Do finer gap mosaics provide a wider niche for Quercus gilva in young Japanese cedar plantations than coarser mosaics? Simulation of spatial heterogeneity of light availability and photosynthetic potentialThis article is one of a selection of papers published in the Special Forum IUFRO 1.05 Uneven-Aged Silvicultural Research Group Conference on Natural Disturbance-Based Silviculture: Managing for Complexity.

Both the size and the density of gaps affect seedling growth, but these two parameters have a reciprocal relationship at a given gap ratio. The objective of the present study was to clarify the appropriate gap mosaic coarseness required to facilitate the growth of Quercus gilva Blume seedlings in Cryptomeria japonica D. Don plantations. The spatial heterogeneity of the photosynthetic photon flux density on the forest floor was predicted using mimicked hemispherical diagrams under five levels of gap mosaic coarseness ranging from the finest gap mosaic with a gap size of 25 m2 and a gap density of 100·ha–1 to the coarsest mosaic of 400 m2 and 6.25·ha–1. Photosynthetic potentials (PP) were calculated by combining the predicted photosynthetic photon flux density and photosynthetic relationships of Q. gilva and two nontree major competitors ( Mallotus japonicus (Thunb.) Muell. Arg. and Miscanthus sinensis Anderss.). The coarser gap mosaic formed a more heterogeneous and bimodal PP frequency and resulted in a wider site in which the three species had high growth potential without considering competition among species. However, an intermediate mosaic with a gap width to mean canopy height ratio of 0.7 formed the widest realized niche in which Q. gilva would show good growth with a relative PP > 70%, whereas the competitor species would be suppressed with a relative PP < 70%.