Nobuhiro Akashi

Dispersion pattern and mortality of seeds and seedlings ofFagus crenata Blume in a cool temperate forest in western Japan

The seed and seedling mortality ofFagus crenata Blume after a mast year (1993) was examined in relation to density and distance from the nearest conspecific adult tree in a mixed conifer-hardwood forest in Ohdaigahara, western Japan. The mortality of fallen seeds during winter amounted to 93.7%, and 79.2% of the current-year seedlings died in the first growing season. The most important factor of death for both seeds and seedlings was predation by vertebrates. The mortality of seeds during winter was positively correlated with sound seed density. The mortality of seedlings was positively correlated with density but not significantly related to the distance from the nearest crown edge of a conspecific adult tree. Mortality patterns varied with stages and spatial scales due to the behavior of predators; it is thus important to investigate the spatial pattern of seeds and seedling mortality at various temporal and spatial scales. After the first growing season, the difference in seedling density between distance classes was not significant at <4m from the nearest adult trees due to density-dependent mortality. However, seedling density was significantly lower in the ≥4 m class than in the <4 m classes.

Geographical variation in the early regeneration process of Siebold's Beech (Fagus crenata BLUME) in Japan

The causes and timing of seed death in early regeneration process of Siebolds beech (Fagus crenata Blume) was studied at 15 sites along a snowfall gradient in Japan, in order to clarify why the seedling density of the species has geographic difference remarkably. Seed production did not significantly differ along the snowfall gradient. Pre-dispersal seed mortality by insect damage was higher at sites with light snowfall than at sites with heavy snowfall, but this only seemed to be a minor factor influencing the population. A large proportion of the viable nuts that fall in autumn ware killed in winter before germination. Winter mortality was much higher at sites with thin snow cover than that at sites with thick snow cover, and this factor was strongly correlated with the geographic variation of seedling regeneration probability. There was little seed mortality by winter desiccation. The main factor contributing to the geographic difference seemed to be a seed predation by rodents in winter. Deep snow cover may reduce the success of rodents finding seeds in winter. Thus the observed relationship between snowpack depth and early mortality may be due to an indirect effect through the process of seed predation.p>

The spatial pattern and canopy-understory association of trees in a cool temperate, mixed forest in western Japan

The spatial distribution pattern of trees and the association between canopy and understory individuals were examined with reference to the distribution of tree crowns in a cool temperate, mixed forest in Ohdaigahara, western Japan. Line transect and contact sampling methods were used to examine the pattern over various spatial scales. These methods are useful to detect patterns over a large study area. The dominance ofChamaecyparis obtusa on steep slopes forming large patches suggested that the distribution of this species is a consequence of landslides. UnderstoryFagus crenata showed a clumped distribution, and the relative coverage of this species was larger in canopy gaps than under a closed canopy. Understory individuals ofAbies homolepis showed a positive association with canopy trees ofF. crenata but a negative association with conspecific canopy individuals. These patterns suggested thatF. crenata regenerates in canopy gaps and is replaced byA. homolepis. The dynamics of these two species are consistent with the process of gap dynamics. The effects of both small- and large-scale disturbance must be evaluated to understand the mechanisms of patch formation and the coexistence of forest tree species.

Simulation of the effects of deer browsing on forest dynamics

I introduced forest dynamics to a deterministic herbivore-vegetation model to examine the effects of vertically stratified forest structure on the dynamics of the herbivore–vegetation system, the resilience and stable states of vegetation, and the interactions between deer populations and vegetation. I constructed a model based on data from field studies performed in Hokkaido, northern Japan. Three phases of state were identified for a given deer density: (1) understory vegetation is maintained with a equilibrium proportion of canopy gaps in the absence of deer; (2) if the equilibrium proportion of canopy gaps is greater than that in the equilibrium state in the absence of deer, the understory vegetation can be maintained; and (3) the understory vegetation cannot be maintained. At the boundary between phases 2 and 3, the herbivore population level had discontinuous effects on vegetation. When the deer density was held below the threshold, forest vegetation had resilience to recover to the equilibrium stable state at the given deer density, although the equilibrium canopy gap ratio and vegetation biomass differed with deer density. However, the forest vegetation–herbivore system could not be maintained in a stable state without artificial deer population management if food limitation was the only mechanism to keep the deer population at low levels. The deer population must be kept below the boundary between phases 1 and 2 to maintain the forest regeneration processes. The level cannot be determined by observing the deer population; careful observation of forest regeneration processes is required.

Lateral and vertical crown associations in mixed forests

The crown-association sampling method was developed to examine horizontal and vertical spatial associations in forest tree communities. We sampled tree crowns along line transects and recorded the lateral contact of neighboring crowns within the canopy and the vertical overlap of crowns between the understory and the canopy. Deviations from random association can be examined for lateral and vertical associations. It is also possible to compare the results for vertical overlap among study sites. Three natural mixed forests were sampled: a warm-temperate rainforest on Yakushima Island (southern Japan), and two cool-temperate mixed forests in Ohdaigahara (central Japan) and on the Shiretoko Peninsula (northern Japan).

Growth response of mature larch trees to simulated vole gnawing: 20-year results

ABSTRACT Growth consequences of Japanese larch caused by vole gnawing were evaluated using artificial girdling of differing severity, ranging from 50% to 95% of circumference. The treatments were applied to 37-year-old trees in a larch plantation in Hokkaido, northern Japan, and the stand was monitored for 20 years. None of the treated trees died; simulated girdling did not appear to cause serious reductions in diameter at breast height or tree height. The artificial scars gradually became occluded by callus, the extent of the scars decreasing by 10–17 percentage points every 5 years. These observations indicate that when thinning mature stands of larch, it is unnecessary to select trees commensurate with their wounding extent, in terms of future volume growth.

Estimation of Population Density for Sika Deer (Cervus nippon) Using Distance Sampling in the Forested Habitats of Hokkaido, Japan

Abstract. Sika deer (Cervus nippon) populations in Hokkaido have irrupted during the last three decades, causing severe damage to agriculture and forestry. Methods have been developed to estimate their population size on large but not on small scales. We estimated deer density using distance sampling to analyze the feasibility of employing the line-transect method in forested habitats on a management-district scale. Spotlight counts were conducted four times each year on the Iburi (IMD; 11 routes; 328.8 km total survey length) and Kushiro (KMD; 10 routes; 352.6 km) management districts in November 2013 and 2014. We observed 206 herds (302 deer) on the IMD and 680 herds (1,173 deer) on the KMD. Density estimates on the KMD, 39.6 ± 4.9 (average ± SE) in 2013 and 15.6 ± 2.3 in 2014, were significantly higher than those on the IMD, the former 8.4 ± 1.9 and the latter 4.4 ± 0.8. The line-transect method using distance sampling was useful for estimating deer density on the management-district scale. However, the large interannual variations of estimated density suggest that we have to specify the factors for annual variation in the future studies.