Negative canopy–understorey interaction shapes the sapling bank of Fagus crenata in a cool-temperate, conifer–hardwood mixed forest

Abstract

Negative density-dependence (NDD) is a major mechanism for shaping forest communities. However, its influence on species coexistence in conifer–hardwood mixed forests remains unclear. We aimed to determine whether NDD influences establishment of the seedling and/or sapling bank of Fagus crenata in an evergreen, coniferous Cryptomeria japonica-dominated stand, focusing on canopy structure (openness, distance from mature plants and species composition). For F. crenata trees, we analysed the spatial pattern using the pair-correlation function g(r) to test for NDD and spatial correspondence to canopy structure in two study plots established in a cool-temperate, old-growth conifer–hardwood mixed forest in western Japan. We also analysed the height growth of offspring and understorey light condition with respect to canopy structure. We detected evidence of NDD among neighbours during the transition of seedlings (age ≤ 4 years) to saplings (age > 4 years, stem length 4 years) was elevated under the canopy of C. japonica, which was lighter than under that of F. crenata. Our results suggest that F. crenata recruitment was affected by NDD at the seedling-to-sapling transition under the conspecifics and allowed for the formation of the sapling bank under C. japonica canopies beyond the drip lines of conspecifics.