on wetland biota through changes in both salinity and water regime, which result from the hydrological and hydrogeological changes associated with secondary salinity. Whilst there have been some detailed studies of these interactions for some Australian riparian tree species, the combined effects on aquatic biodiversity are only just beginning to be elucidated, and are therefore, a future research need.
5.Rainfall/flow-pulses, which are a well-recognized control on ecological function in arid/semi-arid areas, also play an important, though indirect, role through their impact on wetland salinity. Freshwater pulses can be the primary means by which salt stored in both the water column and the underlying sediments are flushed from wetlands. Conversely, increased runoff is also a commonly observed consequence of secondary salinity, and so, wetlands can experience increased surface water inflows that are higher in salinity than under natural conditions. Moreover, changes in rainfall/flow-pulse regimes can have a significant impact on wetland GW–SW interactions. It is possible that in some instances groundwater inflow to a wetland may become so heavy that it could become a major component of the water balance, and hence, mask the role of natural pulsing regimes. However, if the groundwater is low in salinity, this may provide an ecological benefit in arid/semi-arid areas by assisting in maintaining water in wetlands that become aquatic refugia between flow-pulses.
6.There has been almost no modelling of GW–SW interactions in arid/semi-arid wetlands with respect to water fluxes, let alone salinity or ecology. There is a clear need to develop modelling capabilities for the movement of salt to, from, and within wetlands to provide temporal predictions of wetland salinity which can be used to assess ecosystem outcomes.
7.There has been a concerted effort in Australia to collect and collate data on the salinity tolerance/sensitivity of freshwater aquatic biota and riparian vegetation. There are many shortcomings and knowledge gaps in these data, a fact recognized by many of the authors of this work. Particularly notable is that there is very little time-series data, which is a serious issue because wetland salinities are often highly temporally variable. There is also a concern that many of the data are from very controlled laboratory experiments, which may not represent the highly variable and unpredictable conditions experienced in the field. In light of these, and many other shortcomings identified, our view is that the data currently available are a useful guide but must be used with some caution. Copyright © 2008 John Wiley & Sons, Ltd.

http://www.clw.csiro.au/publications/science/2006/sr53-06.pdf

A review of groundwater–surface water interactions in arid/semi‐arid wetlands and the consequences of salinity for wetland ecology

There is a need to generalize water use behavior of eucalypts to facilitate bioengineering and landscape remediation programs in a wide range of Australian environments. A critical question can be stated as a null hypothesis: tree water use per unit leaf area (leaf efficiency) is independent of eucalypt species. This is implicitly equivalent to the hydrological equilibrium hypothesis that states that leaf area is a function of climate, at least in cases where transpiration and growth are limited by soil water. Failure to reject this null hypothesis simplifies (a) the selection of tree species for water balance management, (b) the generation of regional-scale expectations of leaf area index, and (c) the estimation (monitoring) of the effectiveness of plantations in controlling site water balance. The hypothesis was tested with tree water use data collected in natural multi-species stands across Australia, including sites in the wet-dry season tropical woodlands of the Northern Territory, the Mediterranean climate forests of Western Australia, and a woodland system in southern New South Wales receiving an even distribution of rainfall throughout the year. We also tested the hypothesis in a multi-species tree plantation growing on a saline gradient. In each case, we could not reject the hypothesis of constant leaf efficiency among eucalypts. In every case there was a common, strong, linear relationship among tree leaf area and mean daily water use by all tree species in a sample. Single factor (species) analysis of variance did not detect significant differences between leaf water efficiencies of species. For the jarrah forest (Eucalyptus marginata J. Donn ex Sm., E. calophylla R. Br. ex Lindl.), the null hypothesis held in both spring (wet) and autumn (dry) conditions. The null hypothesis held in the mixed species woodland of New South Wales (E. macrorhynca F.J. Muell. ex Benth., E. blakelyi Maiden., E. polyanthemos Schauer.) under summer and autumn conditions, and across five species in the wet-dry tropical woodland (E. miniata A. Cunn. ex Schauer, E. tetrodonta F.J. Muell., E. porecta S.T. Blake, Erythrophleum chlorostachys F.J. Muell., and Terminalia ferdinandiana Exell.). The null hypothesis also held for a plantation of E. occidentalis Endl. and provenances of E. camaldulensis Dehnh. growing on a shallow saline gradient; i.e., leaf water efficiency remained constant across species and varieties despite obvious effects of salinity on the size of individual canopies. We conclude that there is little evidence for rejecting the hypothesis that leaf efficiency does not vary significantly among sympatric eucalypt species in rainfall-limited (soil-water-limited) systems. These findings open the way for useful bioengineering generalities about the hydrological role of trees in the Australian landscape.

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Does leaf water efficiency vary among eucalypts in water-limited environments?

Over the last ten years, there has been a large increase in the number of vegetation studies that have incorporated measurements of the stable isotopic composition of water. There are many methods for measuring the amount of water being used by plants, but until recently it has been difficult to determine from where plants obtained their water. This has been particularly difficult where there is more than one available water source (e.g. where groundwater is shallow or streams are nearby).

The Use of Stable Isotopes of Water for Determining Sources of Water for Plant Transpiration

This study investigated the surface water–groundwater interactions of three semi-arid floodplain wetlands of the lower River Murray (SE Australia) using a combination of hydrometric, natural tracer and geophysical methods. The current management objective for these wetlands is to mimic the natural surface water flow regime by engineering wetting and drying cycles for the benefit of the biota. However, the impact this will have on groundwater processes and wetland salinization is unknown. This study found that when inundated, two of the wetlands were groundwater recharge features, whereas the other was a groundwater throughflow system. After these wetlands were dried, there was a reversal of the hydraulic gradients and all three wetlands became groundwater discharge features. The transformation of these wetlands to groundwater discharge features after the removal of surface water means that there is an increased risk of salinization when wetting and drying cycles are reintroduced. In arid/semi-arid regions of the world where wetlands are in direct contact with saline groundwater, extreme caution should be applied when altering the management of the surface hydrology of the wetlands because increases in salinity can impact upon biodiversity. Copyright © 2009 John Wiley & Sons, Ltd.

Kerryn McEwan

Papers

A review of groundwater–surface water interactions in arid/semi‐arid wetlands and the consequences of salinity for wetland ecology

Does leaf water efficiency vary among eucalypts in water-limited environments?

The Use of Stable Isotopes of Water for Determining Sources of Water for Plant Transpiration

Salinization risk in semi-arid floodplain wetlands subjected to engineered wetting and drying cycles

Managing groundwater and surface water for native terrestrial vegetation health in saline areas