Mangrove encroachment into salt marshes may enhance carbon storage but reduce surface accretion in coastal wetlands
Coastal wetlands are increasingly recognized as carbon (C) sinks capable of storing more C per area than all other ecosystems, particularly in their soils. Warmer temperatures are facilitating poleward expansion of mangroves into salt marshes worldwide. Changes in soil C storage and accretion associated with vegetation state changes are critically important in predicting how coastal wetlands will adapt to sea-level rise and sequester C in the future. We maintained patch-scale (3 ´ 3 m) vegetation (marsh or mangrove) along a gradient of plot-scale (24 ´ 42 m) mangrove density in a coastal wetland (Port Aransas, Texas, USA). We isolated the effects of vegetation state change from abiotic conditions to test how changes in function scale from patch-to-plot-scale.. We tested for differences in microclimate [photosynthetically active radiation (PAR), wind and temperature], soil processes (cellulose and wood breakdown, accretion) and soil stocks [belowground biomass and soil organic matter (SOM)].
Plant canopy in mangrove patches intercepted more PAR (87.66 ± 3.46 %) than marsh patches (7.62 ± 3.46 %; P<0.001), while soil temperature displayed a hump-shaped relationship with plot-scale mangrove density (P = 0.01; r2=0.63). Breakdown rates (k d-1) of cellulose were lower in mangrove (0.005 ± 0.0007) than marsh (0.006 ± 0.0007) patches (P = 0.047), and wood k decreased with increasing plot mangrove density (P = 0.013; r2=0.39). Root biomass (marsh: 1051.86 ± 222.32 g m-2, mangrove: 2131.97 ± 257.47 g m-2; P < 0.001) and percent SOM (marsh: 8.1 ± 0.46%, mangrove: 10.62 ± 0.77%; P = 0.001) were higher in mangrove than marsh patches. Sediment accretion rates were similar in marsh and mangrove patches (0.02 ± 0.003 mm d-1 ;P > 0.05), but decreased with increasing plot mangrove density (P = 0.006; r2= 0.17). Patch-level k was not explained by PAR, root biomass, or organic matter, but wood breakdown was positively related to surface accretion (P=0.008; r2=0.16). Our results indicate that mangroves alter microclimate, decrease organic matter breakdown and increase soil carbon stocks, leading to increased C retention at multiple spatial scales, but also lead to a linear reduction in surface accretion rates at the plot scale.