Ancient rock-avalanches as microrefugia for relict conifers: A geoecological study in the Akaishi Mountains, central Japan

Abstract

Large-scale landslides are fundamental drivers of landscape dynamics that generate distinct abiotic legacies; however, few studies have explicitly linked geomorphological history to long-term ecological processes. This study investigates the geoecological role of a rock-avalanche deposit formed approximately 1100 years ago in the Akaishi Mountains (central Japan). It aims to elucidate how such geomorphic legacies contribute to the persistence of forests dominated by relict conifers. Detailed field surveys revealed a clear differentiation of site conditions within the landslide deposit, distinguishing with-matrix (WM) from no-matrix (NM) boulder slopes. At WM sites, fine-grained material infilling inter-boulder voids has promoted soil development over the past millennium, thereby supporting a closed-canopy mixed forest structurally comparable to the zonal vegetation on adjacent undisturbed slopes. In contrast, NM sites lack fine sediment. These severe edaphic conditions act as a spatial filter, limiting the establishment of competitively dominant broad-leaved species and thereby maintaining an edaphic climax community. NM sites harbor a distinctive assemblage of stress-tolerant relict taxa belonging to lineages that originated in the Tertiary (e.g., Chamaecyparis pisifera), as well as disturbance-dependent species with glacial histories. The latter group includes the glacial relict Picea alcoquiana and the cold-adapted pioneer Larix kaempferi, whose contemporary distributions are largely confined to unstable substrates. Our findings demonstrate that the ancient rock avalanche creates competitor-reduced rocky habitats that replicate the historical niches of these diverse lineages. Furthermore, given the region's ongoing tectonic activity, we propose that spatially dispersed landslide scars form a dynamic habitat network. This network likely facilitates the regional persistence of relict populations through metapopulation dynamics. We conclude that ancient landslide scars are not merely sites of destruction but are critical geoecological structures that enhance landscape heterogeneity and contribute to the conservation of regional floristic diversity.