Abstract
This review highlights four major topics in plant flooding research: the processes
underlying vegetation zonation in the floodplain, the challenges of using model species
to reveal adaptive responses in shoots and roots, the role of micro-organisms in flooded
soils in relation to plant growth, and the molecular regulation of the hormone ethylene
which is heavily involved in the adaptation reaction of flood-resistant plants. Model
species and vegetation strategies are used to unravel mechanisms of vegetation zonation
in the river flood-plain. In the case of woodlands, hydrological conditions determine
to a large extent their zonation patterns under natural conditions. For softwood species,
such as Salicaceae, the interaction between water levels and timing of seed dispersal
is the dominating process determining their establishment success on river banks.
Their strategy is well adapted to irregular, high and prolonged floods. Hardwood species,
Quercus, Fraxinus, Ulmus and Acer, are flood-sensitive and inhabit the higher sites. They mainly have heavy seeds and
germinate under shaded conditions. The most shade-tolerant hardwood species are the
least well adapted to flooding. Anthropogenically influenced parts of the floodplain
are characterized by grasslands with elevation level and management practices determining
the species composition. Low-lying grasslands have flood-tolerant species; elevated
zones are seldom flooded and have flooding-sensitive species. Following Grime (1998[59]), plant species of major vegetation types within the floodplain zone can be divided
into three categories - dominants, subordinates and transients - illustrating the
diversity in plant species in relation to environmental properties. Model species
that are indicative of the different conditions in the various zones are chosen to
help in the understanding of morphological and physiological adaptations at the plant
level. The formation of aerenchymatous roots and the capacity to elongate shoot parts
upon submergence are among the main responses of surviving plants. The role of hormones
in the adaptation reaction is emphasized. Owing to high porosities in roots of flood-tolerant
plants, radial oxygen loss greatly influences nitrification and denitrification processes
in the flooded soil. Nutrient cycles are restored by root-derived oxygen and the oxygenated
rhizosphere is detoxified. A new development in flooding ecology is the unravelling
of the molecular regulation of hormonally controlled processes. The expression of
an ethylene receptor gene in Rumex palustris is highlighted. This paper ends with some suggestions for future flooding research.
Key words
Flooding - floodplains - adaptation - ethylene -
Rumex
- life history strategies - soil biology
