Conifer dominance is mostly restricted to high latitudes, high elevations or nutrient-deficient soils in the humid regions of the world. This is because conifers (gymnosperm) are competitively inferior to broadleaf trees (angiosperm) at the juvenile stage in warm and fertile habitats due to the less efficient water-transporting systems (non-reticulated leaves and tracheids). Yakushima Island, southern Japan, exhibits a conspicuous forest zonation: evergreen broadleaf forests (EBF) at the low elevations (< 1000 m) and mixed forests (MF) dominated by both evergreen conifers and evergreen broadleaf trees at the high elevations (> 1000 m). In this study, we analyzed the data of long-term tree censuses in EBF and MF of Yakushima to compare the tree dynamics and forest structure, to elucidate how conifers and broadleaf trees coexist in MF of Yakushima.
Tree census was conducted every five years from 1993 to 2023 in EBF (600m, 0.4 ha) and MF (at 1200 m, 1.0 ha) of Yakushima. In each tree census, diameter at breast height (DBH) was measured for all living trees > 2.9 cm DBH, and death and recruitment were recorded. Aboveground biomass was calculated from wood density, DBH and tree height. Relative growth rate (RGR) of aboveground biomass, mortality, and recruitment rate were calculated for the six five-year periods. We measured the relative light intensity at 1-m height intervals in a EBF (170 m) and MF (1200 m). To characterize forest structure, we used the data of canopy height profile quantified by a ground-based LIDAR system.
In MF, conifer dominance persisted for 30 years. Mortality and recruitment rate were almost balanced for conifers. There were only a small number of conifers in the small DBH class (2.9-10 cm), but RGR of conifers was higher than that of broadleaf trees. When EBF and MF were compared, mortality of broadleaf trees was higher in MF than in EBF, especially at small DBH. MF exhibited vertically elongated canopy height profiles, with less distinct upper canopy layers, compared to EBF. Relative light intensity was higher in MF than in EBF in the understory below 15 m height, and correspondingly, RGR of five common understory species were higher in MF than in EBF, especially in the small DBH class. These findings suggest that the coexistence of conifers and broadleaf trees is associated with the unique canopy structure of MF, which provides regeneration opportunities for conifers.