Stand dynamics over 15 years including an outlying population of Fagus crenata at the northern margin of its distribution range in Hokkaido, Japan

Abstract

AbstractFagus crenata, a dominant tree species in cool‐temperate deciduous broad‐leaved forest of Japan, is continuously distributed northward to the Kuromatsunai lowland of Hokkaido, beyond which its distribution is patchy and isolated. We analyzed stand dynamics over 15 years (2006–2021) in a plot (0.75 ha) including one such outlying population of F. crenata. At the stand level over 15 years, stem density decreased, but aboveground biomass increased. The dominance of F. crenata consistently increased in stem density and aboveground biomass. Change in the relationship between stem density and mean stem mass indicated a successional nature of the stand undergoing self‐thinning. Increasing F. crenata dominance and the associated compositional shift was due to the higher growth rates and lower mortality of F. crenata compared to other tree species. Judging from tree‐ring analysis, the oldest F. crenata tree in the plot established in 1821, but the density of F. crenata remained low till ca. 1895. The stand started to be affected by persistent disturbance since ca. 1895, with the waves of tree establishment and growth release both peaking at ca. 1930–1960. The F. crenata population mirrored the pattern of the stand, with more pronounced increases during these waves. We suggest that the long‐lasting episode of tree establishment and growth release over ca. 1930–1960 was caused by human disturbance, most likely selective logging for fuelwood. The results support the “northward expansion” hypothesis, implying that F. crenata potentially can become dominant beyond the current northern limit of continuous distribution and that disturbance facilitates the range expansion.