図 7: 勾配の緩急と風況の違い

4.2 砂丘斜面を吹く風の進行経路について

堀田ら（2000）は風洞実験で，堤体風上側斜面

勾配を変えた場合（風下側斜面勾配は 1:1 に固定）

の風速比分布を明らかにした．これは，著者らの

風向 90°の実験にあたる．その結果によると，風

上側の斜面勾配が 1:1 と 1:5 の場合，堤体風下側

の風速比 0.4 以下の範囲がそれぞれ 12h と 10h で

あった．斜面勾配が緩くなることで防風範囲が 2h小さくなったことを示す．

著者らの実験では，X-Z 断面上の斜面勾配が

1:1.7（風向 90°）と 1:4.4（風向 22.5°）のとき，

堤体風下側の風速比 0.5 以下の範囲はそれぞれ

13.5h と 0h である（表 1）． 著者らの風向 90°と堀田らの風上側斜面勾配

1:1 の結果は，ほぼ同程度の防風効果である．し

かし風向 22.5°のときは X-Z断面上の風上側斜面

勾配は同程度にも関わらず，防風効果には大きな

差が生じた．この理由は，砂丘に斜め方向から接

した風は，風上側斜面を斜行して丘頂へ吹いたた

めと推測できる．萩野ら（2008）が海岸の砂丘上

で，風紋の向きから風の流れ方向を推定した例が

ある．それによると，風が砂丘に対して斜め方向

から吹いた後の風上側斜面では，砂丘に沿う風の

方向成分が見られた．今回の実験でも，風上斜面

を吹いた風の進行経路は，主流方向に沿った斜面

長 Sd-a ではなく，Sd-b のような斜行する長い経路

であったと推測する．そのため風の進行経路はよ

り緩勾配になり，砂丘風上側の風速を維持したま

ま風下側へ流れたと考える（図 8）． また堀田らの実験では，風下側の勾配が 1:1 と

比較的急勾配に固定されてあった．このことも防

風範囲が維持される要因であった可能性がある． なお風向が斜めのとき，丘頂の風速ベクトルが

沿岸負方向へ向いたことと，風向 67.5°のとき丘

頂の風速比が沿岸方向で大きく異なった理由なら

びにそのことが風下側の風況に与えた影響につい

ては，今回の実験だけでは不明であった．

図 8: 風上斜面の風の進行経路（推測）

5 おわりに

本研究では風が斜めから吹くときに，防風効果

が小さくなるメカニズムを示した．堀田ら（2000）の実験結果と比較することで，風が斜めから吹く

場合の防風効果は，X-Z 断面上の斜面勾配だけで

は推定できないことが示唆された．次の課題とし

て，以下の項目が考えられる． ① 風上側斜面での風況把握 ② 斜めからの風のとき，丘頂上の風が Y 軸負側

に傾く現象の解明 ③ 風下側の斜面勾配が防風効果に与える影響

以上については，今後数値シミュレーションで

確認したい． 最後に，茨城県鹿行農林事務所林業振興課の

方々には，波崎海岸の砂丘形状の情報を提供して

いただいた．また匿名査読者の意見は，本論の内

容を改善する上で必要なものであった．以上の

方々へ，心よりお礼申し上げる．

引用文献

[1] 萩野裕章・野口宏典・坂本知己（2008）：風食が進

んだ人工砂丘上の飛砂集中過程，日本海岸林学会誌，

7(2)，27-32 [2] 萩野裕章・野口宏典・島田和則・坂本知己（2010）：

風洞実験による人工砂丘防風効果範囲の推定，九州

森林研究，No.63，134-136 [3] 堀田新太郎・畑中勝守・田中寛好・小泉圭右・大

塚香織（2000）：人工砂丘周辺の風の場による砂丘

形状決定に関する研究，砂防学会誌，53(2)，22-33 [4] 樫山徳治・松岡広雄・佐伯正夫（1971）：人工砂丘

の風速減少作用の一例，日林講，82，269-271 [5] 根本茂（1967）：局地風を対象とした風洞模型実験

の相似則，農業気象 22(3)，129-136 [6] 大儀健一・佐藤弘史（2000）：台形周りの風況特性

に関する検討，土木学会第 55 回年次学術講演会，

I-B14

Temporal changes in vegetation and soil environment caused by volcanic activity of Mount Sakurajima

Yukiyoshi Teramoto1* and Etsuro Shimokawa1

Abstract: We examined the temporal changes in vegetation and soil environment caused by volcanic activity of Mount Sakurajima via field investigations and interpreting aerial photographs taken in 1966, 1974, 1984, 1996 and 2006. The proportions of conifer and broadleaf tree areas and the timberline altitude in the study area decreased from 1966 to 1996, subsequent to volcanic activity commencing in 1972. The timberline altitude rose slowly from 1997 to 2006 because of vegetation recovery with the ebb of volcanic activity. The proportions of conifer and broadleaf tree areas decreased little after 1997. The thickness of tephra deposited since 1972, and the dry density and pH of the tephra surface layer, decreased with increasing distance from Minamidake crater.

1 Introduction Vegetation on the hillslope and coastal area of Mount Sakurajima has declined greatly due to the impact of long-term volcanic activity since 1955. Because of the decline of vegetation, the hillslope have experienced accelerated erosion, and consequently debris and mud flows have often occurred in the rivers emanating from them (Shimokawa and Jitousono, 1987). Erosion and debris and mud flows have damaged vegetation and vegetation habitat on the hillslope and coastal areas. Moreover, because of the decline of coastal vegetation, damage from storms and salt has occurred on farms and orchards located on the lower slopes of Mount Sakurajima (Sumi and Suzuki, 1998). The volcanic activity of Mount Sakurajima has also had a great impact on the vertical distribution of vegetation on hillslopes (Teramoto and Shimokawa, 2007). Teramoto and Shimokawa (2007) conducted a vegetation survey ranging from 500 m to 220 m above sea level (a.s.l.) on the northern flank of Mount Sakurajima. They showed that in 2007 vegetation had been severely impacted to around 220 m a.s.l., compared with 1963 when vegetation was impacted slightly by volcanic activity. Vegetation succession in 1963 was more advanced than in 2007. The purpose of this study is to study temporal vegetation and soil environment changes caused by the volcanic activity of Mount Sakurajima. The vegetation decline caused by volcanic activity on hillslopes and in the coastal area is related closely the movement of eroded sediment transported to lower slopes via debris and mud flows. Therefore, clarifying the impact of volcanic activity on the distribution of vegetation and its

habitat from hillsides to the coastal areas of Mount Sakurajima will aid the conservation of hillside and coastal ecosystems. 2 Study area and methods The study area is located on the northern flank of Mount Sakurajima (Figure 1). The area is 2.28 km2 with an altitude ranging from 230 m to 1117 m a.s.l.. The topography lower than 300 m a.s.l. comprises hillslopes and plateaus. Topography from 300 m to 500 m a.s.l. consists of hillslopes and steep slopes. The topography at 500 m a.s.l. and higher is steep slopes. The geology of the study area consists of a 1914 Taisho pumice layer, covered with a soil layer and tephra.

Mount Sakurajima

Minamidake crater

NN

1 km0

Mt. Kitadake

Mt. Minamidake

Soil profile Study area

Figure 1: Location of the study area

Aerial photography was used to clarify temporal changes in vegetation and timberline caused by volcanic activity, and a distribution map of the vegetation was made. Vegetation is divided into conifer, broadleaf tree,

1Faculty of Agriculture, Kagoshima University, 1-21-24 korimoto, Kagoshima 890-0065, Japan *Corresponding author:

－ 58 － － 59 －

海岸林学会誌 ( Journal of the Japanese Society of Coastal Forest ) 9（2）：59 － 62, 2010ARTICLE

herb and bare areas. A distribution map of gully erosion scars was also made. Five pairs of aerial photographs taken in 1966, 1974, 1984, 1996 and 2006 were used for the analysis. The areas of conifer, broadleaf tree, herb, bare land and gully erosion scars were calculated using a distribution map produced from aerial photograph interpretation. Timberline altitude was determined using a distribution map produced by aerial photograph interpretation and a 1:25,000 topographical map produced by the Geographical Survey Institute. Soil and tephra investigations were conducted in December 2009. Surface soil tests and pH measurements were conducted on samples collected from the tephra layer on slopes covered by broadleaf trees. The investigation of tephra distribution and deposition was conducted through observations of eight soil profiles (Figure 1). Soil profiles were sited 2.40 km to 2.95 km from the Minamidake crater of Mount Sakurajima (Figure 1), and were sited on near flat slopes where erosion processes were limited to sheet erosion. According to tephra characteristics such as color, density, hardness and humus content based on the observation of the soil profile, tephra deposition since the 1914 Taisho eruption was divided into two layers (Shimokawa and Jitousono, 1987; Teramoto et al., 2005). The upper tephra layer accumulated since 1972, and the lower layer accumulated from 1914 to 1971 (Shimokawa and Jitousono, 1987). The thickness of tephra deposited since 1972 was measured, and surface dry density was determined using undisturbed cores collected in 55 mm in diameter/60 mm height metal cylinders. pH measurement of the tephra surface layer was conducted as follows. An air-dried sample and pure water were packed in a container. The air-dried sample to pure water weight ratio was 1:2.5. After the container was shaken, pH was measured using a pH meter. 3 Results and discussion 3.1 Temporal changes in vegetation and timberline

caused by volcanic activity Figure 2 shows the distribution maps of conifer, broadleaf tree, herb and bare land, and gully erosion scars derived from interpretation of aerial photographs taken in 1966, 1974, 1984, 1996 and 2006. Conifer, broadleaf tree, and herb and bare land were distributed on the lower, middle and upper slopes of Mount Sakurajima, respectively. The area of broadleaf trees decreased with increase of herb vegetation and bare land. The extent of the gullies increased considerably over this time. Figure 3(a) shows the temporal change in annual

frequency of eruptions from Mount Sakurajima in the period 1955 to 2006. Mount Sakurajima has been active continuously with frequent and lively small-scale ash eruptions since 1955. The volcanic activities of the 1950s and 1960s were relatively gentle. However, since 1972, the activity has become vigorous. Mount Sakurajima erupted annually from 1974 and 1986 more than 200 times. Since 1999, the frequency of eruptions has decreased to about 20 events per year (Kagoshima local meteorological observatory, 1955-2006).

1966 1974 1984

1000800

600

400m

1000 1000

1000 1000

1996 2006

800

800

800 800

600

600

600

600

400m

400m400m

400m

NN

1 km0

Mt. Kitadake Peak

Study area

Gully erosion

Conifer

Broadleaf tree

Bare land and herb

Figure 2: Distribution maps of vegetation

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 20050

100

200

300

400

500

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

400

600

800

1000

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 20050

20

40

60

Gully erosion

Conifer

Bare land and herb

Broadleaf tree

Alti

tude

of

timbe

rlin

e (m

)Pr

opor

tion

in th

e st

udy

area

(%)

Freq

uenc

y of

exp

losio

n

Year

(a)

(b)

(c)

Figure 3: Temporal changes in eruptions from Mount

Sakurajima (a), altitude of timberline (b), and proportion of vegetation area (c)

－ 60 － － 61 －

Figure 3(b) and Figure 3(c) show the temporal changes in timberline altitude, and the areas of conifer, broadleaf tree, herb and bare land, and gully erosion, respectively. The timberline altitude in 1966 was 1000 m, with relatively gentle volcanic activity. Since 1966 hillside vegetation has been damaged or destroyed because of tephra and gas from the Minamidake crater, and the timberline altitude has lowered. The area of gully erosion scars in has increased from 1966 to 1996 in parallel with volcanic activity, and the proportions of conifer and broadleaf tree areas decreased. From 1966 to 1996 the proportion of herb and bare land areas and gully erosion scars increased. From 1997 to 2006 vegetation on hillslopes was restored and the altitude of timberline rose slowly due to a decrease in tephra and gas resulting from an ebb in volcanic activity. After 1997, decrease in the areas of conifer and broadleaf trees, and increase in herb and bare areas and gully erosion scars were negligible. 3.2 Temporal changes in vegetation soil habitat

caused by volcanic activity Figure 4 shows the thickness of tephra deposition since 1972 based on distance from the Minamidake crater. Tephra thickness decreased with increasing distance from the Minamidake crater. Tephra thickness was between 65.5 cm and 110 cm over the distance 2.40 km to 2.95 km from the eruptive crater, with a regression analysis result (distance vs. thickness) of 0.61 (significance level of 1 %).

Distance from the Minamidake crater (km)

Thi

ckne

ss o

ftep

hra

(cm

)

2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.150

60

70

80

90

100

110

120

Figure 4: Thickness of tephra since 1972 according to

distance from Minamidake crater Figure 5 shows the tephra surface layer dry density since 1972 according to distance from Minamidake crater. Dry density decreased with increasing distance from Minamidake crater, reflecting an increase in void ratio with increased distance. This is thought to be due to

root growth by recovering vegetation following the decrease in volcanic activity (Teramoto and Shimokawa, 2007). According to the vegetation characterization at the soil survey sites (Figure 1), the dominant tree species (tree height is no less than 3 m) was Lithocarpus edulis and the chief tree shrub species (tree height is less than 3 m) were Eurya japonica Thunb. and Ligustrum japonicum. Tree height and breast height diameter of the dominant tree species and shrub layers increased with increasing distance from Minamidake crater. Dry density of the surface layer was between 1.25 g cm-3 and 1.89 g cm-3 from 2.40 km to 2.95 km from Minamidake crater. Regression analysis showed that the correlation coefficient between distance from Minamidake crater and surface layer dry density was 0.71, with an ANOVA statistical significance and correlation of 0.1%. Shimokawa and Jitousono (1987) showed tephra thickness and tephra surface layer dry density accumulated in the period 1972 to 1983 decreased with increasing distance from the Minamidake crater. These results are similar to the results of the present study.

Distance from the Minamidake crater (km)

Tep

hra

surf

ace

laye

r dr

y de

nsity

(g/c

m3 )

2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.11.0

1.2

1.4

1.6

1.8

2.0

Figure 5: Dry density of tephra surface layer according

to distance from the Minamidake crater Figure 6 shows post-1972 tephra surface pH according to distance from Minamidake crater. The pH of the surface layer decreased with increasing distance from Minamidake crater. The pH of the surface layer was between 3.71 and 4.38 over the distance 2.40 km to 2.95 km from the Minamidake crater. Regression analysis showed that the correlation coefficient between distance from the Minamidake crater and pH of the tephra surface layer was 0.60, with a statistical significance of 1 %. Fujita and Nakata (2001) showed that as vegetation succession progressed, soil pH decreased. According to the vegetation survey at the soil survey sites (Figure 1), vegetation succession progressed with increasing distance from Minamidake crater.

－ 60 － － 61 －

Distance from the Minamidake crater (km)

pH o

f sur

face

laye

r

2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.13.4

3.6

3.8

4.0

4.2

4.4

4.6

Figure 6: pH of surface layer according to distance from

Minamidake crater 4 Conclusions The results of the present study are as follows. (1) Timberline altitude lowered as Mount Sakurajima volcanic activity commenced from 1972. The timberline rose slowly from 1997 to 2006 because of the recovery of vegetation caused by ebb in volcanic activity. (2) According to the vegetation survey around the soil survey sites, tree height and breast height diameter of the main tree species in the tree and shrub layers increased with increasing distance from Minamidake crater, and vegetation succession progressed with increasing distance. (3) The thickness of tephra, and the dry density and pH of the post-1972 tephra surface layer decreased with increasing distance from Minamidake crater.

References [1] Fujita, E. and Nakata, M. (2001): Changes in vegetation

and soil properties caused by mixture of deciduous broad-leaved trees in Japanese black pine (Pinus thunbergii Parl.) stand at coastal sand dune: A case study in Kaetsu district, Niigata prefecture, Jour. Jap. Forestry Society, 83(2), pp.84-92. (in Japanese with English abstract)

[2] Kagoshima local meteorological observatory (1955-2006): Observed data:

[3] Shimokawa, E. and Jitousono, T. (1987): Rate of erosion on tephra-covered slopes of volcanoes, Trans. Jap. Geomorph. Union, 8, pp.269-286. (in Japanese with English abstract)

[4] Sumi, A. and Suzuki, Y. (1998): Studies on crop growth damage due to ash fall from Mt. Minamidake of Sakurajima (1)- Experimental indications of crop damage due to ash fall -, Jour. Jap. Soc. Natural Disaster Science, 17(2), pp.167-176. (in Japanese with English abstract)

[5] Teramoto, Y., Shimokawa, E. and Jitousono, T. (2005): Effects of the difference of volcanic activity on erosion rate by sheet erosion in Sakurajima volcano, Jour. Jap. Soc. Erosion Control Engineering, 57(5), pp.65-68. (in Japanese)

[6] Teramoto, Y. and Shimokawa, E. (2007): Impact of volcanic activity on vertical distribution of vegetation on the hillslope of Mount Sakurajima, Jour. Jap. Soc. Coastal Forest, 7(1), pp.19-23. (in Japanese with English abstract)

－ 62 － － 63 －

〔Received…April…8th,…2010……Accepted…September…13th,…2010〕