PS 7-72
Mangrove encroachment into salt marshes may enhance carbon storage but reduce surface accretion in coastal wetlands

Monday, August 10, 2015
Exhibit Hall, Baltimore Convention Center
Sean P. Charles, Florida International University, Miami, FL
John S. Kominoski, Florida International University, Miami, FL
Anna R. Armitage, Department of Marine Biology, Texas A&M University at Galveston, Galveston, TX
Hongyu Guo, University of Houston, Houston, TX
Carolyn A. Weaver, Department of Life Sciences, Texas A&M University - Corpus Christi, Corpus Christi, TX
Steven C. Pennings, Department of Biology and Biochemistry, University of Houston, Houston, TX
Ashley Whitt, Department of Marine Biology, Texas A&M University at Galveston, Galveston, TX

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.