ABSTRACT
Wetlands differ signifi cantly in their water source and seasonal hydrologic regime. Hydrological patterns (i.e., fl ooding frequency, duration and hydroperiod) infl uence physical and chemical characteristics (e.g., salinity, oxygen and other gas diffusion rates, reduction-oxidation potential, nutrient solubility) of a wetland. In return, these internal parameters and processes control fl ora and fauna distribution as well as ecosystem functions. Plants, animals and microbes are often oriented in predictable ways along the hydrological gradient (Fig. 1). Conversely, the biotic component affects the hydrology by eventually modifying fl ow or water level in a wetland.[1,2] Species also infl uence nutrient cycles and other ecosystem functions.[3]
A LANDSCAPE PERSPECTIVE
Although the hydrology is part of the ecological signature of an individual wetland, wetlands are considered as neither aquatic nor terrestrial systems. They have characteristics from both systems and are defi ned as ecotones placed under this dual infl uence.[4] Because wetlands are located at the interface of multiple systems, they assure vital functions (e.g., wildlife habitats) benefi cial at the landscape level. Reduction of wetland area often reduces biodiversity in the landscape.[2,5] Increases in biodiversity occur when wetlands are created or restored in a disturbed landscape.[6]
WETLAND FLORA
Wetland plants are adapted to a variety of stressful abiotic conditions (e.g., immersion, wave abrasion, water level fl uctuation, low oxygen conditions). Identical adaptations
to common environmental features have led taxonomically distinct species to sometimes look similar in terms of morphology, life cycle and life forms.[7] Traditionally, wetland plants have been classifi ed into groups of different life forms, primarily in relation with hydrological conditions. Helophytes are defi ned as plant species with over-wintering buds in water or in the submerged bottom.[8] They are differentiated from hydrophytes in that their vegetative organs are partially raised above water level.[8] Hejny’s classifi cation is based on relatively stable vegetative features that determine the ability of wetland plants to survive two unfavorable conditions, cold and drought.[7] This classifi cation uses the types of photosynthetic organs present in both the growth and fl owering phases. Other classifi cations include both life form and growth form. When a species has a range of growth form, it is classifi ed under the form showing the greatest achievement of its potential.[7]
Plant communities in wetlands can be more or less homogeneous, mosaic-like, or distributed along a gradient resulting in a clear zonation of species. A gradient exists if one or several habitat parameters change gradually in space. This phenomenon is common in fresh water marshes that present a gradient of water depth and water saturated soils (Fig. 1). Such a gradient is often accompanied by differences in peat accumulation that is infl uenced by waves or currents. General principles of the zonation of aquatic plants have been largely described.[7,9,10] Littoral vegetation can belong to several types of communities, which derived from the general principle that, from deeper water to the shore, we may expect successively submergent, fl oating, and emergent macrophytes. The most important habitat factor is water depth, depending on slope and peat accumulation.[10] Other factors may be poor irradiance caused by high turbidity or exposure to waves or fl ow.[7,10]
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Riparian ecosystems are found along streams and rivers that occasionally fl ood beyond confi ned channels or where riparian sites are created by channel meandering in the stream network. Riparian or bottomland hardwood forests contain unique tree species that are fl ood tolerant. Species distribution is associated with fl oodplain topography, fl ooding frequency, and fl ooding duration.[11,12] In SouthEastern U.S. bottomland, seasonally fl ooded forests are colonized by Platanus occidentalis (sycamore), Ulmus americana (American elm), Populus deltoides (cottonwood) and are fl ooded between 2% and 25% of the growing season (Fig. 1). Other species such as Fraxinus pennsylvanica (green ash), Celtis laevigate (sugarberry) and Carya aquatica (water hickory) colonized bottomlands that are fl ooded by less than 2% of the growing season.[11] Freshwater marshes are dominated with emergent macrophytes rooted in the bottom with aerial leaves (i.e., helophytes). Species such as Typha (cattail), Phragmites (reed grass) and Scirpus (bulrush) are often clonal. A plant community is usually organized in sequence of patches that are dominated by one species. The second plant groups are the
rooted plants with fl oating leaves (Nymphaeid). Lotus (Nelumbo) and water lilies (Nymphea) have very identical morphology (i.e., similar leaves and fl owers) but a genetic analysis showed that lotus is more closely related with plane-tree than with water lilies.[10] Submerged plants include elodeids (i.e., cauline species whose whole life cycle can be completed below the water surface or where only the fl owers are emergent) and isoetids (i.e., species growing on the bottom whose whole life cycle can be completed without contact with the surface). Submerged species include species such as coontail (Ceratophyllum demersum) and water milfoil (Myriophyllum spp.). Plant species found in salt marshes are called halophytes (i.e., plants which complete their cycle in saline environments). A saltmarsh can be divided into low, middle, and upper marsh, according to fl ooding frequency and duration. Each zone is dominated by different plant species according to their tolerance to saline immersion.[10]
A dominant competitive species-often a clonal species-can modify the theoretical zonation. Change in water chemistry (i.e., eutrophication) or hydrology may favor a
Fig. 1 Species distribution along the hydrological gradient in a freshwater marsh.
