森林立地学会誌 森林立地41（2），1999，93〜102
Jpn.J.For.Environment41（2），93−102
1999
1995年に宮崎県で起こったスギ人工林の干ばつ被害
西村五月＊・讃井孝義＊＊
＊南九州大学園芸学部・＊＊宮崎県林業総合センター
1995年秋，宮崎県南部のスギ林に枯損が発生し，枯損実面積は41ヘクタール，枯損推定本数は8万本に達した。原因として当該年夏期の無降水状態が考えられたが，被害木にはしばしば暗色枝枯病の病徴が見られた。同期の平均風速，気温とも平年の値より大きかった。被害は宮崎層群南部地域に限られ，隣接する日南層群では少なかった。日南層群の地形は起伏が穏やかで，山頂部や尾根筋に平坦面も見られ，黒色土が堆積している。宮崎層群は急斜面で谷密度が高く，浸食が激しい。宮崎層群では土壌が礫土化しており，有効土層は日南層群より小さい。また，宮崎層北部の無被害地帯と，南部被害地帯との間には地形の違いはなかった。地形図の解析結果と被害分布の間には関係が認められ，被害は斜面の一定方向に発生が多く，尾根にも多発していた。被害発生とその他の地形要因などの関係から，今回の被害は海からの風の吹き込みの影響が大きいと推察した。土壌試料を採取時重量と飽水時重量の差で比較すると，被害区は無被害区よりもA層，B層ともにその差が小さい傾向にあり，最大容水量の違いが考えられた。透水指数は被書区のA層が大きく，無被害区は小さい傾向にあった。しかし，B層では著しい差はなかった。また，最小容気量は被害区の方がやや大きい傾向にあった。以上の結果から，干害は長期にわたる無降水状態の継続の他に，土壌母材や山地地形，土壌の物理性，樹体や地表からの水分消失条件，スギの暗色枝枯病に対する感受性の差異などが複合的に組み合わさって発生することが推測された。
Satsuki
Nishimura and Takayoshi Sanui : Drought damages upon artificial Cryptomeria japonica stands in 1995 at
Miyazaki Prefecture
In the autumn of 1995, high mortality
of Cryptomeria trees occurred in southern part of Miyazaki Prefecture. It was
estimated that damaged square was about 41 hectares and the number of mortality
were 800 hundreds. The drought was considered to be the main cause of the
damage, but we often observed symptoms of guignardia dieback on dead trees. In
this area, it only rained a little in the summer of 1995. At the same period.
the mean annual wind velocity and mean annual temperature were higher than
ordinary years. Severe damages occurred in the southern area of Miyazaki group,
but damage in the Nichinan group adjacent to the northern and western
territories of the Miyazaki group were not severe.
In the Nichinan group, hill slopes are
characteristically gentle with little soil eroded areas, and accumulation of
black soil was observed. In contrast, hill slopes in the Miyazaki group are
typically steep, and have a lot of soil eroded areas. In the Miyazaki group,
soil is characterized by containing a lot of gravel, and the effective soil
depth is shallower than in the Nichinan group. There were no topographical
differences between damaged and non-damaged areas in the Miyazaki group.
We tried to analyze the relationship
between the damage and topographical characteristics by mapping the damaged
sites. The map showed us that damage was concentrated on a particular direction
on slopes near ridge tops. We speculate that damage occurred due to the wind
from the sea under a particular condition.
Weight-differences of soil core samples
between field moisture condition and saturated condition in damaged areas were
less than those in non-damaged areas both A and B horizons. Percolation index
of the A horizon was larger in the damaged area than in the non-damaged area.
No differences in percolation index in the B horizon were observed between
damaged and non-damaged areas. The maximum water holding capacity of damaged
area soil was slightly lower, and the minimum air capacity of it was slightly
higher than those of non-damaged area soil. Those results show that water
holding capacity of damaged areas soil is smaller than it of non-damaged areas.
From the above mentioned
investigations, it can be concluded that the damage of Cryptomeria stands in
1995 was caused by a water deficiency that derived from the complex factors
such as meteorological condition, geological features, soil moisture condition
associated with soil physical properties, and susceptibility to guignardia
dieback disease.