## 2 Experimental Section  
### Study Site  
Twin Cays, Belize (16\({}^{\circ}\)50\({}^{\prime}\) N, 88\({}^{\circ}\)06\({}^{\prime}\) W) is a small archipelago comprised of 92 ha of mangrove forest that lies within the lagoon of the Meso-American Barrier Reef [26]. The mangroves forests of Twin Cays can be divided into two main vegetation zones: (1) Forests fringing the ocean dominated by _Rhizophora__mangle_, which are approximately 3 to 6 m tall and occupy approximately 100,000 m\({}^{2}\) of the islands; and (2) Dwarf or scrub stands (less than 1.5 m) of _R.__mangle_ and well developed microbial mats that are associated with ponded areas of the interior of islands which cover \(\sim\)190,000 m\({}^{2}\) of the islands area [31; 32] (Figure 1). _Avicennia germinans_ and _Laguncularia racemosa_ also occur on the islands in areas of higher elevation, but are excluded from this study because sufficient data are not available to create detailed C budgets for these vegetation zones. Dwarf or scrub trees within the islands interiors show pronounced growth enhancements when fertilized with P (see insert in Figure 1), indicating P limitations to primary productivity in these areas, in contrast to nitrogen limitation to growth observed in the taller seaward fringing forests [14].  
Data for the construction of C budgets for Twin Cays, Belize have been acquired over the last 25 years due to the presence of the Smithsonian's field station at nearby Carrie Bow Cay [26] and a range of research initiatives, including the Caribbean Coastal Marine Productivity program (CARICOMP [33]), that have been supported by the Smithsonian Institution and other funding agencies. In order to construct a C budget for different forest zones (seaward fringing forests and landward scrub forests) and for nutrient enriched (fertilized) and control (non-fertilized) plots, we have collated and integrated results from a range of studies that have used a wide array of approaches (Table 1). Much of the data are from long term fertilization experiments that were commenced on Twin Cays in scrub forest in 1988 [34] and extended to cover both fringe and scrub forests in 1995 [14]. Briefly, the fertilization experiments were conducted at three locations in both fringing forests and scrub forests (Figure 1), At each location and in each forest zone nine plots were fertilized with nitrogen (N, applied as urea) and nine fertilized with slow release phosphorus fertilizer (P) with nine remaining as unfertil controls [14]. In each plot a focal tree was assessed to determine rates of tree growth. As there were no differences among control and P fertilized trees in the fringing forests for many measured parameters and relatively small differences among N fertilized and control trees in the fringing forests [14,15,35], and because in the scrub forests there were small differences between N fertilized and control trees, but very large differences between P fertilized trees and control trees [14,15,35,36], we chose to focus on contrasts among fringe and scrub forests and within the scrub forest between P fertilized plots and control plots (Table 1).  
Figure 1: Aerial image of the study site, Twin Cays, Belize in 2006. Study locations are indicated by yellow arrows. The inset provides a detailed view of one of the study locations. Scrub and fringe forests are indicated as well as the phosphorus (P) fertilized scrub trees.