Riverine floodplains are typically characterized by high productivity. Productivity is enhanced in many flood-plain areas by the continued import and retention of nutrient-rich sediments from headwater regions and lateral sources, increased water supply (especially in arid regions), and more oxygenated root zones as a result of flowing waters. The flood pulse advantage has long been recognized, with ancient Egyptians setting taxes based on the extent of the annual flood.

Primary productivity of unaltered, seasonally flooded ecosystems is generally higher than that of floodplain forests that are permanently flooded or those with stagnant waters. Despite the theoretical basis for increased floodplain productivity due to pulsing, it has been difficult to confirm. More recent studies tend to point toward the idea that seasonal flooding can be both a subsidy and a stress. In the southeastern United States, aboveground NPP was similar for upland hardwood, bottomland hardwood, and Taxodium-Nyssa forests. The reason for this may be that for some sites, subsidies and stresses occur simultaneously and cancel one another. As a result, flood intensity and duration affect soil moisture, available nutrients, anaerobiosis, and even length of growing season in a complex and nonlinear 'push-pull' arrangement. When hydrology is altered rapidly, above-ground productivity is less than in natural forest communities with nearly continuous flooding (Figure 6).

Aboveground biomass in floodplain forests ranges between 100 and 300 tha~ , although there is one report of a forest in Florida where biomass exceeds 600 tha-1. Leaves account for only 1-10% of the total aboveground biomass. Belowground biomass has been sampled rarely and varies greatly, but reported values tend to be somewhat lower than the 20% of total biomass often cited for upland species. Total aboveground biomass production (leaves plus stem wood) ranges from 668 to 2136gm~2yr~\ with leaves accounting for approximately 47% of the production. Although it has been reported that there are no latitudinal patterns in NPP, litterfall production of Taxodium forests in the United States shows a curvilinear relationship with latitude with a maximum occurring at about 31.9° N. In northern Australia, litterfall in Melaleuca forests has been reported to be 2-3 times greater than that in forests in the southern part of the continent. Changes to natural hydrologic regimes decrease litter production by half. As a result of the high productivity, generally associated with floodplain forests, carbon sequestration is particularly important there.



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Mean growing season water depth (cm)

Mean growing season water depth (cm)

Figure 6 Relationship between aboveground net primary productivity (NPP) of floodplain forests of the southeastern United States and mean water depth during the growing season.

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