有珠山2000年噴火による林地への泥流堆積が根圏のガス交換に与える影響  [in Japanese] Effect of Rainfall-induced Mudflow Sediment on the Gas Exchange of Tree Root Systems after the 2000 Eruption of Mt. Usu  [in Japanese]

有珠山2000年噴火により生じた降雨型泥流の堆積が根圏のガス交換に与えた影響を検討するため,降雨型泥流の起きた斜面下部の2カ所の林地を対象に,樹木の枯死状況と堆積物の通気性を調べた。枯死率がほぼ100%の調査区では,火口噴出降下物が約10cm堆積した上を泥流堆積物が焼く20cmの厚さで覆う二層構造をなし,枯死率36%の調査区では,降下物のみが約10cmの厚さで堆積していた。すなわち地表の土砂堆積物構造により枯死状況が異なっていた。現地観測によるpFの変動状況と,室内試験より得られたpFと相対ガス拡散係数D/D_0の関係から,高枯死率の調査区の泥流堆積物はD/D_0が0.02以下で,降下堆積物のD/D_0は常に0.005以下で,両層ともガス拡散がほとんど起こらなかったと推察された。一方,低枯死率区における降下堆積物のD/D_0は無被害森林土壌表層と同程度で,測定期間の半分で0.02以上を示し,ガス拡散が十分に行われていたと推察された。以上より,降雨型泥流堆積物とこれにより被覆された降下堆積物は通気性が低いために根圏と大気とのガス交換が阻害され,根系が酸素欠乏した結果,樹木群の枯死が生じたと考えられた。

Mt. Usu, in southern Hokkaido, Japan, erupted in March 2000, and in July 2000 a rainfall-induced mudflow occurred on the northern slope of Mt. Konpira, near to the area that was active in the recent eruption. To investigate the effect of this mudflow sediment on gas exchange by tree root systems and tree mortality rates, gas diffusion rates were measured at two study sites at the foot of this slope in July 2001. The tree species, their diameter at breast height, and sediment thickness were also scrutinized. The mortality rate depended on the surface sedimentary structure after the recent eruption. At the first site, where the mortality rate was almost 100%, the rainfall-induced mudflow sediment was about 20 cm thick on a pyroclastic fall deposit about 10 cm thick. While the D/D_0 values in the pyroclastic fall deposit covered with the mudflow sediment were less than 0.005 during September 2001, the values at the site not covered by the mudflow exceeded 0.02. This suggests that the mudflow sediment cover retarded gas exchange in the rhizosphere, asphyxiating root systems ; consequently, the trees died after the rain-induced mudflow occurred.