Elsevier

Aquatic Botany

Volume 86, Issue 3, April 2007, Pages 213-222
Aquatic Botany

Interactive effects of salinity and water depth on the growth of Melaleuca ericifolia Sm. (Swamp paperbark) seedlings

https://doi.org/10.1016/j.aquabot.2006.10.002Get rights and content

Abstract

Melaleuca ericifolia Sm. (Swamp paperbark) is a common tree species in freshwater and brackish wetlands in southern and eastern Australia. The survival of this species in many wetlands is now threatened by increased salinity and inappropriate water regimes. We examined the response of 5-month-old M. ericifolia seedlings to three water depths (exposed, waterlogged and submerged) at three salinities (2, 49 and 60 dS m−1). Increasing water depth at the lowest salinity did not affect survival, but strongly inhibited seedling growth. Total biomass, leaf area and maximum root length were highest in exposed plants, intermediate in waterlogged plants and lowest in submerged plants. Although completely submerged plants survived for 10 weeks at the lowest salinity, they demonstrated negative growth rates and were unable to extend their shoots above the water surface. At the higher salinities, M. ericifolia seedlings were intolerant of waterlogging and submergence: all plants died after 9 weeks at 60 dS m−1. Soil salinities increased over time, and by Week 10, exceeded external water column salinities in both the exposed and waterlogged treatments. In exposed sediment, ∼90% of plants survived for 10 weeks at 60 dS m−1 even though soil salinities reached ∼76 dS m−1. No mortality occurred in the exposed plants at 49 dS m−1, and small but positive relative growth rates were recorded at Week 10. We conclude that at low salinities M. ericifolia seedlings are highly tolerant of sediment waterlogging, but are unlikely to tolerate prolonged submergence. However, at the higher salinities, M. ericifolia seedlings are intolerant of waterlogging and submergence and died rapidly after 5 weeks exposure to this combination of environmental stressors. This research demonstrates that salinity may restrict the range of water regimes tolerated by aquatic plants.

Introduction

Almost all wetlands across Australia have been subject to some form of hydrological modification, ranging from complete drainage or inundation to more subtle changes in the wetting and drying regimes as a result of abstraction for agriculture and domestic supply or changes to catchment hydrology (National Land and Water Resources Audit, 2001). Linked to these hydrological changes is a suite of other threats, the most important of which is likely to be salinisation (Williamson, 1998). In Australia, an estimated 80 important wetlands are already affected by secondary salinisation and this number is likely to rise to 130 by 2050 (National Land and Water Resources Audit, 2001).

Melaleuca ericifolia Sm. (Swamp paperbarks) are common wetland plants across much of Australia (Bird, 1962). Ecologically important Melaleuca-dominated wetlands have been cleared across much of their range (Greenway, 1998), and remaining communities are further threatened by saline intrusions and inappropriate water regimes (Finlayson and Oertzen, 1993).

Dowd Morass State Game Reserve is a large (1540 ha) wetland (38°09′S, 147°12′E) in the Ramsar-listed Gippsland Lakes region of southeast Victoria. It provides an example of a Melaleuca-dominated wetland subjected to inappropriate water regime, having been flooded almost continually since 1973. It is subject also to saline intrusion from the oceanic Gippsland Lakes, resulting in salinity fluctuations ranging from ∼1.3 to ∼27‰ [Department of Sustainability and Environment (DSE), unpublished data recorded between 1991 and 2001]. These values are equivalent to ∼2 and ∼49 dS m−1 using a conversion factor of 0.55 (TPS instruments, Brisbane, Qld.).

The genus Melaleuca is reputedly tolerant to intermittent waterlogging under non-saline conditions (Bell, 1999, Lockhart et al., 1999, Carter et al., 2006). Waterlogging exerts its main effect on plants by reducing the diffusion of oxygen into the soil, resulting in sediment anoxia, which in turn leads to a decline in shoot growth and increased rates of leaf abscission (Kozlowski, 1997). Under waterlogged conditions, root biomass is reduced more than shoot biomass, resulting in a higher shoot-to-root ratio (Kozlowski, 1997). This shift in biomass distribution may compromise the capacity of the roots to meet the transpiration and nutrient demands of the canopy.

Tolerance to waterlogging in Melaleuca may be due to a number of physiological adaptations, including development of aerenchyma (Carter et al., 2006) and the production of adventitious roots (Sena Gomes and Kozlowski, 1980, Lockhart et al., 1999, Carter et al., 2006). Melaleuca quinquenervia seedlings may tolerate complete submersion for short periods through a range of adaptations, including changes in leaf morphology and rapid increases in height (Lockhart et al., 1999). However, growth and survival is reduced in Melaleuca halmaturorum after 6 weeks of submergence (Denton and Ganf, 1994). To our knowledge, the response of M. ericifolia seedlings to submergence has not been examined.

Salinity imposes a water deficit by reducing soil osmotic potential. To maintain turgor, plants must lower their leaf water potential (Taiz and Zeiger, 1991). To minimise water loss, stomatal resistance is often increased. In turn, this can lower rates of photosynthesis and slow growth (Kozlowski, 1997). Over time, salts can accumulate in leaves to toxic levels and cause premature senescence (Kozlowski, 1997). Although both shoot and root growth is reduced by salinity, shoot growth is usually more affected. As a result, and in contrast to the case with waterlogging, shoot-to-root ratio decreases under saline conditions (Munns and Termaat, 1986).

Melaleuca are reported to be highly tolerant to salinity (Bell, 1999, Niknam and McComb, 2000). In a trial of 20 Melaleuca species, Van der Moezel et al. (1991) reported that 82% of non-flooded seedlings survived a salinity of 63 dS m−1. From the distribution pattern of M. ericifolia in the Gippsland Lakes region of Victoria, Bird (1962) suggested the field limit of the species to salinity to be about 25–30‰ (∼45–55 dS m−1).

The responses of Melaleuca plants to waterlogging or submergence are likely to be altered under saline conditions. The ability of most plants to tolerate salinity is compromised when combined with soil hypoxia resulting from waterlogging (Barrett-Lennard, 2003). As the availability of oxygen in the soil is reduced, ion selectivity at the root is impaired and this leads to an increased flux of Na+ and Cl to the shoot, which further increases the rate of leaf senescence (Barrett-Lennard, 2003). Salinity can also impair the ability of a plant to produce aerenchymous roots in response to flooding, compounding oxygen deficits at the root and escalating the flux of Na+ and Cl to the shoot (Barrett-Lennard, 2003).

This study examines the response of M. ericifolia Smith (Swamp paperbark) seedlings to three levels of salinity (2, 49 and 60 dS m−1) at each of three water depths (exposed sediment, waterlogged sediment and submerged). We hypothesised that increasing salinity and water depth would reduce plant growth and survivorship. We predicted that at low salinity, growth would be greatest in exposed plants, followed by waterlogged plants and be lowest in submerged plants. As salinities increased, the response to water depth would shift, with waterlogged and submerged plants demonstrating a greater sensitivity to salinity than exposed plants.

Section snippets

Species description

Melaleuca ericifolia Smith is a small (2–9 m), erect, open to bushy shrub or small tree (Costermans, 1998). It is capable of sexual and vegetative reproduction (Ladiges et al., 1981). It is common in fresh to brackish waters in coastal wetlands across southern and eastern Australia, and can dominate the riparian vegetation to form dense, multi-branched thickets (Bird, 1962).

Salinity and water depth treatments

The response of M. ericifolia seedlings to three water depths was examined across three salinity levels using a partly

Physiochemical properties of the water column and sediments

The salinity of the water column was within 10% of target values across all salinity levels over the duration of the experiment. Soil salinities did not always match that of the water column, and tended to be higher than the water column in the exposed and waterlogged treatments (Table 1). At the end of the experiment (Week 10), soil salinities in the exposed treatment were ∼500%, 43% and 27% higher than the water column salinities in the 2, 49 and 60 dS m−1 salinity treatments, respectively.

Responses to water depth

Increasing water depth at the lowest salinity did not affect plant survival but strongly inhibited plant growth. Total biomass, leaf area and the number of growth points were highest in exposed plants, lower in waterlogged plants and lowest in fully submerged plants. Although waterlogging initially reduced RGR, this effect was not apparent by Week 10. We conclude that there is some degree of adaptation to waterlogging in M. ericifolia seedlings and this may enable them to tolerate extended

Acknowledgments

We would like to thank Leesa Hughes for her excellent technical assistance, Andrew Schultz (Parks Victoria) and Eleisha Keogh (Department of Sustainability and Environment) for technical advice concerning Dowd Morass and Associate Professors Ralph McNally and Gerry Quinn for statistical advice. The following people generously donated their time to help during the two harvests: Julian La Brooy, Suzan Ghantous, Katherine Harrison, David Kerr, Heath Matheson, Ann and Daniella Mayer and family,

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