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publications > paper > development of allometric relations for three mangrove species in South Florida for use in the greater everglades ecosystem restoration > results and discussion
Development of allometric relations for three mangrove species in South Florida for use in the Greater Everglades Ecosystem restoration
Results and discussion
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.
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).
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).
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.
U.S. Department of the Interior, U.S. Geological Survey
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