Mast seeding and its spatial scale in Fagus crenata in northern Japan

Seed production of Fagus crenata was visually monitored at 159–321 districts in the Tohoku Regional Forest Office, which covers almost all the natural forest of this species in north-eastern Japan (ca. 200 km × 400 km), for 12 years from 1989 to 2000. Flowering and seed production of F. crenata were also investigated at three stands using seed traps during the same period, to calibrate visually assessed data with actual seed production, and to estimate factors affecting seed production of this species in more detail. Geographical distributions of seeding in this region were classified into four types: mast seeding throughout the study area, in northern districts, in southern districts, and in no districts, and these types occurred in alternating years. Mast seeding throughout the study area occurred only twice. These two events might possibly be explained by higher temperature in mid-June of the preceding years. There were 11 groups of districts, within which seed production was synchronized significantly. Districts of each group were significantly aggregated by the scale of 60–190 km. Five of the groups were found in the northern part, four were in the southern part, and two were in the middle part. The results suggest that F. crenata innately synchronizes its seed production throughout the study area. Such seeding synchrony is disturbed, however, to form groups of smaller-scale synchrony. Causal factors may include local events such as an outbreak of seed-eating insects, as well as spatial variation in weather conditions.