Effects of gap size on the understory in mature beech forests with dense dwarf bamboo

ABSTRACT Canopy gap mosaics, characteristic features of mature forests, exert substantial influence on understory and environmental variables. Previous studies have documented that larger gaps correlate with enhanced diversity of understory, known as gap size effect (GSE), particularly in tropical and subtropical forests. However, this relationship remains poorly understood in cool-temperate forests where dwarf bamboo dominates the understory exclusively. Dwarf bamboo can change this relationship through various mechanisms in understory. This study aimed to elucidate GSE and its mechanisms considering the presence of dwarf bamboo. In two cool-temperate forests, Kayanodaira and Ohshirakawa, central Honshu, Japan, gaps of various sizes were selected. In each gap, the estimated number of herb-layer spp. (height < 1 m) and shrub-layer spp. (height ≥1 m and diameter at breast height ≤5 cm) as understory was measured. Simultaneously, major environmental variables such as light intensity at three heights, dominance of dwarf bamboo, and litter thickness were also measured, and their means and variations were calculated. In two study sites, the estimated number of herb-layer and shrub-layer spp. increased with gap size. Moreover, this GSE was found to be influenced by dwarf bamboo from both negative and positive perspectives. As negative way, dense dwarf bamboo cover intercepts light and weakens the positive effect of increased light in larger gaps. As positive way, larger gaps lead to greater spatial heterogeneity in bamboo biomass, which enhances microenvironment variation and promotes niche differentiation. These findings suggest that forests dominated by dwarf bamboo exhibit a unique mechanism of GSE.