Recently, forest dieback around the coastal lake Sugari-oike, National Natural Monument of Japan, has rapidly increased. To clarify the factors affecting the forest dieback, we examined species composition, stand structure, and mortality at the lake-edge and landward areas for 3 years. We also examined water quality, and measured how much the water level varied within a year. Forest dieback did not affect specific species, and there was no bias in the diameter at breast height of dead individuals. Mortality in the lake-edge area was more than 10% per year, which was considerably greater than that reported for other temperate broad-leaved evergreen forests. Mortality for the landward area was 2.8% per year, significantly lower than that in the lake-edge area. The basal area of the landward plots increased slightly during the census period, while that of the lake-edge plots decreased, reflecting the high mortality. Water level fluctuated greatly within a year, rising by 40 cm at its peak. The water has been eutrophic for 20 years, suggesting that eutrophication did not cause the recent dieback. We consider that the period during which trees on the lake edge are flooded is increasing. To confirm this, future studies should investigate water-level fluctuations over multiple years, and also soil conditions. This would help to delineate the long-term dynamics of the forest and would further our understanding of the causes of dieback in this area.

最近，日本国家自然保护区沿海湖泊Sugi-oike周围的森林荒芜迅速增加。为了弄清影响森林枯死的因素，我们研究了3年中湖沿和陆上地区的物种组成，林分结构和死亡率。我们还检查了水质，并测量了一年内水位变化了多少。森林枯萎不会影响特定物种，死者的胸高直径没有偏差。湖边地区的死亡率每年超过10％，大大高于其他温带阔叶常绿森林的报告死亡率。陆上地区的死亡率为每年2.8％，大大低于湖边地区的死亡率。在普查期间，陆上土地的基础面积略有增加，湖边地块减少，反映了高死亡率。一年内水位波动很大，最高时上升了40厘米。水已经富营养化了20年，表明富营养化并没有导致近期的死亡。我们认为湖边树木被淹的时期正在增加。为了证实这一点，未来的研究应调查多年的水位波动以及土壤状况。这将有助于描述森林的长期动态，并有助于我们进一步了解该地区森林枯竭的原因。这表明富营养化并没有导致近期死亡。我们认为湖边树木被淹的时期正在增加。为了证实这一点，未来的研究应调查多年的水位波动以及土壤状况。这将有助于描述森林的长期动态，并有助于我们进一步了解该地区森林枯竭的原因。这表明富营养化并没有导致近期死亡。我们认为湖边树木被淹的时期正在增加。为了证实这一点，未来的研究应调查多年的水位波动以及土壤状况。这将有助于描述森林的长期动态，并有助于我们进一步了解该地区森林枯竭的原因。