## 4 Discussion
### Secondary Forest Recovery and Biomass Accumulation  
Understory vegetation is influenced by the development of overstory trees through light and belowground competitions for water and nutrient resources [18]. The understory biomass temporarily increases with the canopy losses, but it reaches a peak [24] or decreases [18; 25; 28] with overstory recovery. In turn, overstory tree recovery is influenced by dense undergrowth through shading [19] and belowground competition [20; 21], especially during the sapling stage. At this site, understory biomass increased for six years after the disturbance, but decreased from the eighth year after the disturbance with the recovery of overstory trees (Figure 1a).  
Because of the large contribution of overstory biomass to total biomass [18], except for the initial stage of succession, the biomass accumulation rates in the whole forest ecosystems are greatly influenced by the recovery rate of overstory trees. Total AGBs in temperate or boreal forests without the recovery of canopy-forming trees within 12 years after disturbance were 2.3-5.9 Mg\(\cdot\)ha(Table 3; [20; 45; 46]). In contrast, biomass accumulations four to six years after clearcutting of hardwood forest ecosystems with rapid establishment of pioneer trees were 7.9-32.6 Mg\(\cdot\)ha\({}^{-1}\) of AGB [47] or up to 34.6 Mg\(\cdot\)ha\({}^{-1}\) of total biomass [24] (Table 3). At this site, where canopy-forming trees started to emerge six years after disturbance, AGB of 2.7-5.8 Mg\(\cdot\)ha\({}^{-1}\) and total biomass of 6.0-11.2 Mg\(\cdot\)ha\({}^{-1}\) was observed 2-10 years after the disturbance (Figure 1a), which was similar to the case without the establishment of canopy-forming trees (Table 1). The slow stand establishment is probably the result of dense cover by _Rubus ideaus_ and _Solidago gigantea_, which compete with the saplings of canopy-forming trees.  
The biomass accumulation rates of overstory trees are influenced by abiotic factors of temperature and belowground water and nutrient resource availability [42], as well as biotic factors of above-described competition. In this study site, the root zone thickness (A horizon) was only about 0.10 m. Below the layer, there was a root-impermeable layer of pumiceous volcanoes regosols [31; 34]. The soil is characterized by rapid drainage and low nutrient retention which is common with sandy soils. Forests on sandy soils, which have low water and nutrient holding capacity, tend to have low biomass accumulation rates [42]. The AGB in the study site 10 years after disturbance is similar to another Japanese site, but the RGB is about half that of the other Japanese site (Table 3). At the study site, the low water and nutrient availability is also responsible for the slow overstory recovery and overall biomass accumulation, but additional research is needed to confirm this effect.  
Increasing mortality after 2013 (Figure 3) was probably caused by inter-specific and intra-specific competition (self-thinning), which can also result in slow biomass recovery of overstory trees. [PERSON] et al. [29] reported that tree biomass decreased temporarily 15 years after disturbance in paper birch (_Betula papyrifera_) stands because of self-thinning in British Columbia.