A general pattern of trade-offs between ecosystem resistance and resilience to tropical cyclones.

Tropical cyclones drive coastal ecosystem dynamics, and their frequency, intensity, and spatial distribution are predicted to shift with climate change. Patterns of resistance and resilience were synthesized for 4138 ecosystem time series from n = 26 storms occurring between 1985 and 2018 in the Northern Hemisphere to predict how coastal ecosystems will respond to future disturbance regimes. Data were grouped by ecosystems (fresh water, salt water, terrestrial, and wetland) and response categories (biogeochemistry, hydrography, mobile biota, sedentary fauna, and vascular plants). We observed a repeated pattern of trade-offs between resistance and resilience across analyses. These patterns are likely the outcomes of evolutionary adaptation, they conform to disturbance theories, and they indicate that consistent rules may govern ecosystem susceptibility to tropical cyclones.

Quantifying how changing mangrove cover affects ecosystem carbon storage in coastal wetlands.

Despite overall global declines, mangroves are expanding into and within many subtropical wetlands, leading to heterogeneous cover of marsh-mangrove coastal vegetation communities near the poleward edge of mangroves' ranges. Coastal wetlands are globally important carbon sinks, yet the effects of shifts in mangrove cover on organic-carbon (OC) storage remains uncertain. We experimentally maintained black mangrove (Avicennia germinans) or marsh vegetation in patches (n = 1,120, 3 × 3 m) along a gradient in mangrove cover (0-100%) within coastal wetland plots (n = 10, 24 × 42 m) and measured changes in OC stocks and fluxes. Within patches, above and belowground biomass (OC) was 1,630% and 61% greater for mangroves than for recolonized marshes, and soil OC was 30% greater beneath mangrove than marsh vegetation. At the plot scale, above and belowground biomass increased linearly with mangrove cover but soil OC was highly variable and unrelated to mangrove cover. Root ingrowth was not different in mangrove or marsh patches, nor did it change with mangrove cover. After 11 months, surface OC accretion was negatively related to plot-scale mangrove cover following a high-wrack deposition period. However, after 22 months, accretion was 54% higher in mangrove patches, and there was no relationship to plot-scale mangrove cover. Marsh (Batis maritima) leaf and root litter had 1,000% and 35% faster breakdown rates (k) than mangrove (A. germinans) leaf and root litter. Soil temperatures beneath mangroves were 1.4°C lower, decreasing aboveground k of fast- (cellulose) and slow-decomposing (wood) standard substrates. Wood k in shallow soil (0-15 cm) was higher in mangrove than marsh patches, but vegetation identity did not impact k in deeper soil (15-30 cm). We found that mangrove cover enhanced OC storage by increasing biomass, creating more recalcitrant organic matter and reducing k on the soil surface by altering microclimate, despite increasing wood k belowground and decreasing allochthonous OC subsidies. Our results illustrate the importance of mangroves in maintaining coastal OC storage, but also indicate that the impacts of vegetation change on OC storage may vary based on ecosystem conditions, organic-matter sources, and the relative spatiotemporal scales of mangrove vegetation change.

Nutrient enrichment shifts mangrove height distribution: Implications for coastal woody encroachment.

Global changes, such as increased temperatures and elevated CO2, are driving shifts in plant species distribution and dominance, like woody plant encroachment into grasslands. Local factors within these ecotones can influence the rate of regime shifts. Woody encroachment is occurring worldwide, though there has been limited research within coastal systems, where mangrove (woody shrub/tree) stands are expanding into salt marsh areas. Because coastal systems are exposed to various degrees of nutrient input, we investigated how nutrient enrichment may locally impact mangrove stand expansion and salt marsh displacement over time. We fertilized naturally co-occurring Avicennia germinans (black mangrove) and Spartina alterniflora (smooth cordgrass) stands in Port Aransas, TX, an area experiencing mangrove encroachment within the Northern Gulf of Mexico mangrove-marsh ecotone. After four growing seasons (2010-2013) of continuous fertilization, Avicennia was more positively influenced by nutrient enrichment than Spartina. Most notably, fertilized plots had a higher density of taller (> 0.5 m) mangroves and mangrove maximum height was 46% taller than in control plots. Fertilization may promote an increase in mangrove stand expansion within the mangrove-marsh ecotone by shifting Avicennia height distribution. Avicennia individuals, which reach certain species-specific height thresholds, have reduced negative neighbor effects and have higher resilience to freezing temperatures, which may increase mangrove competitive advantage over marsh grass. Therefore, we propose that nutrient enrichment, which augments mangrove height, could act locally as a positive feedback to mangrove encroachment, by reducing mangrove growth suppression factors, thereby accelerating the rates of increased mangrove coverage and subsequent marsh displacement. Areas within the mangrove-marsh ecotone with high anthropogenic nutrient input may be at increased risk of a regime shift from grass to woody dominated ecosystems.