Long-term forest dynamics in response to climate change in northern mixed forests in Japan: A 38-year individual-based approach

Climate change is expected to cause a directional change in northern mixed forests from sub-boreal conifer-dominated to cool-temperate broad-leaved trees dominated forests due to species-specific responses to the changing climate regime. However, few studies have examined both the long-term effects of climate (temperature, precipitation and wind disturbance) and non-climate (stand structure and topography) factors. In this study, we analyzed forest census data from 15 forest plots (17.5 ha, 8460 trees) in two primeval reserve areas over a 38-year period (1979-2016) using individual-based or grid-based state-space models to evaluate the effects of climate and non-climate factors that affect the demographic parameters (growth rate, mortality, and recruitment rate) of the three coniferous and seven deciduous broad-leaved species. Two climate factors during the growth period differentially affected the growth rate of trees: high temperature and high precipitation had negative effects on coniferous species and positive effects on broad-leaved species, respectively. The change of stand basal area and its interaction between precipitation in non-growth period affected the growth rate of some species. Furthermore, the mortality of large conifers was strongly affected by a typhoon in 2004. The dominance of coniferous species has declined especially in plots with west-facing slopes and from snowfall that has largely diminished since 1979. In northern Japan, the temperature and precipitation of the growth period are increasing, and snowfall is decreasing. Our results indicated that this climate change reduced the growth rates of coniferous species and increased those of broad-leaved species. Moreover, wind disturbances will increase, which will accelerate the change in species composition by removing dominant coniferous trees. This study is the first to show these directional changes in the species composition of northern mixed forests from conifer-dominated to broad-leaved tree-dominated based on the long-term tree demography.