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Mangrove trees are invading saltmarshes at subtropical ecotones globally, but the consequences of this vegetation shift for ecosystem sustainability remain unknown. Using the Coastal Wetland Equilibrium Model (CWEM) to simulate vegetation survival and sediment accretion, we predict that black mangroves,Avicennia germinans, can build soil elevation by 8 mm yr⁻¹, four times greater than saltmarshes at the same site, a finding that is broadly consistent with field measurements of elevation change. Mangroves build elevation more rapidly than saltmarshes by producing much greater live and labile belowground biomass, but when mangroves drown, they abruptly lose elevation due to the large volume of quickly decomposing necromass following flood‐induced mortality. Under certain conditions, young mangroves can accumulate root mass faster than mature trees and, therefore, gain elevation more rapidly, but neither saltmarshes nor mangroves of any age survived a centenary sea‐level increase of 100 cm. The acceleration of sea‐level rise that coastal marshes are encountering raises the question of how coastal wetlands should be optimally managed and these results provide managers with predictive information on wetland building capacity of mangroves versus marshes.

 

Aim: Global change is expected to modify the magnitude and trajectory of organic matter decomposition in mangrove ecosystems. Yet, the degree and direction of that change is unknown, especially considering the large C storage potential that mangroves provide. We performed a systematic review of primary literature to examine the relationships between genus-specific litter quality, latitude or other global change proxies and decomposition of mangrove litter fractions. Location: Global. Time Period: 1976–2021. Taxon: Mangroves. Methods: We compiled a dataset of 480 decomposition rates, including species, litter fraction, latitude, and relevant biophysical data. We investigated the influence of genera, tissue type, latitude, and global change proxies on decomposition rates using linear models and qualitative approaches. We also performed calculations to determine the potential importance of the decomposition process on the root litter biomass C pool in the context of blue C significance. Results: Collectively, latitudinal relationships suggest that factors other than temperature, such as tissue type and genus, may regulate decay rates within mangroves' distributional range. Decay rates of leaf litter, roots, and wood converged on a value of 0.009 ± 0.0005, 0.002 ± 0.0001, and 0.001 ± 0.0003, respectively, across continents and geomorphological settings. Our calculations suggest that small changes in decomposition rate will not elicit large changes in blue C storage potential. Conclusions: The main drivers behind variability in mangrove biomass decay rates detected across the distributional range remain uncertain. However, the small latitudinal range that mangroves inhabit and the submerged environment within which litter decomposes suggest that decay depends on species-specific responses or biotic interactions among species to global change drivers. Few studies have examined global change impacts directly, and variability in decay and lack of representation of some mangrove groups in the literature suggest that implications for blue C are important to consider. 

Nutrient enrichment is a major driver of environmental change in mangrove ecosystems. Yet, nutrient enrichment impacts on physiological processes that regulate CO2 and water fluxes between mangrove vegetation and the atmosphere remain unclear. We measured peak growing season photosynthesis (A) and respiration (R) in black mangrove (Avicennia germinans) leaves that had been subjected to long-term (8-year) nutrient enrichment (added N, added P, control) in north Florida. Previous results from this site indicated that Avicennia productivity was N-limited, but not P-limited. Thus, we expected that N addition would increase light saturated net photosynthesis at ambient CO2 (Anet), intrinsic water-use efficiency (iWUE), maximum rate of Rubisco carboxylation (Vcmax), and leaf dark respiration (R), while P addition would have little effect on any aspect of photosynthesis or respiration. We expected that increased photosynthesis and respiration would be most apparent immediately after N addition and in newly formed leaves. Indeed, Anet and Vcmax increased just after N addition in the N addition treatment; these increases were limited to leaves formed just after N addition. Nonetheless, over time, photosynthetic parameters and iWUE were similar across treatments. Interestingly, R measured at 25 °C increased with N addition; this effect was consistent across time points. P addition had little effect on R. Across treatments and time points, Vcmax,25 (Vcmax standardized to 25 °C) showed no relationship with R at 25 °C, but the maximum rate of electron transport for RuBP regeneration standardized to 25 °C (Jmax,25) increased with R at 25 °C. We conclude that N addition may have small, short-lived effects on photosynthetic processes, but sustained effects on leaf R in N-limited mangrove ecosystems.