Search

Periods of low water level ('drawdown') may kill mature submersed aquatic plants and their vegetative propagules. Re-establishment from a sediment-based seed bank may be limited by desiccation, high temperatures and increased salinity, environmental changes that may often accompany drawdown. Vallisneria australis (Hydrocharitaceae) is a common, submersed clonal wetland plant in southeastern Australia, which reportedly reproduces mainly by vegetative spread. Although vegetative growth in this species is inhibited by drying and salinity, it is not known how such conditions influence the germination of sediment-stored seed, and so whether re-colonisation from seeds can occur after drawdown. Hence, this study examined whether drying, high temperature or high salinity reduced the germination of V. australis seeds. Germination was slowed by drying, with significant germination still occurring 20-30 weeks after re-wetting. The final percentage germination of seeds that had been dried for 8, 16 or 32 weeks and then rewetted was approximately twice that of seeds that remained wet. Moreover, sediment-stored seeds germinated only after drying, suggesting that drawdown would promote germination of V. australis in the field. Final germination of wet-stored seeds was 44 % at 25 °C but only 7 % at 35 °C, indicating that germination may be inhibited over summer in southeastern Australian wetlands. Although percentage germination declined with increasing salinity, some seeds still germinated (∼10 %) at the highest salinity tested, 23 dS m-1. Variability in the time V. australis seeds take to germinate may represent a mechanism that increases the opportunities for regeneration in an unpredictable environment. The demonstrated tolerance of seeds to drying and salinity suggests that, even in salinised wetlands, V. australis may be able to recolonise from the sediment-based seed bank following the loss of mature plants after a drawdown, provided viable seeds are present.

Essential oils of paperbarks, Melaleuca spp., have been shown in laboratory studies to inhibit bacterial activity and slow the rate of cellulose decay. Field (Gippsland Lakes, south-eastern Victoria, Australia) and glasshouse experiments were conducted to test the hypothesis that leaves of Melaleuca ericifolia SM. (the swamp paperbark) suppressed the decay of leaf litter under conditions existing or mimicking those in natural wetlands. Under field conditions, neither brown nor green M. ericifolia leaves, at loadings that would normally occur in a typical paperbark wetland and over a range of leaf-mass ratios, significantly affected the decay rate of two common freshwater macrophytes, Triglochin procerum (water ribbon) and Phragmites australis (common reed). In contrast, glasshouse experiments showed that Melaleuca ericifolia leaves (both intact and ground, and both brown and green) suppressed decay of T. procerum when placed in moist conditions on the sediment surface. However, no inhibitory effect was observed when leaves were flooded. Purified, commercially available Melaleuca essential oil (extracted from Melaleuca alternifolia leaves) decreased the rate at which T. procerum leaves decayed under glasshouse conditions by as much as 43 %. As with entire leaves, the inhibitory effect of purified Melaleuca essential oil was greatest when the leaves were placed in moist, but not submerged, conditions. Experiments designed to test the possibility that the inhibition was due to a simple, physical (coating) effect rather than metabolic inhibition of microbes showed clearly that the effect was not due simply to the hydrophobic nature of essential oils. This study indicates that M. ericifolia leaf litter in freshwater wetlands probably exerts only a small effect on the rate at which leaves from other wetland plant species decay. Any inhibitory effect is likely to be greater after water levels drop (e.g., seasonally in a Mediterranean climate) and remaining leaf material is left moist on the surface of water logged sediments than when material is totally submerged, as during the wetland's high water phase.

Leaf transport - especially differences among species from diverse taxonomic groups - is generally less well understood than are the other phenomena that influence the fate of leaves in streams, such as conditioning by bacteria and fungi and fragmentation and consumption by invertebrates. To address this topic, we compared the transport behaviour of entire leaves from five indigenous species of tree in a naturally forested, upland stream in south-eastern Australia: two cool temperate rainforest taxa (Nothofagus cunninghamii (HOOK) OERST. and Atherosperma moschatum LABIL.), two sclerophyllous taxa (Acacia melanoxylon R. BR. and Eucalyptus regnans F. MUELL.) and one taxon from the ecotone between the two forest types (Lomatia fraseri R. BR.). Laboratory experiments indicated that, irrespective of flow regime, rainforest leaves sank markedly more slowly than did sclerophyllous leaves. Eucalyptus regnans leaves, for example, in moving water typically sank within 2 days of immersion, whereas Nothofagus cunninghamii leaves in moving water had a mean time before sinking of 8 to 67 days. These differences in flotation behaviour were reflected in field experiments that used marked leaves to quantify transport down a first-order stream in the study area. The field experiments showed the stream to be highly retentive, with leaves from no taxon travelling more than 100m in six hours. Leaves from rainforest taxa, however, were transported longer distances in a given time than were sclerophyllous leaves, and in some of the latter cases (e.g., Acacia melanoxylon) retention commenced in distances as short as 5 m. There was some evidence that small leaves were transported greater distances than were large leaves; leaf texture and flexibility (a reflection of leaf morphology) also influenced transport distance, but the characteristics of the leaf margins seemingly did not. The ecological significance of these findings is that differential transport will influence the relative contribution made by various species to the pool of coarse particular leaf matter occurring in a given reach of a stream. It will also influence the amount of material available to downstream ecosystems and the formation of allochthonous fossil leaf assemblages.

Essential oils are a common component of the leaves of many plants, and may be a factor that, to date, has been overlooked as a determinant of leaf decay in natural environments. Cellulose sheets and dried leaves of Platanus sp. (common plane tree) treated with essential oil extracted from Melaleuca alternifolia CHEEL and buried in soil in laboratory mesocosms decayed at a significantly slower rate than did control materials not treated with the oil. Inhibition of bacterial activity may account in part for this effect on organic matter decay, since Melaleuca essential oil at concentrations of greater than 0.001% v/v markedly inhibited the growth of the bacterium, Escherichia coli. Melaleuca essential oil was lethal to starved bacteria (E. coli, Staphylococcus aureus and Pseudomonas aeruginosa), especially at higher concentrations (>0.I % v/v) of the oil. EC 50 values, determined with the Microtox assay, were c. 0.01 % v/v, a value equivalent to an essential oil concentration of about 100mg/l. Essential oil derived from Melaleuca leaves and twigs may have a significant role in organic matter processing and nutrient cycling in coastal wetlands by slowing the decomposition of organic detritus and inhibiting or killing microbes.