## 2 Experimental Section
### Primary Productivity and Respiration of the Canopy, Mats and Macroalgae  
Tree gross primary production (GPP) was estimated by addition of canopy net primary productivity (NPP) to canopy respiration [13]. Leaf area index (LAI) of all forest plots was determined using hemispherical photography analysed with the Hemiview program (Delta T Devices, Cambridge, UK). Canopy NPP was estimated by multiplying the LAI by the mean photosynthetic rate measured in the field (obtained from [35]), which was then scaled up to an annual NPP per plot assuming 10 h per day of photosynthetic activity. Respiration rates of three canopy leaves from each plot were measured at night (in darkness) with the LiCor 6400 standard leaf chamber (Li-Cor, Lincoln, NE, USA). Because LAI incorporates variations in leaf angle (assesses the projected leaf area), and mangroves are known to have steep leaf angles to avoid excess solar radiation and limit water loss [37; 38] we also assessed the total leaf area per m\({}^{2}\) by harvesting all leaves within a 1 m\({}^{2}\) quadrat through the canopy in the fertilization experimental plots (9 per treatment for each forest zone within each of the three study locations). We used this direct measure of leaf area to scale up leaf respiration rates to an annual rate canopy respiration rate by multiplying leaf respiration per leaf area (Table 1) by total leaf area per m\({}^{2}\)\(\times\) 24 h\(\times\) 365 days. GPP of the microbial mats was obtained from [39; 40]. We assumed 100% cover of the soil surface by these microbial communities [3], but differing productivity over forest zones due to variation in mangrove canopy cover and thus light reaching the benthos [39]. No values of GPP for the mats were available for the P fertilized trees. We therefore used values for the fringe forest zone anticipating that \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Forest Attributes**} & \multicolumn{3}{c}{**Forest Type**} \\ \cline{2-4}  & **Fringe** & **Scrub** & **P Fertilized Scrub** \\ \hline Density of trees No. m\({}^{-2}\) (\(N=5\)) & 0.51 \(\pm\) 0.23 & 0.23 \(\pm\) 0.09 & 0.23 \(\pm\) 0.09 \\ \hline Tree Height (\(N=9\)) & 3.3 \(\pm\) 0.2 & 1.19 \(\pm\) 0.10 & 2.24 \(\pm\) 0.22 \\ \hline LAI m\({}^{2}\)\(\cdot\)m\({}^{-2}\) (\(N=9\)) & 1.82 \(\pm\) 0.07 & 0.39 \(\pm\) 0.06 & 1.70 \(\pm\) 0.77 \\ \hline Direct measure of canopy leaf area (m\({}^{2}\)\(\cdot\)m\({}^{-2}\)) & 2.62 \(\pm\) 0.30 & 1.17 \(\pm\) 0.33 & 4.61 \(\pm\) 0.94 \\ \hline Surface area stems (m\({}^{2}\) tree) & 1.3 \(\pm\) 0.3 & 0.06 \(\pm\) 0.01 & 0.46 \(\pm\) 0.09 \\ \hline Aboveground root length (m length per tree) & 62 \(\pm\) 20 & 7.4 \(\pm\) 4.0 & 39 \(\pm\) 16 \\ \hline Surface area aboveground roots & 5.4 \(\pm\) 1.7 & 0.59 \(\pm\) 0.29 & 2.8 \(\pm\) 1.1 \\ (m\({}^{2}\) root area m\({}^{-2}\)) & 40 \(\pm\) 20 & 8 \(\pm\) 8 & 24 \(\pm\)13 \\ \hline Surface area of habitat for macroalgae & 0.68 \(\pm\) 0.14 & 0.22 \(\pm\) 0.02 & 0.53 \(\pm\) 0.08 \\ (m\({}^{2}\) root area m\({}^{-2}\)) (\(N=9\)) & 10.3 \(\pm\) 0.4 & 7.4 \(\pm\) 0.1 & 12.1 \(\pm\) 0.7 \\ (\(N=9\)) & 1.41 \(\pm\) 0.06 & 1.61 \(\pm\) 0.04 & 1.98 \(\pm\) 0.12 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters used in estimating components of the carbon budget of mangrove forests of Twin Cays, Belize. Density of trees is from [PERSON] _et al._[33]; leaf photosynthetic carbon gain is from [PERSON] _et al._[35] and Cheeseman and Lovelock [36]. Values are means \(\pm\) standard errors. the similar canopy cover (LAI) limited productivity of the mats in the treatment. GPP of the macroalgal community that is associated with mangrove roots (comprised mainly of _Bostrychia tenella_, _B. montagnei_ and _Catenella repens_) was estimated from measures of maximum rates of electron transport from chlorophyll fluorescence using a PAM 2100 ([PERSON], Effeltrich, Germany) and the relationship between carbon gain and electron transport described by [PERSON] _et al._[41].