Effects of canopy gaps on the genetic structure of Camellia japonica saplings in a Japanese old-growth evergreen forest

The genetic structure of Camellia japonica saplings was investigated in relation to canopy conditions in an old-growth evergreen forest in Tsushima, Japan. To elucidate effects of canopy gaps on genetic structure, a 1 ha study site was divided into 20 x 20 m quadrats, which were classified into a gap quadrats (GAP), closed canopy quadrats (CLS) and mixed quadrats. Five GAP quadrats and six CLS quadrats were analyzed separately. Isolation-by-distance was tested by examining the correlation between genetic distance and geographic distance. A significant positive correlation was detected for GAP quadrats, whilst that for CLS quadrats was significantly smaller and not significantly different from zero. On the other hand, an analysis using Moran's I spatial autocorrelation coefficients indicates that the genetic structure is weaker in GAP quadrats than in CLS quadrats in short distance classes. The values were significantly positive for both types of quadrat. These results, along with our field observations on flowering, suggest that canopy gaps affect the genetic structure of C. japonica saplings in two distinct ways. First, canopy gaps may promote flowering and mating in an isolation-by-distance manner within canopy gaps. Second, canopy gaps may promote seed production and resulting overlap in seed shadows may weaken fine-scale genetic structures.