Geoscience Reference
In-Depth Information
Wetland vegetation
6
6.1 Plant adaptations
and northern white-cedar (
Thuja occidentalis
).
Cordgrass (
Spartina
sp.) delivers so much
oxygen to its roots, in fact, that iron and man-
ganese in adjacent soil are often oxidized into
rusty streaks (Welsch et al. 1995). During the
night, when photosynthesis stops but respira-
tion continues, oxygen stored in aerenchyma is
gradually consumed (Lahring 2003).
Another way to deliver oxygen to the roots
is via pneumatophores, as in some mangroves
(Fig. 6-2), or woody “knees” in swamp cypress
(Fig. 6-3). These structures grow upward from
lateral roots near the surface and project above
ground into air. The exact function of pneu-
matophores has been debated, but it seems
clear they are involved with root respiration
(Dugan 2005).
Other trees, such as green ash and red man-
grove (
Rhizophora mangle
), have enlarged
pores on the bark called lenticels, which allow
oxygen diffusion to submerged roots. Certain
trees grow shallow lateral roots above the wetter
soil level, and other trees develop adventitious
roots, which are extra roots that sprout from the
stem above the wetter soil level (Welsch et al.
1995). In mires, the slow accumulation of peat
and upward growth of the substrate gradually
raise the surface above the water table so that
more oxygen is available.
Large trees growing in soft, water-saturated
soil or water have a further structural challenge,
which is simply to remain standing. Stilt or prop
roots and trunk buttresses serve an architectural
Vegetation is one of the key attributes of wet-
lands. Aquatic and wetland plants, known as
hydrophytes, must cope with a lack of oxygen
in the soil as well as potential l ooding and
drying, lack of nutrients, low pH, high salinity,
and other limiting factors that may be present
in certain kinds of wetlands. The primary
problem is providing oxygen to roots submerged
in water or growing in anaerobic soil or sedi-
ment (Dugan 2005). Many plants have responded
with special adaptations that allow them not
only to survive, but to thrive, under these stress-
ful conditions. Some of the more important
plant adaptations are discussed here, and other
hydrophyte characteristics are mentioned below.
6.1.1 Structural adaptations
Among the most important features are struc-
tures designed to move oxygen from the leaves
and stems above water to the roots in the satu-
rated zone. Extremely porous tissue, called aer-
enchyma (pronounced air-ENK-a-ma), has large
air-i lled spaces in which gases may diffuse
rapidly. Aerenchyma stores oxygen and allows
it to move from leaves to roots and rhizomes.
Aerenchyma also allows the reverse transport of
methane from the anaerobic zone to the air.
Many common wetland plants have aerenchyma,
such as cattail (Fig. 6-1), water-hemlock (
Cicuta
maculata
), green ash (
Fraxinus pennsylvanica
)
