Seed dispersal and seedling establishment of Rhus trichocarpa promoted by a crow (Corvus macrorhynchos) on a volcano in Japan

Abstract

The interaction between frugivorous birds and trees producing bird‐dispersed seeds in devastated areas has been considered to be weak, owing to the paucity of avifauna and/or food resources for birds. Here, we present evidence that strong interactions between birds and plants may promote the enlargement of tree distribution on harsh environments. The summit of Mount Koma, northern Japan, was denuded by the 1929 volcanic eruption. Vegetation cover gradually decreases from the bottom (secondary forest) to the top (bareground) of the mountain. We recorded 48 bird species in the four seasons of 2001, along a 5‐km line transect on the southwestern slope of the mountain. Birds faeces collected along the transect contained seeds of more than 14 plant taxa. Five of the 14 taxa were bird‐dispersal tree species (Rhus trichocarpa, Sorbus commixta, Prunus ssiori, Prunus maximowiczii and Prunus sargentii) and were established in the summit area. Most faeces were derived from Corvus spp. (mostly C. macrorhynchos) and Turdus naumanni. In particular, the seeds of R. trichocarpa were found mostly from the faeces of Corvus spp. and the seeds of Gaultheria miqueriana, a shrub species, were only from T. naumanni. Rhus trichocarpa retained fruits on the canopy at all times of the year, and crows could feed on them even when food resources were poor in winter. Rhus trichocarpa seedlings established well near rock at higher elevation, while they occurred mostly under the larch canopy of larches at lower elevation. Crows mostly utilized tree canopies and rocks as perches in respective habitats. Therefore, seedlings should be abundant in specific habitats at different elevations. Size‐class distribution of seedlings suggested that seedling mortality was lower at higher elevation where open sites were more abundant. These findings indicate that strong mutual advantages for C. macrorhynchos and R. trichocarpa on denuded areas play an important role on revegetation.