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3 Functions of Coastal Forests Against Tsunami

3 Functions of Coastal Forests Against Tsunami

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O. Ichihashi et al.

Fig. 23.5 The result of belt transects survey in Idoura, Miyagi Prefecture (After BCMoE



Shorelines and Land Cover Change of Coasts

The earthquake and tsunami had a great impact on the coast as shown by the recession of many shorelines due to the tsunami and the loss of sandy beaches and coastal

forests behind them. Much of the shoreline which had receded showed some signs

of recovery, while the recovery was slow at other coasts such as river mouth bars

and pocket beaches.

Figure 23.6 shows the land cover areas of sand dunes, sand dune vegetation,

coastal forests, and coastal artificial structures in the beaches (680.5 km) in 2000s

(before the earthquake), 2011 (just after the earthquake), and 2014. Compared with

in 2011 and in 2014, the land cover statuses of sand dunes, sand dune vegetations,


Survey of Impact of the Great East Japan Earthquake on the Natural Environment…






ha 3 000




Sand dunes Sand dune


2000s (Before GEJE)






2011 (After GEJE)


Fig. 23.6 Change of land cover status behind the shorelines of the beaches in the tsunami-struck

natural coast (After BCMoE 2013, 2014, 2015, forthcoming)

and coastal forests were reduced respectively by 225 ha, 92 ha, and 496 ha. On the

other hand, the areas of coastal artificial structures (e.g., tide embankments) were

increased by 331 ha during the same period.


Seagrass and Seaweed Beds

Heavy impact due to the GEJE was given to the sandy and/or muddy seabed, and the

majority of seagrass beds were lost because seagrass generally grow on the sediment of sand and silt (Fig. 23.7). Despite this, by 2015, some recoveries of seagrass

beds were observed in various parts of the coasts with rather dense cover of seagrass. In contrast, seaweed beds, which generally grow on rocks and stones, were

little affected by the tsunami and other events (Fig. 23.7).


Changes at Specific Sites of Monitoring Site 1000 Sites

Tidal Flats in the 18 Sites

The tsunami caused by the GEJE carried away a large number of living organisms

from tidal flats, disrupted or destroyed reed beds, and reduced the areas of tidal flats.

As a result of this survey, the number of species found increased in many sites in


O. Ichihashi et al.

Fig. 23.7 Accumulated

distribution areas of

seagrass (Zostera spp.) and

seaweed (Phaeophyceae

spp.) beds in Iwate

Prefecture to northern

Fukushima Prefecture

before the GEJE, the

satellite images and aerial

photographs were acquired

in 2003 to 2011, and after

GEJE, the satellite images

and aerial

photographs were acquired

in 2011 to 2015 (After

BCMoE forthcoming)




ha 3,000


1 000


Seagrass beds

Before GEJE

Seaweed beds

After GEJE

2015, though the community compositions at some sites were very different from

those before the earthquake (Fig. 23.8). Furthermore, in contrast with the recovery

of the number of species, the population densities have not recovered since 2012 in

many sites. It seemed to take some more time until the populations recover to the

level as before the earthquake.

Seagrass Beds in the Five Sites

The tsunami had carried seagrass away with the sediments of sand and silt where

seagrass generally grow on with rootstocks spread on the seabed. Moreover, the

change of light conditions due to the ground subsidence also influenced the growth

of the remained seagrass. Thus, even after the earthquake, the coverage of the seagrass has not generally recovered in the five sites although the coverage has increased

in some other sites. It revealed that large variation of recovery processes exists

among the seagrass beds in the tsunami-flooded area.

Seaweed Beds in the Four Sites

Unlike seagrass, seaweed grows firmly fixed to rocks and stones with rhizoid, and

the tsunami rarely carried them away in the four sites. Even if their phyllodes are

torn away, they are able to regenerate soon. However, it was observed that the numbers of species and species compositions were changed in some sites after the


Survey of Impact of the Great East Japan Earthquake on the Natural Environment…

































Fig. 23.8 Comparison of the number of species of benthos at various tidal flat sites before and

after the earthquake. The data acquired in 7th National Surveys on the Natural Environment

(BCMoE 2007) was used as the pre-earthquake data and the data acquired in this monitoring survey (BCMoE 2013, 2014, 2015, forthcoming) was used as the post-earthquake data (the number of

species in qualitative research)


O. Ichihashi et al.

earthquake. Overall, the coverage of seaweed in 2015 was similar to or larger than

before the earthquake in the surveyed sites.

Seabird Breeding

The GEJE occurred in March, not in a breeding season for seabirds in the four surveyed sites. Therefore, the earthquake and tsunami seemingly had no direct impact

on seabirds although soil runoff and vegetation changes, which might disturb the

seabird by changing the environments for breeding, were observed in some parts of

the surveyed islands. The indirect impacts from these changes (i.e., soil runoff and

vegetation changes) on seabirds are still unclear. Thus, further monitoring is needed.


Important Habitat Maps

Based on the results of the conducted surveys and experts interviews, “Important

Habitat Maps” (Fig. 23.9) were created to show the statuses of the natural environment after the earthquake focusing on the important habitats for living organisms in

order to conserve the natural environment. The maps define 11 habitats4 as “important habitats” in the tsunami-flooded areas from four perspectives.5

Since it was urgently required to identify the important environment that need

consideration and utilize this information in the reconstruction projects, the

“Important Habitat Maps” were first created in March 2014 based on the survey

results obtained in 2012. This information was updated as “Important Habitat Maps

2015” in March 2016 with in enlarged area (i.e., Fukushima Prefecture).


Information Platform

All of the data sets of the series of surveys (Table 23.1) are opened to access at the

web site, “Shiokaze Natural Environment Log” .

These data sets suggested that organisms and ecosystems were seemed to be

rather adapted to the natural disturbance, such as earthquake and tsunami, than devastated, showing quite rapid recovery, although they are still in the middle of recovery process.


Eleven important habitat: (1) seaweed beds, (2) seagrass beds, (3) tidal flats, (4) sand dune (sandy

beach and sand dune vegetations), (5) natural vegetation in coastal cliff areas, (6) remind forest

areas, (7) formerly forested area (mosaic-like varied disrupted environment), (8) marsh (salty and

freshwater marsh vegetation), (9) grassland (seminatural grassland), (10) uncultivated paddy field,

and (11) open waters (e.g., rivers and lakes).


Four perspectives: (1) important places for inhabitation and growth of rare species, (2) places with

high biodiversity, (3) places with high potentials of nature, and (4) important places to interact with



Survey of Impact of the Great East Japan Earthquake on the Natural Environment…


Fig. 23.9 Important habitat map (first edition of Sendai Bay Coastal Area) (After BCMoE 2014)

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