The Plant Growth Inhibitor Nagilactone Does Not Work Directly in a Stabilized Podocarpus nagi Forest

Nagilactones isolated from Podocarpus nagi are known for poisonous physiological activities to organisms. To clarify the allelopathic potential of nagilactone to wild plants, the seasonal changes in nagilactone contents in throughfall, leaf litter, and soil were monitored monthly in a podocarp forest at Mt. Mikasa, Nara City, central Japan. The average annual nagilactone concentrations in throughfall, leaf litter, and surface soil at 0–10 cm depth were 4.7 × 10^–12 g/ml, 3.8 × 10^–4 g/g and 3.1 × 10^–7 g/g, respectively. The nagilactone flux from canopy leaves to the forest floor via throughfall was 5.2 × 10^–2 g/ha/yr, which was far smaller than that via leaf litterfall: 765.3 g/ha/year. The disappearance of nagilactones from leaf litter and the nagilactone accumulation in the Ao layer and mineral soil were also studied. Nagilactone in leaf litter rapidly disappeared and its relative disappearance rate was 5.8/yr. Nagilactone concentration in mineral soil also decreased rapidly with an increase in soil depth and could not be detected in soil at 40–100 cm depth. The total mass of nagilactones was 177.3 g/ha in the Ao layer and 105.0 g/ha in mineral soil. These quantities in nagilactone dynamics give a very rapid turnover time of 0.37 year (4.4 months) in a soil nagilactone pool, implying the possibility of the reabsorption of nagilactone by P. nagi, while the allelopathic effects of nagilactone to other plants in the podocarp forest are questionable.