Linking Scrub Mangroves Long-Term (2012–2021) Foliar Net Primary Productivity and their Spatial Distribution in the Everglades (Florida, USA): A Conceptual Model

Abstract Mangrove ecotypes reflect the critical role of environmental factors in regulating their functional and structural properties. Although structural differences across riverine, fringe, basin, and scrub ecotypes (e.g., tree height, species diversity, basal area) have been quantified, there remains a knowledge gap regarding the extent of interactions among these factors at regional and local scales, and the net productivity of these systems, especially in scrub mangrove wetlands. Here, we evaluate decadal foliar Net Primary Productivity (NPP F ) of monospecific Rhizophora mangle scrub mangroves (tree height < 2.5 m) in the lower Everglades (Taylor Slough) to assess the local interactions among hydroperiod, soil stressors regulating plant growth, and soil resources controlling NPP F . Using a combination of leaf-tagging techniques, we determined that scrub mangroves’ NPP F ranged from 0.22 ± 0.01 to 1.12 ± 0.02 MgC ha −1 yr −1 (overall mean: 0.46 ± 0.02 MgC ha −1 yr −1 ) from 2012 to 2021. This productivity is 80% lower than the annual NPP F contribution of R. mangle reported for riverine ecotypes at the same latitude (25° N) within the Everglades (1.8–2.2 MgC ha-1 yr-1). Because these scrub and riverine ecotypes are influenced by the same climate and located in an oligotrophic karstic setting, the lower NPP F in the scrub ecotype is explained by low phosphorus (P) availability in both soil and porewater and long duration of inundation (> 450 h month −1 ), causing high soil sulfide concentrations (0.6–4.0 mM). Low total P concentrations (0.10 ± 0.02 mg g −1 ), both high N:P (range: 79.8 ± 1.6—88.9 ± 5.7) and C:N ratios (128.0 ± 10.4–131.5 ± 8.5) in senescent R. mangle leaves reflect this low fertility when compared to R. mangle leaves in riverine mangroves, where TP availability is higher. Despite the lower scrub mangrove NPP F , the long-term accumulation of leaves and dead wood, forming patchy mounds with higher relative elevation (> 30 cm), enables the establishment of propagules that trigger mangrove expansion into freshwater and brackish areas, thereby outcompeting other vegetation. Based on this colonization and expansion pattern, we propose a conceptual model to explain the current spatial landscape-level configuration of scrub mangroves and to forecast the potential impact of reengineering freshwater flow in Southern Everglades coastal areas.