participate in the floodplain's food 

 chains, nutrient cycles, and import-export 

 pathways. 



The major factor contributing to the 

 high productivity of the floodplain forest 

 is the pulsing of the wet-dry cycle. 

 Conner and Day (1976) made an analogy 

 between these floodplain forests and the 

 tidal marshes in terms of the positive 

 effects of fluctuating water levels: 



"This periodic flooding acts somewhat 

 in the same manner as tidal flooding 

 in saline marshes, in that fluctuat- 

 ing water levels are energy subsidies 

 which control variations in hydric 

 conditions, temperature, nutrient 

 levels, and available oxygen (Hester 

 1973; Butler 1975)." 



Bottomland hardwood communities that 

 either are permanently flooded with slow- 

 moving to stagnant water, or are regularly 

 damaged by unusually high and irregular 

 destructive floods are not as productive 

 as communities that undergo periodic mod- 

 erate floods. This has been illustrated 

 clearly (Figure 43) by Odum (1978), who 

 graphically compared the productivity of 

 stagnant, seasonally flooded, and abra- 

 sively flooded systems with a regional 

 average of all wetland and upland forest 

 types . 



Communities in permanently ponded 

 conditions, or on sites where poor drain- 

 age leads to continuously high water 

 tables and the accumulation of acidic peat 

 soils, have lower productivities primarily 

 because of low nutrient turnover, due to 

 anoxia, nitrogen limitation, and low pH. 

 Brown et al. (1979) and Conner and Day 

 (1976) presented data that demonstrate the 

 reduced productivities of still water 

 systems. 



Productivity values gleaned from the 

 literature for 19 upland and bottomland 

 forest types are presented in Table 16 and 

 generally support the concept of a flood 

 subsidy depicted in Figure 43. A second 

 verification of this concept is shown in 

 Figure 44, where productivity data from 

 sites in several zones are plotted (from 

 Gosselink et al. 1981). Gosselink et al. 

 (1981) stated: 



Slowly 

 flowing 

 Stress -• Subsidy- 



Abrasive 



flooding 



— Stress 



a gradient of 

 compared with 



Figure 43. The effect of 

 flooding on productivity as 

 a regional level that might be expected in 

 the absence of standing or flooding water. 

 The graphic model takes the form of a 

 stress-subsidy curve. For southern swamps 

 Conner and Day (1976) estimated annual net 

 production for stagnant, slowly flowing 

 and seasonal flooding conditions as of the 

 order of 0.2, 0.7, and 1.2 kg dry matter 

 per square meter, respectively (Odum 

 1978). 



"Forest production appears to peak at 

 the once-per-year flood frequency if 

 flooding is during the winter because 

 this regime furnishes the optimum 

 environment for plant growth in terms 

 of nutrient input by flood waters, 

 summer soil moisture, and possibly 

 aerobic conditions during the summer 

 leading to inorganic nutrient release 

 from organic debris." 



Primary productivity data in the lit- 

 erature are much more common for the tree 

 canopy and woody subcanopy (small trees 

 and shrubs) components of floodplain com- 

 munities than for the herbaceous, aquatic 

 vascular, and nonvascular components. 

 Aquatic plant productivity in river chan- 

 nels and drainage tributaries, permanent 

 ponds, and temporary sloughs and swales 

 has received the least attention. Brinson 

 and Wharton (1979a) suggested that the 

 productivity of alluvial stream communi- 



81 



