Based on a comparison of the number of individuals captured before and after 2017, both Apodemus species, A. speciosus and A. argenteus, increased dramatically after the mast seeding of S. borealis (Fig. 5). Conversely, E. smithii did not show such an increase. Therefore, the mast seeding significantly contributed to the increase in the populations of A. speciosus and A. argenteus, but did not affect the reproduction of E. smithii. The response in abundance of the two Apodemus species in this study is similar to that seen after the mast seeding of trees. Miguchi (1988) reported that Microtus montebelli and A. speciosus had an outbreak in the year following the masting of Fugus crenata in Japan. Similarly, Zwolak et al. (2016) revealed that in Poland, the numbers of A. agrarius and Myodes glareolus (a species of the vole) increased after the masting of F. sylvatica. In the case of bamboo, Bovendorp et al. (2020) reported an increase in the number of rodents after the mast seeding of the subfamily Bambusoideae in Brazil. In addition, Shimada et al. (2019) conducted a rodent–capture survey at other S. borealis deadlands in 2017, the same year as in this study, and the findings revealed that in the year following the masting, A. speciosus and A. argenteus increased, while E. smithii remained low density. The findings of this study showed a faster population increase in Apodemus species than that in Shimada et al. (2019). The response time differs owing to the differences in the habitat conditions of the rodents at the study sites. Our study site is a secondary broad-leaved forest with the original rodent populations, whereas that of Shimada et al. (2019) is a planted coniferous forest, to which numerous rodent individuals migrated from the surrounding original habitat after the mast seeding. Regarding E. smithii, which did not increase in our study, a large outbreak, however, was observed in Tokushima Prefecture, western Japan. This phenomenon occurred in the same year as the simultaneous fruiting of dwarf bamboo but was limited to some survey sites and considered not directly related (Tanaka 1967). E. smithii gets nourishment from the green part of plants and the starch of seeds (Kaneko 2005). Since more than 76% of the nutrients of S. borealis seeds are nitrogen-free extracts (carbohydrates) (Shimada et al. 2019), it is entirely possible that voles eat them as food. However, E. smithii prefers moist areas (Tanaka and Shibata 2006), so that its habitat is more limited than that of the two Apodemus species. Therefore, despite the large supply of S. borealis seeds, the E. smithii population may have not increased because of the difficulty of external immigration, which is likely to occur in A. speciosus and A. argenteus. In addition, Conrod and Reitsma (2015) reported that in an American forest, where three species of rodents existed together, only Napaeozapus insignis was no longer captured due to interference among the species after masting. Fasola and Canova (2000) also found that the exclusion of a species of the genus Apodemus increased the population density of bank voles, while the vole exclusion had little effect on the Apodemus species, which implies that asymmetrical competition was established between the two species in Italy. These facts suggest that in this study area, E. smithii may have been suppressed by A. speciosus and A. argenteus, both of which increased rapidly in the population density, and could not inhabit the surveyed areas.
It is well known that rodent populations increase significantly in the year following tree mast events, but decrease after 2 years (Miguchi 1988; Zwolak et al. 2016, 2018). Conversely, our study found that in 2019, 2 years after the mast seeding of S. borealis, the increased populations of A. speciosus and A. argenteus remained the same (Fig. 2). This difference may be attributed to the fact that the food supply associated with the mast seeding of S. borealis was more persistent than that of the trees. It is also reported that most of the fallen acorns are eaten by rodents or the black bear (Ursus thibetanus), even in the case of a good harvest year, and then the rest germinates the following spring (Kikuzawa 1988; Ida and Nakagoshi 1996). However, we observed many S. borealis seedlings even in 2019 at the survey sites examined in this study, suggesting that most of its seeds have remained on forest floors for a few years.
In this study, regardless of the species (A. speciosus and A. argenteus) or sex of rodents, the body mass were not significantly different before and after mast seeding (Fig. 6). These results suggest that the increase in food sources due to mast seeding did not contribute to body mass gain. It is also worth noting that the body mass decreased slightly after mast seeding (Fig. 3), which seems to be slightly similar to the pattern (Krebs 2013) that in the increasing population of rodents, females mature earlier and become smaller. On the other hand, from 2018 to 2019 (the period after mast seeding), there was a significant increase in the mean body mass of A. speciosus adult males (Fig. 3). These results can be explained by the fact that there were many S. borealis seeds on forest floor at the end of the 2-year period, which may have resulted in higher intake by the rodents in 2019. Conrod and Reitsma (2015) showed that the mean body mass of adults for three rodent species, Peromyscus sp., Napaeozapus insignis, and Myodes gapperi, was heavier in the post-masting spring than in the pre-masting spring of trees. However, in the current summer following this response, N. insignis disappeared completely from their study area, and then the mean body mass of M. gapperi and Peromyscus sp. was found to have decreased—probably due to low food-availability and density-dependent competition for the vacant niches. In addition, Scarlett (2004) found a significant mean body mass loss in P. leucopus in the first of the two straight years of low acorn yield, compared to the masting year, but no difference in the second of the two straight years of low acorn yield, which suggests that factors other than acorn availability may affect the body mass change of the rodents. Therefore, patterns of the body mass change associated with masting are not consistent for dwarf bamboo and trees. Further investigations are needed to explain the mechanism, including the dynamics of other food sources.
The proportion of adults was already high at the time of new capture in both A. speciosus and A. argenteus (Fig. 4), suggesting immigration. It is evident that juveniles of A. speciosus tend to disperse and move away from their birth sites (Miguchi 1988; Sekijima 2008). Therefore, the proportion of juveniles in the study area may have decreased because of their natal dispersal. Moreover, the survey plots used for the capture process in this study were flatter and had a higher density of dead S. borealis culms than the surrounding areas did, suggesting that a high seed density was produced. The relative dominance of food sources in the plots may have led to the increase in the migration from the surroundings. In fact, Miguchi (1988) found that an increase in acorn supply due to the masting of F. crenata resulted in the concentration of A. speciosus, which is likely to actively get around. Ogawa et al. (2017) categorized the effects on the population dynamics of rodents depending on the abundance of seeds as food sources: (1) increased growth, (2) increased survival, and (3) increased immigration. Among them, the third effect, increased immigration, was observed in Pinus strobus seeds, which were particularly abundant. Therefore, the effect of S. borealis seeds on rodent population is considered similar to that of P. strobus seeds. Furthermore, males tended to have a higher proportion of adults than females in both A. speciosus and A. argenteus (Fig. 4). These results may also be explained by the differences in mobility (females < male).
In this study, the proportion of juveniles was different between males and females in both 2018 and 2019, with higher values being recorded for females (Fig. 4). In A. argenteus, female juveniles increased when seeds, which they ate as food, were abundant and conditions for female breeding were good (Shibata and Kawamichi 2009). In the survey sites used in this study, the food amount was much higher than before due to the mast seeding of S. borealis, and the breeding conditions for females in A. argenteus as well as A. speciosus improved. Therefore, the S. borealis masting clearly caused an increase in female juveniles. If so, since this food abundance did not change between 2018 and 2019, the breeding female rodents were still in good condition after 2 years of mast seeding. Furthermore, it has been reported that the sex ratio of A. argenteus captured usually tends to be higher in males (Kinoshita and Maeda 1961; Miyao et al. 1963). In this study, the results were the opposite—more females were caught in each year, which may be due to the unusual (the 120-year cycle) mast seeding of S. borealis.