Region-wide mass mortality of Japanese oak due to ambrosia beetle infestation: Mortality factors and change in oak abundance

摘要

Insect and pathogen disturbances can result in a decline in the abundance of host tree species, leading to changes in tree species composition at a regional scale. Quercus crispula and Q. serrate are dominant oak species in Japanese secondary forests, which are mostly composed of abandoned coppice. In recent decades, these two oak species have suffered mass mortality from Japanese oak wilt disease (JOW), which is caused by the ambrosia beetle Platypus quercivorus. This study analyzed factors affecting oak mortality during a JOW epidemic, and quantified the impact of the epidemic on forests at a regional scale in Toyama Prefecture, central Japan using data from 101 plots in the National Forest Inventory. Plot measurements before the epidemic (1999-2003) and re-measurements after the epidemic (2012-2014) were used for analysis. Before the epidemic, Q. crispula was the most dominant species in the natural forests and was distributed at a wider elevation range than Q. serrata (the third most dominant species). The mortality rate during the epidemic was 68% for Q. crispula and 21% for Q. serrata. The mortality of both species positively related with plot-level oak volume, suggesting that stand development after abandonment of coppice forests was an important factor in the epidemic. The mortality of Q. crispula related inversely with elevation and was especially low above 1000 m, probably because of the low reproductive success of P. quercivorus at high elevation, suggesting that global warming was a factor increasing the mortality from JOW. The total volume of Q. crispula in the study region decreased by 56% during the epidemic, while that of Q. serrata increased by 13%. After the epidemic, the dominance of Q. crispula in the natural forests decreased below that of Fagus crenate and Q. serrata. There were few regenerating Q. crispula and Q. serrata in the canopy gaps formed by dead oak because of their low numbers of saplings. Regenerating trees comprised various shade-tolerant canopy tree species and shrub species, and forests will change toward multispecies mixed, uneven-aged forests. This study comprehensively quantified forest change due to a JOW epidemic at a region-wide scale, and revealed that the epidemic was caused by multiple interactions among coppice abandonment, global warming, ambrosia beetles, and oak trees.