## 4 Aboveground Net Primary Production  
At least eight methods have been used to measure the mangrove aboveground net primary production (ANPP): (1) biomass increment + litterfall; (2) CO\({}_{2}\) gas measurements + litterfall; (3) field-based, radiation-based models; (4) gas exchange; (5) growth curve/growth increment; (6) light attenuation; (7) light interception; and (8) litterfall only. Using all data and methods, the mean ANPP averaged 3.7 (4.6 \(\pm\) 1 SD) g C m\({}^{-2}\) d\({}^{-1}\) with a median of 2.4 g C m\({}^{-2}\) d\({}^{-1}\) and a range of 0.0- 35.0 g C m\({}^{-2}\) d\({}^{-1}\). However, there were significant differences between methods (one-way ANOVA on ranks, H = 162.35, df = 8, \(p\) < 0.001), with the light attenuation and light interception methods yielding greater rates of ANPP than most of the other methods (Table 1). The rates of ANPP were highly variable among methods as well as globally (Table 1). These methods have been partially or fully adapted from methods used to measure the terrestrial forest ANPP and thus have the same pitfalls and advantages in estimating mangrove ANPP. Traditionally, ANPP is measured indirectly by measuring and summing (1) the biomass of the incremental growth of tree stems and (2) litterfall. Both the field-based, radiation-based and the growth curve/growth increment models incorporate the Farquhar model of C3 photosynthesis and several empirical models of stomatal response fitted to eddy covariance data, which are measures of gas exchange from the forest floor to above the canopy. Whole-ecosystem methods include the use of remote sensing and chlorophyll fluorescence techniques which are increasingly used in forest ecology. Litterfall is by far the most common method used because it is inexpensive and easy to measure, but measures only leaf production. Assuming that the biomass increment/litterfall method is the most reliable measurement of mangrove ANPP, the mean values (Table 1) indicate that litterfall accounts for only 55% (2.2/3.9 \(\times\) 100) of the total ANPP. The litterfall (Table 1) was greater (2.2 g C m\({}^{-2}\) y\({}^{-1}\) = 8.0 Mg C ha\({}^{-1}\) y\({}^{-1}\)) than previous estimates of 4.7 Mg C ha\({}^{-1}\) y\({}^{-1}\)[123], 5.2 Mg C ha\({}^{-1}\) y\({}^{-1}\)[4], and 4.7 Mg C ha\({}^{-1}\) y\({}^{-1}\)[124]. Other studies have estimated lower percentages (33-38%) of the mangrove ANPP [4; 125]. The light attenuation and light interception methods are based on several untested assumptions that translate into overestimates of ANPP. And indeed, both methods provided mean values significantly greater than \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Method** & **Size** & **Mean** & \(\pm\)**1 SD** & **Max** & **Min** & **Median** \\ \hline Biomass increment + litterfall b,c & 70 & 3.9 & 2.6 & 11.8 & 0.0 & 3.2 \\ \hline CO\({}_{2}\) exchange + litterfall d & 4 & 1.4 & 2.1 & 4.4 & 0.0 & 0.7 \\ \hline Field-based, radiation-based model c,d & 3 & 3.1 & 0.3 & 3.6 & 3.0 & 3.0 \\ \hline Gas exchange a,b,c & 8 & 8.9 & 5.4 & 16.6 & 2.3 & 6.7 \\ \hline Growth curve/growth increment b,c & 14 & 3.6 & 2.8 & 9.3 & 0.7 & 2.4 \\ \hline Light attenuation a & 36 & 12.8 & 9.2 & 30.7 & 1.9 & 9.0 \\ \hline Light interception a,b & 18 & 10.0 & 8.7 & 35.0 & 3.7 & 6.9 \\ \hline Litterfall d & 338 & 2.2 & 1.4 & 8.1 & 0.0 & 1.9 \\ \hline \end{tabular}
\end{table} 
Table 1: Mean and median rates of mangrove ANPP (g C m\({}^{-2}\) d\({}^{-1}\)) based on different methods. Different letters as superscripts in methods indicate significant differences (\(p\) < 0.001) using [PERSON]’s test in pair-wise comparisons test post-ANOVA. DW data were converted to CORG assuming 48% C content [8]. References: Biomass increment + litterfall: [7; 8; 54; 55; 56; 70; 71]. CO\({}_{2}\) measurements + litterfall: [11]. Field-based, radiation-based biogeochemical model: [68]. Gas exchange: [8; 59; 69]. Growth curve analysis: [69; 70]. Growth increment: [72]. Light attenuation/light interception: [7; 8; 43; 73; 74; 75; 76; 77; 78; 79].