Abstract: The growth of Sakhalin fir (Abies sachalinensis) seedlings, an important forest tree species in northern Hokkaido, Japan, is significantly affected by competition from surrounding vegetation, especially evergreen dwarf bamboo. In this study, we investigated the height and root collar diameter (RCD) growth of Sakhalin fir seedlings under various degrees of cover by deciduous vegetation and evergreen dwarf bamboo. Generalized additive models were used to quantify the effects of canopy cover and forest floor cover on the relative growth rates of these two parameters. The canopy cover of Sakhalin fir seedlings had a nonlinear negative effect on both the height growth of seedlings in the subsequent year and the RCD growth in the current year, given the general growth pattern in this species, where height growth ceases in early summer and RCD growth continues until autumn. Height growth declined sharply after the canopy cover rate exceeded 50%, while RCD growth declined rapidly between 0 and 50% canopy cover rate. The forest floor cover had a greater negative impact on RCD growth than on height growth. These results suggested that Sakhalin fir seedlings respond to vegetative competition by prioritizing height growth for light acquisition at the expense of diameter growth and possibly root growth for belowground competition. The cover of evergreen dwarf bamboo reduced the height growth of fir seedlings significantly more than the cover of deciduous vegetation. This difference is likely due to the timing of light availability. When competing with deciduous vegetation, Sakhalin fir seedlings exposed to light during the post-snow melt and early spring before the development of the deciduous vegetation canopy can photosynthesize more effectively, leading to greater height growth. The results of this study highlighted the importance of vegetation control considering the type of vegetation for successful Sakhalin fir reforestation. Adjusting the intensity and timing of weeding based on the presence and abundance of dwarf bamboo and other competing vegetation could potentially reduce weeding costs and increase biodiversity in reforested areas.

Key words: Abies sachalinensis, Competition, Crown cover, Forest floor cover, Generalized additive models (GAM), Relative growth rate