Regeneration process biased toward under-crown Sasa-lacking habitat of sparse Abies mariesii forest with dense undergrowth of dwarf bamboo on a snowy mountain, northern Japan

In sparse Abies mariesii forests with an undergrowth of dense dwarf bamboo (Sasa kurilensis) in subalpine areas of snowy mountains in northern Honshu, Japan, A. mariesii saplings are more abundant in Sasa-lacking patches under the crowns of A. mariesii canopy trees than in outside-crown areas densely covered with Sasa. To discuss the mechanism of such under-crown-biased regeneration, we examined the underlayer light conditions, population structure, seed dispersal, and the survival and growth of seedlings and saplings in four habitats with different canopy and undergrowth conditions, i.e. under-crown lacking Sasa (UCLS), under-crown with Sasa (UCWS), outside-crown lacking Sasa (OCLS), and outside-crown with Sasa (OCWS). Sapling banks were observed in all four habitats. The light condition in the UCLS was almost equivalent to or inferior to that in the OCWS. The survivorship of seedlings and saplings did not differ among the habitats. The increment of sapling trunk length in the UCLS was equivalent to that in the OCWS. These results indicate that the presence of A. mariesii canopy trees did not significantly improve the light condition, survival, or growth of A. mariesii seedlings and saplings. Instead, abundant seed supply under the crowns contributed to higher population densities and under-crown-biased regeneration. The existence of a sapling bank in the OCWS suggests the possibility of successful A. mariesii regeneration in this habitat even without the simultaneous death of Sasa. The dynamics of sparse A. mariesii forest may progress through shifting of the canopy and undergrowth conditions (e.g. UCLS to OCWS via OCLS).