Adaptations of Floodplain Vegetation

Due to the alternating wet-dry environment experienced by trees growing on floodplains, they have developed a variety of physiological and morphological adaptations that allow survival during flooding. Initially, stimulation of alcohol dehydrogenase (ADH), enzyme activity may provide a temporary means to support essential metabolic functions. The anaerobic pathway is less efficient than the aerobic pathway (39 moles ATP per mole hexose vs. 3 moles ATP per mole hexose), but provides an energy resource while anatomical changes are occurring.

The seeds of floodplain tree species require oxygen for germination, and even those species that can grow in permanently to nearly permanent flooded conditions (e.g., Taxodium and Nyssa) require moist, but not flooded, soil for germination and establishment. Occasional drawdowns are necessary for the survival of tree species. Rapid stem elongation, such as been observed with Nyssa aqua-tica, allows the seedling to get its crown above the water surface of subsequent floods. The dispersal and survival of many wetland tree seeds is dependent upon hydrologic conditions. Taxodium and Nyssa seeds are produced in the fall and winter between the periods of lowest and highest streamflows, giving the seeds the widest possible range of

Table 1 Mean structural and aboveground productivity characteristics of floodplain forests

Aboveground NPP

Table 1 Mean structural and aboveground productivity characteristics of floodplain forests

Aboveground NPP

No. of

Density

Basal area

Biomass

Area

species

(no. ha1)

(m2ha-1)

(tha1)

Leaf

Wood (thayr1)

Totala

Southeastern USA

13

1242

45.0

302

5.36

7.78

13.26

Northeastern USA

10

970

26.1

150

North Central USA

5

546

29.5

Western USA

5

310

27.5

Central USA

12

405

33.5

290

4.20

2.50

8.70

Europe

1237

26.5

314

3.48

17.88

Central America

10

726

49.9

118

11.61

Caribbean

27

3359

42.4

224

15.55

South America

89

687

33.0

413

Africa

26

Southeast Asia

9.15

Australia

12

493

260

aTotal NPP does not always equal leaf plus wood as some sources only report total.

aTotal NPP does not always equal leaf plus wood as some sources only report total.

hydrologic conditions. Overall, seed production of many wetland species seems to be linked to the timing and magnitude of hydrologic events.

Stem hypertrophy, commonly called butt swell or buttressing, is characterized by an increase in diameter of the basal portion of the stem and is common in Taxodium, Fraxinus, Nyssa, and Pinus species. Basal swelling can extend from just above the ground level to several meters depending upon the depth and duration of flooding. Swelling generally occurs along that portion of the trunk that is flooded seasonally. Increased air space in the swollen portion of the stem allows increased movement of gases within the plant. Ethylene production has been documented to play a regulatory role in altering growth and stem anatomy of woody plants, and has been found to be higher in flooded Fraxinus stems with well-defined hypertrophy than those without stem hypertrophy. Lenticel hypertrophy has long been associated with flooding and acts to increase internal gas transport from the stem to the roots. Duration of flooding does not appear to affect the number of lenticels formed but does affect the size. The formation of hypertrophied lenticels under anoxic conditions also appears to be induced by ethylene. Other commonly observed features in flooded environments include buttress roots and knees. Buttress roots appear as fluted projections at the base of mature trees and extend for several feet from the trunk outward and down into the soil. Because of the shallow nature of root systems in saturated or flooded soils, these buttress roots are thought to provide additional support to the tree. Knees are common in Taxodium spp. in the southeastern United States. Their function has not been confirmed, although there is some speculation that they also serve in stability of the tree. In Australia, Melaleuca trees on floodplain sites have modified bark structures such as papery bark with internal longitudinal air passages that allow them to tolerate flooded conditions.

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