## 4 Discussion  
### Understory Species after Disturbance  
A drastic change of species composition was observed at the study site. _Rubus ideaus_, a biennial cancel _Rosaceae_ species with ca. 1 m in height and the dominant species immediately after the disturbance, is known to become dominant after the disappearance of overstory species in boreal or cool-temperate forests in Europe and North America [50]. In northern Japan, this species similarly dominated after disturbance without undergrowth of _Sasa_ spp. (dwarf bamboo) [51], which is a competing species against _Rubus ideaus_[51]. The expansion and dominance of _Rubus ideaus_ are expected to be common features in severely disturbed boreal and cool-temperate forests. The _Rubus_ biomass increases according to an increase in light intensity [52] and soil disturbance [53]. _Sasa_ spp. also increases distribution according to an increase in light intensity by gap formation or forest disturbance [3; 51; 54], but the relation between _Sasa_ distribution and site conditions, such as soil moisture or fertility, is unclear.  
Later dominant species, _Solidago gigantea_, is an _Asteraceae_ species with ca. 1 m in height. This species is native to North America and represents a common invasive species in central to northern Europe and eastern Asia, including Japan [55]. In Hokkaido Island, this species was introduced as an ornamental before the 1950s [56]. This species is light-demanding, grows in wide ranges of soil moisture and nutrient conditions, and is often found in open sites such as wetlands [55], the forest margin, and gaps including those at disturbed sites [57], abandoned farmland and grassland [58], and ski slopes [59]. At the study site, _Solidago gigantea_ also formed dense and monospecific communities, accounting for about 70% of the understory biomass (Figure 1b), as reported by [55]. In previous studies, _Rubus ideaus_ had been reported to dominate until trees began to form a canopy in Hokkaido (for 10-15 years) [51], and North America (for 10 years) [60]. At the site studied here, however, _Rubus ideaus_ had dominated for only four years after the disturbance before being replaced by _Solidago gigantea_ (Figure 1b). The expansion of _Solidago gigantea_ during secondary forest succession has not been reported and might be a new phenomenon related to human activities, but the reasons are unclear and require further research  
The earlier peak biomass of _Rubus ideaus_ (Figure 2) is partly attributable to the phenology of this species. The aboveground part is biennial. The primocane (first-year stem) grows from spring through autumn, but the floricane (second-year stem) develops leaves in the spring and produces fruits and dies in the summer [61]. The early defoliation of floricane causes the peak biomass to be observed during the early summer [2]. In contrast, _Solidago gigantea_ continued to grow until it bloomed [55] in August at the study site. The nutrient condition in the soil can also affect the timing of peak biomass. N deficiency accelerates senescence of leaves [62]. On the other hand, later peak biomass in 2009 during _Rubus ideaus_ dominance is likely caused by the improvement of soil nutrient conditions, which as a result did not accelerate senescence.