Effects of snowmelt timing on survival of beech saplings (Fagus crenata) in snowy mountains of northern Japan
In snowy mountains of northern Japan, snowmelt is delayed due to high snow accumulation, hence deciduous canopy trees usually unfold leaves in early spring when snowpack remains on the forest floor. This leaf emergence of canopy trees prior to snowmelt may strengthen shading effects on understory seedlings. In this study, to elucidate effects of delayed snowmelt on understory seedlings and saplings of predominant deciduous tree species in snowy mountains, we investigated environmental factors and survival of seedlings (current-year seedlings) and saplings (saplings with stem length less than one meter, other than current-year seedlings) of beech in two sites, “low-altitude site” (alt. 450m, snow depth:1-2m) where snowmelt occurs simultaneously with or prior to leaf emergence of canopy and “high-altitude site” (alt. 890m, snow depth:4-5m) where snowmelt occurs following leaf emergence of canopy, in Hakkoda mountains, northern Japan. Eight transects were set up in each site, and snowmelt duration (duration from leaf emergence of canopy to snowmelt) per quadrat, leaf-emergence duration (duration from snowmelt to leaf emergence of beech seedlings or saplings), and survival for beech seedlings and small- and large- beech saplings (saplings with stems length shorter than the median of stem-length distribution and those longer than the median) were investigated.
Survival rates from leaf-emergence period to autumn for beech seedlings decreased significantly as both snowmelt duration and leaf-emergence duration increased in both sites. Mortality of beech seedlings due to herbivory was observed in the sites, but the percentages of this mortality were small, 16 to 18 %. Also survival rates from leaf-emergence period to the following spring for small beech saplings decreased significantly as both of the two factors increased in both sites. Effects of light environment (diffuse sky radiation) on the survival of the small saplings were insignificant in both sites. On the other hand, effects of the three factors on survival were significant for large beech seedlings in low-altitude site: effects of light environment were positive and those of other two factors were negative. In contrast, all of these factors were insignificant for large beech seedlings in high-altitude site. These results indicate that delayed snowmelt is a significant factor decreasing survival of beech seedlings and saplings on forest floor in snowy mountains. Effects of the three factors diverged between the sites in large saplings, suggesting that leaf emergence of canopy trees prior to snowmelt may strengthen shading effects on survival of large seedlings together with heterogeneity of light environments.