## 3 Gross Primary Production
North and south of the equator, the modelling indicates that the GPP declines to intermediate rates (3.3-6.6 g C m\({}^{-2}\) d\({}^{-1}\)) in subtropical regions along the east coast of the Americas, Africa, and Australia. A relatively lower GPP (\(\leq\)2.7 g C m\({}^{-2}\) d\({}^{-1}\)) is seen along the northern Pacific coast of North America and Western Australia as well as the Middle East. The mangrove  
Figure 2: Spatial distribution of mangrove ecosystem GPP (g C m\({}^{-2}\) y\({}^{-1}\)): (**a**) global distribution of GPP estimates averaged by 0.5\({}^{\circ}\)\(\times\) 0.5\({}^{\circ}\); (**b**–**g**) closeup of GPP in (**b**) Everglades National Park, (**c**) Gurupi-Pria Marine Extractive Reserve, (**d**) Edumanom National Forest, (**e**) Sundarbans National Park, (**f**) Terusan, and (**g**) Deception Bay, respectively; (**h**) latitudinal distribution of mangrove GPP and terrestrial GPP estimated by DGVMs (dynamic global vegetation models), RS (remote sensing), and MTE (model tree ensemble product); and (**i**) longitudinal distribution of mangrove GPP (g C m\({}^{-2}\) d\({}^{-1}\)). Reproduced from [47] with permission of Elsevier B.V.  
GPP tends to be higher on eastern coasts than on western coasts of the continents; there is no longitudinal gradient. The mangrove GPP in the northern hemisphere is 31% higher than in terrestrial ecosystems and 63% higher in the southern hemisphere. The tendency for a higher GPP on eastern coasts was attributed to lower salinity variations and the higher atmospheric moisture brought in by upwelling and oceanographic processes that may favour plant growth. The optimal temperature was about 26 \({}^{\circ}\)C, which is somewhat higher than the cross-ecosystem optimal temperature of 20.3 \({}^{\circ}\)C [49]. However, it did fall within the optimal air temperature range suggested for tropical ecosystems [50]. The models indicate that the optimal salinity range is 20-34 for mangroves [47]. Mangrove GPP is relatively high compared to the rates estimated in most terrestrial ecosystems (Figure 3).The globally averaged GPP for mangroves (6.0 g C m\({}^{-2}\) d\({}^{-1}\)) is greater on average than model estimates for shrublands (2.6-3.2 g C m\({}^{-2}\) d\({}^{-1}\)), natural grasslands (1.6-2.2 g C m\({}^{-2}\) d\({}^{-1}\)), and croplands (2.4-2.9 g C m\({}^{-2}\) d\({}^{-1}\)). A higher GPP was estimated only for broad-leaved evergreen trees (7.4-7.9 g C m\({}^{-2}\) d\({}^{-1}\)), with some variation among model estimates. The mangrove GPP exceeded the terrestrial production at nearly all latitudes, except near the equator where GPP rates are comparable. The decrease in GPP with latitude is more significant than that of mangroves, mostly due to the natural decrease in the proportion of broad-leaved evergreen forests. A process-based, biogeochemical model (Mango-GPP) was recently developed to improve GPP simulations of mangrove ecosystems, integrating mangrove physiological processes with light-use efficiency [51]. The model results were compared to empirical  
Figure 3: Comparison of annually averaged mangrove GPP with empirically-derived GPP data (MANGROVE) estimated in other plant functional types based on remote sensing (RS), model tree ensemble (MTE), and dynamic global vegetational (DCVM) models. BARE: bare soil; GRASS_MAN: managed grasses; GRASS_NAT: natural grasses; SHRUBS: shrubs; TREES_BD: broad-leaved deciduous trees; TREES_BE: broad-leaved evergreen trees; TREES_ND: needle-leaved deciduous trees; and TREES_NE: needle-leaved evergreen trees. Reproduced from [47] with permission from Elsevier B.V.  
observations with an eddy covariance, with the modelled versus observed total annual GPP resulting in an \(\tau^{2}\) of 0.96. The observed annual mangrove GPP ranged from 4.0 to 7.4 g C m\({}^{-2}\) d\({}^{-1}\) with an average of 5.8 g C m\({}^{-2}\) d\({}^{-1}\).