production (NPP), and respiration (R) of 

 Florida mangroves are given in Table la. 

 Compared to net primary production (NPP) 

 estimates from other ecosystems, including 

 agricultural systems (E.P. Odum 1971), it 

 appears that mangroves are among the 

 world's most productive ecosystems. 

 Healthy mangrove ecosystems appear to be 

 more productive than sea grass, marsh 

 grass and most other coastal systems. 



Further examination of Table la re- 

 veals several possible tendencies. The 

 first hypothetical tendency, as discussed 

 by Lugo et al. (1975), is for red mangroves 

 to have the highest total net production, 

 black to have intermediate values and 

 white the lowest. This conclusion assumes 

 that the plants occur within the zone for 

 which they are best adapted (see section 

 3.2 for discussion of zonation) and are 

 not existing in an area with strong limit- 

 ing factors. A scrub red mangrove forest, 

 for example, growing under stressed condi- 

 tions (high soil salinity or low nutrient 

 supply), has relatively low net produc- 

 tivity (Teas 1979). The pre-eminent posi- 

 tion of red mangroves is shown by the 

 comparative measurements of photosynthesis 

 in Table lb; measurements were made within 

 canopy leaves of trees growing within 

 their zones of optimal growth. 



A second noteworthy tendency is that 

 red mangrove GPP decreases with increasing 

 salinity while GPP of black and white 

 mangroves increases with increasing 

 salinity up to a point. Estimates of Hicks 

 and Burns (1975) demonstrate that this may 

 be a real tendency (Table lc). 



Data presented by Miller (1972), 

 Carter et al. (1973), Lugo and Snedaker 

 (1974), and Hicks and Burns (1975) sug- 

 gest a third hypothetical tendency, 

 assuming occurrence of the species within 

 its adapted zone. It appears that the 

 black mangrove typically has a much higher 

 respiration rate, lower net productivity, 

 and lower GPP/R ratio than the red man- 

 grove. This can be attributed at least 

 partially, to the greater salinity stress 

 under which the black mangrove usually 

 grows; this leads to more osmotic work. 



These three apparent tendencies have 

 led Carter et al. (1973) and Lugo et al. 

 (1976) to propose a fourth tendency, an 

 inverted U-shaped relationship between 

 waterway position and net mangrove com- 

 munity productivity (Figure 6). This 

 tendency is best understood by visualizing 

 a typical gradient on the southwest coast 

 of Florida. At the landward end of the 

 gradient, salinities are very low, 

 nutrient runoff from terrestrial eco- 

 systems may be high and tidal amplitude is 

 minor. At the seaward end, salinities are 

 relatively high, tidal amplitude is rela- 

 tively great and nutrient concentrations 

 tend to be lower. At either end of the 

 gradient, the energetic costs are high and 

 a large percentage of GPP is used for 

 self-maintenance; at the landward end, 

 competition from freshwater plant species 

 is high and at the seaward end, salinity 

 stress may be limiting. In this scenario, 

 the highest NPP occurs in the middle 

 region of the gradient; salinity and tidal 

 amplitude are high enough to limit compe- 

 tition while tidal flushing and moderate 

 nutrient levels enhance productivity. 

 Hicks and Burns (1975) present data to 

 support this hypothesis. 



In addition to these hypotheses 

 generated from field data, there have been 

 two significant, published attempts to 

 derive hypotheses from mathematical simu- 

 lation models of mangroves. The first 

 (Miller 1972) is a model of primary pro- 

 duction and transpiration of red mangrove 

 canopies and is based upon equations which 

 utilize field measurements of the energy 

 budgets of individual leaves. This model 

 predicts a variety of interesting trends 

 which need to be further field tested. 

 One interesting hypothesis generated by 

 the model is that maximum photosynthesis 

 of red mangrove stands should occur with a 

 leaf area index (LAI) of 2.5 if no accli- 

 mation to shade within the canopy occurs; 

 higher LAI's may lead to decreased produc- 

 tion. Another prediction is that red 

 mangrove production is most affected by 

 air temperature and humidity and, to a 

 lesser degree, by the amount of solar 



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