Global drivers are changing coastal wetland plant composition (identity and cover) leading to increased transitional areas worldwide. Many subtropical salt marshes dominated by low-stature herbaceous species are transitioning to woody mangroves with uncertain ecological outcomes including how changes in dominant plant species affect coastal ecosystem organic matter retention, sediment erosion, biogeochemical processes, and belowground biomass during storms. We experimentally manipulated patch-scale (3 × 3 m) cover of black mangroves (Avicennia germinans) and salt marsh plants (e.g., Spartina alterniflora, Batis maritima) in fringe and interior locations of ten plots (24 × 42 m) to create a gradient in mangrove cover in coastal Texas, USA. Hurricane Harvey, made direct landfall over our site on 25 August 2017, providing a unique opportunity to test how plant composition mitigates hurricane effects on organic matter processing rates. We measured A. germinans and S. alterniflora natural plant material (litter) breakdown on soil surfaces and organic matter standard substrates (green and red tea) in subsurface soils of marsh and mangrove patches pre- and post-hurricane to test for hurricane effects on organic matter breakdown.
Results/Conclusions
Breakdown rates of A. germinans litter and fast- (green tea) and slow-decomposing standard substrates (red tea) were similar pre- and post-hurricane. Post-hurricane, fast-decomposing green tea breakdown rates were more rapid in the interior compared to the wetland fringe, which was similar to pre-hurricane cellulose substrate breakdown. Microbial respiration rates of A. germinans and S. alterniflora litter were similar across marsh and mangrove patches within fringe and interior wetlands. Although organic matter breakdown rates were similar pre- and post-hurricane, spatial patterns of litter mass loss have changed following the hurricane. Pre-hurricane, A. germinans litter had up to 2× higher breakdown rates in mangrove than marsh patches in both fringe and interior wetlands, whereas post-hurricane we measured up to 2× higher A. germinans and S. alterniflora litter breakdown rates in fringe than interior wetlands for both mangrove and marsh patches. Post-hurricane declines in soil nutrients, specifically homogenization of pre-hurricane differences in patch-level soil phosphorus, likely explain a loss of plant identity effects but a gain of location effects on organic matter processing in coastal wetland ecosystems.