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Biomass versus stem height and DBH

Both stem height and DBH were excellent predictors of total above-ground biomass for all three species (Figures 2, 3) with total variance explained (R²) greater than 0.92 in all cases (Table 1). DBH yielded R²s that were slightly higher than those for stem height. However, we consider the difference to be insignificant. The best fits were higher for Laguncularia than for either Avicennia or Rhizophora. Given these results, and the fact that DBH is measured very accurately and with great ease in the field, whereas stem height is very difficult to measure non-destructively, we consider only DBH for the remainder of the study.

Figure 2. Total dry biomass as a function of DBH for the three mangrove species. Avicennia = diamonds with solid line, Laguncularia = squares with dotted line, and Rhizophora = triangles with dashed line. [larger image]

Figure 3. Total dry biomass as a function of stem height for the three mangrove species. Symbols as in Figure 2. [larger image]

Stem, branch, leaf, and prop root biomass versus DBH

Highly significant relationships were found for all components of above-ground biomass and DBH for all three species. In general, regressions for stem biomass had higher variance explained (R²s > 0.95) than did regressions for branch and leaf biomass (Table 1 and Figures 4-6). The latter two components of biomass were much more variable. No differences were found among species with respect to total stem biomass and DBH (Figure 4). However, Rhizophora seems to allocate more biomass to branches than either Avicennia or Laguncularia over the entire range of DBHs measured (Figure 5). Rhizophora also seems to allocate more biomass to leaf tissue than Avicennia and Laguncularia, but only at larger DBHs (Figure 6). For Rhizophora, prop root biomass was significantly related to DBH (Figure 7).

Figure 4. Stem dry biomass as a function of DBH for three mangrove species. Symbols as in Figure 2. [larger image]

Figure 5. Branch dry biomass as a function of DBH for three Florida mangrove species. Symbols as in Figure 2. [larger image]

Figure 6. Leaf dry biomass as a function of DBH. Symbols as in Figure 2. [larger image]

Figure 7. Rhizophora prop root biomass as a function of DBH. [larger image]

Biogeographic comparisons

Our equations give the lowest estimate of biomass for all three species when compared to results from other studies (Table 2, see our Figures 8-10 for references). A mangrove with a given DBH will have a greater predicted biomass near the equator than one with the same DBH that is growing in a location to the north or south of the equator. The differences are least for Laguncularia and greatest for Rhizophora . For example, Laguncularia with a DBH 10 cm is predicted to have 60 kg dry mass in French Guiana (Fromard et al. 1998), 50 kg dry mass in the Yucatan of Mexico (Day et al. 1987), and 45 kg dry mass in the Florida Everglades (the present study, see Figure 8). Unfortunately the studies by Fromard et al. (1998) and Day et al. (1987) spanned a small range in DBH (1-10 cm). Therefore we could not compare to the largest Laguncularia trees we sampled (18 cm). For Avicennia, specimens 10 cm DBH are predicted to be equal in biomass for French Guiana and Florida (35 kg), and both of these areas will be less than predicted for Mexico (67.5 kg, see Figure 9). As DBH increases for Avicennia, the predicted biomass for French Guiana and Florida also diverge (Figure 9). At a DBH of 20 cm, Avicennia in French Guiana are predicted to weigh some 246 kg, whereas in Florida the same size stem is predicted to weigh a mere 136 kg (Figure 9). The differences are most striking however for Rhizophora (Figure 10). At smaller size classes (<10 cm DBH) differences are indicated with stems in Australia, Malaysia, French Guiana and Puerto Rico predicted to have more biomass than stems in Florida, Mexico or Brazil (Figure 10). Larger stems (>15 cm DBH) were not measured by many researchers so comparisons are limited to French Guiana, Florida, Australia and Malaysia. A Rhizophora in Florida with a 20 cm DBH stem is predicted to have approximately 140 kg of above-ground dry biomass (this study). Rhizophora from northern Australia, French Guiana and Malaysia are predicted to have from 300 - 350 kg of dry biomass (Figure 10).

The general outcome of the model comparisons is that allometric relations differ by species and region and do not necessarily follow latitudinal or general area trends. The biomass values generated with allometric equations should be considered with caution when used to extrapolate outside of the size range sampled or from areas with inherently different environmental parameters (for example, salinity, nutrients, hydrological exchange, stem density, net primary productivity, and herbivory).

Figure 9. Predicted total biomass for Avicennia germinans based on the allometric equations from Day et al. (1987) as shown by dashes line, from Fromard et al. (1998) as shown by dotted line, and by this study as shown by solid line. Predicted values have been calculated and plotted only for the range in DBHs reported by each study. [larger image]

Figure 10. Predicted total biomass for Rhizophora spp. based on the allometric equations from this study and other studies as shown in the legend. [larger image]

Using the equations to assess the Everglades restoration

Mean sediment salinity predicted change in biomass relatively well for Laguncularia but not for Rhizophora or Avicennia (Figure 11). This is not totally unexpected as Laguncularia is the least tolerant species. Both Avicennia and Rhizophora have broad salinity tolerances with Avicennia capable of surviving in hypersaline conditions (Pool et al. 1977). Plot biomass decreased with increasing sediment salinity for Laguncularia. Based on predictions of the hydrological models used in CERP (Fennema et al. 1994, Langevin et al. 2005), we expect salinities to decrease as freshwater inflows increase. Thus, we should be able to monitor an increase in biomass of Laguncularia in these plots as CERP proceeds.

Figure 11. Change in biomass as a function of mean sediment porewater salinity for plots along the Harney River in Everglades National Park. The regression equations for Avicennia (squares) and Rhizophora (diamonds) are not significant. The regression for Laguncularia is significant. The regression equation is: Change in biomass = -1.691*(mean salinity) + 26.905, r2 = 0.38, p < 0.01. [larger image]