Properties of stumps that promote the growth and survival of Japanese cedar saplings in a natural old-growth forest

Abstract

In a natural forest that has experienced logging and lacks large fallen logs, cut stumps are expected to become a suitable substrate for regeneration. We investigated the properties of stumps that promoted the natural regeneration of coniferous species in a natural old-growth forest dominated by Cryptomeria japonica (L.f.) D. Don. A comparison of C. japonica saplings growing in three microsites (ground, stumps, and fallen logs) revealed that sapling density was highest on the stumps; 75% of saplings were distributed on stumps, which occupied only 3% of the projected forest area. The mortality of saplings during the 3-year study period did not differ significantly among the three microsites. Six properties of stumps were quantified: height, diameter at ground level, decay class, species taxon, type (i.e., cause of occurrence: logging or natural disturbance), and light conditions. Of these properties, the type of stump was the most influential. The density of saplings was significantly greater on stumps produced by logging than on naturally broken stumps. Taller stumps exhibited a significantly higher density of saplings; indeed, there were no saplings on stumps less than 60 cm in height. Saplings on stumps were concentrated in higher positions on the stump, and survivorship tended to be greater when their root location was higher. We concluded that the height of the microsite was an important factor for regeneration of C. japonica at this research site, where saplings were under competition with dense undergrowth vegetation. Most of the fallen logs were less than 60 cm in height and it appeared unlikely that they would function as regeneration sites for C. japonica.