Aeration has an important influence on the chemical status and biota of soils; it also greatly affects the species of vascular plants that can grow on them (Grosse et al., 1998). The diffusion rate of oxygen in water is low (some 10 000 times less than in air) and any oxygen present in flooded soils is rapidly exploited by the bacterial population. Consequently roots growing in frequently or permanently flooded sites often encounter saturated soils in which oxygen partial pressure is zero (anaerobic). Under such conditions root oxygen demand is not met from the rhizosphere (the region immediately surrounding the roots - see Section 5.5.1 - which normally provides a protective oxygenated sheath against the toxic effect of anaerobic conditions), and can be satisfied only by an internal supply from a part of the plant exposed to air, akin to using the plant as a snorkel. Adaptations which improve the oxygen supply of tree roots in wetlands include a variety of aerial roots (pneumatophores) including the prop or stilt roots of red mangroves Rhizophora spp. (see Section 4.2.3) that grow from the trunk down into the mud, the short pencil-shaped peg roots of the black mangroves Avicennia spp. which stick up like snorkels, the knees (a loop of root arching into the air) of a range of tropical hardwoods and the temperate bald or swamp cypress Taxodium distichum, as well as the plank and buttress roots of other trees and the formation of aerenchyma (air passages through the plant tissue) in submerged plant structures (Crawford, 1990; Thomas, 2000). Adventitious roots, produced near the water's surface, or at the top of saturated soil, are grown by a diverse range of trees from eucalyptus of the southern hemisphere to the tupelos, willows, elms and ashes of the north. These can be used to replace roots that rot when flooded in the growing season.
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