case study 18.2  Swamp paperbark: a coloniser of flooded, saline wetlands

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M. Denton, G. Ganf and B.J. Atwell

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Figure 1 A dense stand of swamp paperbark (Melaleuca halmaturorum var. halmaturorum F Muell. Ex Mig.) showing trees growing in Bool Lagoon, South Australia, together with other hydrophytes. All root systems are totally inundated. (Photograph courtesy Department of Environment, Heritage and Aboriginal Affairs, South Australia)

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Figure 2 Mean shoot biomass of two-year-old Melaleuca halmaturorum grown in totally waterloggeed soil (white), waterlogged soil with shoots 50% (green) or 100% (black) submerged. (Based on Denton and Ganf 1994)

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Figure 3 Scanning electron micrograph of a root of Melaleuca halmaturorum after roots and the lower half of shoots had been flooded for 14 weeks. Extensive aerenchyma have formed through breakdown of cortical cell layers. Scale bar = 100 µm (Micrograph courtesy M. Denton)

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Figure 4 Adventitious roots (arrowed) on plants described in Figure 3. These roots formed when only roots were inundated, not when the entire shoot was flooded (Photograph courtesy M. Denton)

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Figure 5 Deuterium enrichment (δ2H) of plant water (straight lines indicate mean values) and soil water in February (late summer) and October (late spring). The point at which the deuterium signature of plant water corresponds to that of soil water (vertical lines cross the depth profile) indicates at which depth roots are currently sourcing transpired water (Based on data from L. Mensforth)

Melaleuca halmaturorum var. halmaturorum F. Muell. ex Miq., commonly referred to as swamp paperbark, forms impressive dense woodlands around permanent and ephemeral wetlands in southeastern Australia (Figure 1). Interdunal areas fed by discharge water and a permanent wetland (Bool Lagoon) provide environments where the interactive effects of waterlogging and salinity on swamp paperbark have been investigated.

Bool Lagoon has stands of mature trees but poor recruitment of new plants led to concerns that regulation of water levels might be needed to stimulate establishment of new seedlings (Denton and Ganf 1994). Adaptations to flooding in mature plants do not always manifest themselves in juvenile growth stages, when well-aerated soil is sometimes necessary for survival. To test this, swamp paperbark juveniles up to two years old were flooded for as much as 14 weeks by adding sufficient water to inundate roots and cover either the lower 50% of stems or the entire shoot. Leaves died on flooded stems, demonstrating physiological damage probably caused by O2 deficits. However, plants returned to well-drained conditions often produced new leaf initials, showing that the woody stems survived flooding better than foliage.

Total submergence killed at least 70% of plants and dramatically impaired growth of the few survivors (Figure 2), suggesting that O2 transport from aerial parts to roots was essential for seedling survival. Development of lacunae able to sustain internal gas transport (aerenchyma) in roots of plants half-submerged for 14 weeks (Figure 3) is powerful evidence for transport of O2 as a factor in survival. Plants with access to atmospheric O2 grew slowly in spite of waterlogged soil conditions (Figure 2) and regenerated when drained. Roots also responded morphologically to waterlogging, generating superficial adventitious laterals (Figure 4) able to exploit O2 in water circulating above the soil surface.

Ontogeny was a critical factor in flood tolerance: four-month-old plants succumbed to submergence within six weeks while one-year-old and two-year-old plants had increasing levels of tolerance. Lowering water levels in the lagoon was therefore implemented to encourage establishment of swamp paperbark seedlings while considering the potential impact on flood-dependent species such as Triglochin procerum. This strategy relieved waterlogging but reduced runoff and increased evaporation, exacerbating another common hazard for wetland plants, salinity. Salt levels rose nine-fold. Con-sequences of this salt can be severe for young plants but mature swamp paperbark have mechanisms to tolerate salinity.

Swamp paperbark growing in an interdunal swamp adapt to saline groundwater by modifying their pattern of root development and thus water extraction. Ratios of stable isotopes of hydrogen (1H v. 2H) and oxygen (16O v. 18O) provide a powerful technique for estimating patterns of water extraction from soils with complex hydrology. Isotope discrimination has been applied to swamp paperbark where winter rainfall recharges surface soil with fresh water bearing a distinctive isotope signature. Meanwhile, subsoil discharge of saline water provides an alternative source of water. The small proportion of water in which deuterium (2H) replaces hydrogen is measured in soil water and sap from twigs to show which position in the soil water profile matches the deuterium signature (δ2H) in sap.

In February, sap water matched most closely soil water from 40 cm below, whereas in October, sap was composed largely of water extracted from surface soil (Figure 5). That is, winter rains provided a fresh water recharge that roots could exploit but in summer deeper groundwater had to be exploited as a source of water. Very saline groundwater (c. 60 dS m–1) was therefore not taken up by paperbarks when non-saline water was present. In summer, evaporation raised surface salt levels as roots tapped progressively deeper sources.

Lateral roots proliferated where water extraction was occurring fastest (Figure 6). For example, up to July most new laterals grew below 30 cm but by September abundant root growth at the surface corresponded with extraction of surface water. In this environment, waterlogging and salinity impose selection pressures on roots interactively. Fine surface roots late in winter would, for example, improve uptake of non-saline water and improve root activity by access to better water with better O2 status. The swamp paperbark adapts to waterlogged, saline habitats with a range of anatomical and morphological modifications, particularly in roots.

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Figure 6 Appearance of new roots through a season showing proliferation of roots when groundwater rises (water depth in meters shown at bottom of each graph) roots initiate in progressively deeper soil as winter approaches (February to July), followed by a burst of surface root growth after winter rains recharge surface water levels (Based on Mensforth and Walker 1996)

References

Denton, M. and Ganf, G.G. (1994). ‘Response of juvenile Melaleuca halmaturorum to flooding: management, implications for a seasonal wetland, Bool Lagoon, South Australia’, Australian Journal of Marine Freshwater Research, 45, 1395–1408.

Mensforth, L.J. and Walker, G.R. (1996). ‘Root dynamics of Melaleuca halmaturorum in response to fluctuating saline groundwater’, Plant and Soil, 184, 75–84.

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