Estimating the Water Requirements for Plants of Floodplain Wetlands

floodplains survive there because they have evolved strategies for lowwater use.In contrast, aquatic and wetland plants rarely have water-conservingstrategies. Submerged plants, for example, have leaves rarely more thana few cells thick and virtually no protective outer cuticle, so theydehydrate rapidly in the sun. Wetland plants with aerial leaves, such asemergent and floating-leafed macrophytes, have a thick or waxy cuticleon their leaves but transpire readily.Most aquatic plants ‘ride out’ the dry inter-flood periods by strategiesother than physiological adaptations. One strategy that is typical ofannuals is avoidance, exemplified by short life-span and setting seeds,hence reliance on the seed-bank. Another strategy, typical of perennials,is to enter a low-activity or no-growth phase. In this, water loss isrestricted by leaf-shedding, or by complete canopy senescence and dieback.The plant survives with its sensitive generative tissues buried inthe sediment, or as hard-coated seeds or some other propagules.Aquatic plants have a wider range of propagules than do terrestrialspecies, with rhizomes, corms, tubers, turions, spores and nodalfragments as well as seeds.The long-term survival of these propagules depends on being buried inprotective sediments and on the sediments being protected fromdisintegration by trampling or machinery. In general, water-conservingstrategies are better developed in shrub and tree species occurring onthe infrequently-flooded parts of floodplains.Adaptations, tolerance and stressDifferences in adaptation to resource availability means species arefound in particular sequences, ie. at different positions on theenvironmental gradient from ‘flooded’ to ‘dry’. This is evident inzonation (Figure 7), the concentric patterns of species in the littoralzone around a billabong or up a riverbank. On a larger scale, a similardistribution can be seen across a floodplain.The degree of adaptation can be described as obligate or facultative;and adapted or tolerant. In the floodplain wetland context, speciesdependent on aquatic conditions to provide resources, support andopportunity for regeneration are obligate species, whereas those thatcan survive on wet muds (at least temporarily) after flood recession andstill flower and set seed, are facultative. Thus, submerged plants such asVallisneria which die of desiccation on flood recession are obligate forinundation, whereas many species of milfoil, Myriophyllum spp., thatcan grow on wet muds, especially during cooler conditions, arefacultative. Similarly, species may be flood-adapted, meaning they havespecific adaptations that allow growth under flooded conditions,whereas species that survive but are not adapted to grow are floodtolerant.An equivalent situation occurs in relation to dry conditions onthe higher parts of a floodplain, where plants may be drought-adaptedor drought-tolerant.The tolerance range of a species to a particular component of the waterregime, such as depth, can be inferred in various ways, includingspecific investigations. Descriptive generalisations such as growth-formcan be helpful (Figure 8).22 Estimating the Water Requirements for Plants of Floodplain Wetlands