Evaluation of secondary dispersal in a large-seeded tree Aesculus turbinata: a test of directed dispersal

Gap characteristics and gap phase replacement of major tree species were examined in two primary old-growth (mean DBHs of the canopy trees were 45.2 and 56.1 cm) and four secondary developing (range of mean DBH of the canopy trees was 23.5–39.9 cm) beech (Fagus crenata) stands in the Daisen Forest Reserve, southwestern Japan, and these were analyzed in relation to stand development as expressed by the difference of mean DBH of canopy trees. Percentage gap area (percentage of total gap area to total surveyed area) and mean and maximum gap size varied widely and ranged from 1.7 to 20.0%, from 19.4 to 162.8 m2, and from 35.7 to 585.1 m2, respectively. Mean percentage gap area and mean gap size were significantly greater in old-growth than in developing stands. However, they and maximum gap size might not increase linearly with stand development, and the gap feature of less developed stands was greater than that of later stages in developing stands. The cause was a higher formation rate, in younger developing stands, of gaps formed by simultaneous death (multiple trees falling down in domino fashion) which tends to produce larger gaps. In developing stands mean DBH of gapmakers (canopy trees that formed a gap) was smaller than that of canopy trees, though the inverse trend might be found in old-growth stands. Three typical types of death or injury states of gapmakers such as standing dead, trunk broken and uprooted were found in every study stand and the difference in stand development may not cause stand-to-stand variations for them. Importance of F. crenata (the most dominant species) in the canopy layer increased and its importance in the understory layer decreased with stand development. Shade-intolerant Quercus mongolica in the canopy layer was more important in younger than in old-growth stands, and there was no Quercus regeneration in old-growth stands. Acer mono consistently appeared, though in much less abundance than other species, in both canopy and understory layers of all study stands. Sub-canopy layers, which are mainly formed by sub-canopy tree species such as Acanthopanax sciadophylloides and Acer japonicum, may gradually develop with stand development.p>