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Biology

Room to Grow

Asaeda, Takashi, et al. “Mangrove plantation over a limestone reef–Good for the ecology?.” Estuarine, Coastal and Shelf Science 173 (2016): 57-64.  doi:10.1016/j.ecss.2016.02.017

 

https://en.wikipedia.org/wiki/Ecological_values_of_mangroves

Figure 1: Mangrove Wetland https://en.wikipedia.org/wiki/Ecological_values_of_mangroves

Mangroves (figure 1) are intricate coastal wetland ecosystems in the subtropics and tropics.   They are home to many species. Their intricate root system (figure 2) provides protection for the life they house and for the coastline.   They trap sediment, contaminants, and are a sink for carbon. Mangroves are able to withstand natural intertidal zones. Unfortunately however, they have difficulty surviving the forces of human needs for roads, recreational fishing holes, and land clearing.

Figure 2: intricate root system https-//pixabay.com/en/mangrove-philippines-trees-nature-1227352/

Figure 2: intricate root system https-//pixabay.com/en/mangrove-philippines-trees-nature-1227352/

Mangrove habitats have decreased by greater than one -third (~170,000 km2) since 1996. Recognition of the problem has prompted restoration efforts.   Restoration is not well defined so many attempts have been unsuccessful. Problems attributing to a disappointing results include: inconsistent government regulations, lack of understanding of the underlying problems with the mangrove, blindly planting without accounting for how big seeds will grow, lack of understanding of local sedimentary structures and tidal ranges, and not involving the local communities. Additional, although more unavoidable, hindrances include: seedlings drowning or being eaten by crabs, damage from floating debris and waves, and macro-algae defoliation.

SMART (Specific, Measurable, Achievable, Relevant, and Time-bounded) project planning is a tool that can be helpful is determining the success of a restoration project at providing the ecological needs the area.   The final assessment is done once a mangrove wetland has reached steady state- ideally a few decades after the restoration was initiated.  Unfortunately, not every case of restoration is approached using SMART guidelines which makes assessment of success difficult.

Mangrove wetland restoration project (figure 3) that have implemented SMART have similar designs and results.  One of the keys to their success is that projects were initiated in areas where mangroves already existed. In general the planted trees were much less diverse than the natural system, the overall biomass was greater in the restoration area because the trees were smaller but they were also more concentrated, and planted mangroves lacked a natural mosaic pattern often described of a natural mangrove forest.  Recovery of inhabitants was within five years for fish, provided that tidal creeks were established; longer time spans were needed for other ecosystem members to return.

Figure 3: Mangrove restoration area https-//pixabay.com/en/photos/mangroves/

Figure 3: Mangrove restoration area https-//pixabay.com/en/photos/mangroves/

In general trying to plant mangroves where they would not naturally form is difficult. However, two success stories of multiple decade efforts at restoring mangrove wetlands on a limestone reef are on the islands of Olango and Banacon in the Phillippines.

Banacon is the smaller of the two islands; it is home to ~400 households. For the past 60 years maintenance programs have existed for continuous planting of Rhizophora stylosa.  As a result, 96% of the island is mangrove with plots ranging from 100 to 40,000 m2.  Similar to projects assessed using SMART, the planted mangrove areas on Banacon did not replicate the co-dominance of Avicennia marina and Sonneratia alba found in the natural mangroves.

Olango is about 10 times larger than Banacon. It has sandy beaches, rocky shorelines, mud flats, coral reefs. In the southern end is a bay filled with limestone sediment and shallow water table with seawater like salinity.   Residents and non-governmental organizations have been planting R. stylosa there since the 1980’s.

By studying the planted mangrove systems on Banacon and Olango, investigators seek to determine if 1) the planted mangrove forests impact the surrounding natural areas, and 2) if the diversity in the planted mangrove is vastly difference than the natural system. 

Methods:

Fieldwork was done between October 2011 and April 2014. A geographical distribution map of mangrove plots was created by helicopter. The map was used to determine ecological survey sites.  In the selected survey sites zonation patterns of the natural mangrove systems were established by determining community structure perpendicular to the shoreline in 100m2 quadrants. The planted area was assessed in 100m2 quadrants that were picked at random.

Plants were categorized based on height and trunk thickness as seedlings, saplings, or mature trees. Populations was counted and identified. A range finder was used to determine distance between the ground and the lowest branch, the ground and the canopy, and the ground and the treetop. Girth (circumference) and diameter at 1.3m height were also measured.   Information collected was used to calculate species abundance, percentage occurrence, and concentration. Canopy coverage was determined using an upward facing camera, and software was employed to determine how much light reached the ground.

Results:

On the island of Banacon the planted mangrove area was dominated by R. stylosa. In the natural area scientists found equal dominators: R. stylosa, A. marina, and S. alba, in addition to other species with more minor roles.

On the island of Olango, natural mangroves occurred at low elevations near seawater and high elevations landward. The lower elevations had equal parts R. stylosa and A. Marina, while the high elevation had diverse speciation but was dominated by A. Marina. The planted area in the low elevation had dense young (~10 years old) planted R. stylosa with some S. alba, R. apiculata, and A. marina. The natural mangrove areas were less dense than the planted ones.

On both islands R. Stylosa had a negative relationship with seedlings and saplings of other species.

Discussion:

The answer to the researchers question about if planted areas impact naturals ones is yes.   In examples presented here, the R. stylosa that had been planted in restoration areas seems to cause an increase in R. stylosa populations in near by natural mangrove areas.   Another impact on surrounding areas is related to the shape of the perimeter. The perimeter of the planted mangrove forest is smoother than that of the natural system. The variability in the natural system results in a rigid edge that dissipates wave energy better than the planted systems. Because the planted mangroves do not dissipate energy as well, the surrounding areas will be impacted harder by waves.

The answer to whether there is a difference in diversity between planted and natural mangrove areas is that diversity is much less in planted areas. In general, there is a trend of planting bias towards one dominant species, which plays a huge role in the lack of diversity of planted systems. As a result, development of diverse ecosystems and colonization of other species is hindered. For example, on Olango, areas of less dense R. stylosa trees displayed greater diversity. One problem brought upon by dense populations in planted areas is a canopy so thick that light is prevented from reaching the ground and thus inhibits the next generation of growth; it is usually a problem starting after the first decade of restoration. R. stylosa is practically good at thick sun-blocking canopies because its branches are more tightly packed than species like A. marina and S. alba.

Mangrove planting has economic and ecologic value but it is important that is done right. Well functioning ecosystems should be biologically diverse; in theory, greater diversity means greater resistance to change brought upon by unpredictable circumstances.

If you are planning on implementing your own restoration project it is important to keep in mind that you can improve rehabilitation by planting diversely and assuring sufficient space is provided so the trees have room to grow. You should understand site-specific details so thinning and other responses may be predicted.   It is also important to understand how the system will evolve over multiple decades, and the importance of including features like tidal creeks.

Rehabilitation projects can be very helpful for the natural environment, but if you have not done your research they may not reach their full goodness potential.

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