Paper: Guo et al. 2016. Coastal regime shifts: rapid responses of coastal wetlands to changes in mangrove cover. Ecology. DOI: 10.1002/ecy.1698.
Due to global changes in climate, vegetation communities worldwide are shifting. Ecosystem functions and properties may be altered greatly by shifts between dominant plants with different characteristics. On land, for example, woody plants encroaching on grassland or savanna systems can alter diversity, albedo (the reflective properties of the habitat), temperatures, seedling recruitment, etc. It is likely that shifts from salt marsh grass habitats to mangrove-dominated habitats would similarly alter ecosystems. Coastal wetlands provide ecosystem services, including shoreline protection, carbon storage, and support of higher trophic levels. Threats to these habitats include climate change, rising sea levels, increasing nutrient loads, land-use change, and overfishing. Mangroves are encroaching into salt marshes on a global scale, but this phenomenon has been little studied by comparison to other changes to vegetative communities. Black mangroves have been known to periodically expand into salt marshes during time spans with warm winters but will die back when there are severe freezes. Cover of mangroves in Texas increased by 74% from 1990-2010. They are expected to replace salt marshes along much of the U.S. coast of the Gulf of Mexico before 2100. But we don’t know what that will mean for ecosystem functions.
Researchers conducted a manipulative experiment in which they varied the mangrove cover in 10 plots that measured 24 m by 42 m (79 ft by 137 ft). These plots were all located on Harbor Island, Port Aransas, TX. Each plot had 112 cells (3m by 3m or 9ft by 9ft), each of which was cleared of mangroves to create plots ranging in mangrove cover from 0% to 100%. The researchers monitored microclimate and plant community composition, collected soil cores, measured light intensity, surveyed wrack (the deposition of algae, plant leaves, and seagrass), and measured sediment deposition. They also conducted bird counts.
Scientists observed an inverse relationship between wind speed and mangrove cover: when wind speed was highest, the mangrove cover was lowest. Light interception increased as mangrove cover increased. The relationship between mangrove cover and temperature was variable; the average air (at a level 1 m above the ground) and soil temperatures increased and then decreased with increasing cover, with a max at 50-70% cover.
Salt marsh plants expanded in gaps between mangroves. After two years, marsh vegetation cover was 80% at 0% mangrove cover, but dropped to 20% at 50% mangrove cover, then declined more gradually at higher cover. Beta diversity (a measure of the differentiation of species between habitats) increased with mangrove cover, peaking when mangrove cover was 22%, and then gradually declining.
Wrack decreased from 13% to 2% as mangrove cover increased. Where mangrove cover was <30% wrack penetrated further into plots than when mangrove cover was greater.
Sediment accretion declined steadily with mangrove cover as did soil organic content.
The number of birds observed declined as mangrove cover increased, with a rapid loss between 0% and 33% mangrove cover then plateauing. Most of the birds observed were herons, rails, ibis, and sandpipers.
Clearly changes in mangrove cover that can occur quickly with freezes or gradually as mangroves encroaching into salt marshes have an impact. In comparison to grassy salt marsh plants, mangrove trees are tall and woody and therefore act as more of a windbreak and buffer for wave energy contributing to shoreline protection. With mangrove cover of just 30%, the majority of wind attenuation occurred. Light interception increased with mangrove cover, meaning that the understory received little light when mangrove cover was high. The hump-shaped relationships of air temperature and soil temperature to mangrove cover are likely due to the loss of cooling from wind combined with the addition of cooling from increased shade.
Mangroves normally outcompete salt marsh plants, so when mangroves are maintained at lower cover, salt marsh plants can expand into the open space. However, at intermediate cover of mangroves, salt marsh plants didn’t expand as much as expected, so it can be inferred that mangroves inhibit salt marsh plant growth beyond the extent of their canopies. This may be achieved through shadow-casting or their root systems. Diversity being greatest at intermediate cover of mangroves suggests that complete encroachment will greatly reduce community diversity.
The decrease in wrack with increasing mangrove cover suggests fringe mangroves trap wrack deposits, which may influence where nutrients are deposited from the decomposition of the wrack. With more nutrients in the fringe zone, those trees could grow higher, produce more leaves, and extend root systems. The extra leaves that fall and the denser roots could lead to changes in sediment accretion.
The authors admit some complications with the results of their bird counts based on scale, but that the decrease in birds as mangroves increase has been seen before.
The researchers speculate that many of the results may be linked. For example, both increased wrack trapping and higher temperatures with higher mangrove cover could increase the organic matter content in soil.
Shifts between vegetation communities can change wetland characteristics and functions. Managers should consider some of these results, especially relating to intermediate states and non-linear relationships, both of which may allow for conservation goals to be achieved in areas that are experiencing shifts. It will depend upon the priorities of the managers though how to maintain (or not maintain) mangrove cover in their area. Many areas will warrant further study, such as the impacts of changing vegetation communities on their inhabitants and the specific effects on various ecosystem services. But as usual, this work gets us a few steps closer to understanding.