Topic 6.3: Freshwater Swamp Forests

Due to the fact that oxygen diffuses about 10,000 times slower through water than through air, flooded soils are anaerobic, which has a multitude of changes. One big change is that the rates of decomposition of organic matter decline markedly with flooding, which often results in the accumulation of its partial decomposition products referred to as peat. Organic soils, or histosols, are widespread in the tropics in low-lying areas with long duration flooding. In places, the peats can be more than 10 m thick and have accumulated for many centuries.

Growth rates of trees in flooded forests are impeded by the same factor that limits organic matter decomposition, lack of oxygen. Survival under these conditions requires trees to produce roots that are porous, to prevent the accumulation of toxic ethanol, and to promote the diffusion of oxygen down from above the water line through air filled tissues, referred to as ‘aerenchyma.’ Trees in swamp forests often develop swollen bases or large buttresses, which increase the surface area above the water line into which oxygen can diffuse. Although swamp forest trees must be adapted to flooding, they can nevertheless be sensitive to changes in hydroperiod, depth or season of flooding, and other factors that influence the soil redox potential.

A primary consideration in managing swamp forests is avoiding soil damage.  Because the trafficability of soil decreases exponentially with water content, timber yarding can be a major challenge in swamp forests.  Where the duration of inundation (= hydroperiod) is long and decomposition rates are therefore slowed, organic matter accumulates and yarding problems intensify. Ground-based yarding can result in channelization and increased drainage as well as in impoundments where hydroperiods are extended and flooding depths are increased.  Both sorts of hydrological changes may influence forest composition and growth in ways that are not silviculturally or environmentally desirable.

Intentional draining of peatswamps to facilitate timber harvests or to convert them for agricultural purposes results in huge emissions of carbon and should be avoided. When flooded soils are drained, oxygen becomes available to the microbes that decompose organic matter, which they do quickly. Even faster are the effects of the soil-burning fires to which drained peats are susceptible.