Dynamics of species diversity in a Japanese warm-temperate secondary forest

To examine the dynamics of species diversity during secondary succession, growth, death and recruitment of trees were monitored for 39 years (1966 to 2005) in two plots (P-1 and P-3, 40 m × 40 m each) in a Japanese warm-temperate secondary forest recovering from clearcutting that occurred in the 1910s. Typhoons in the early 1990s seriously damaged P-1 and moderately damaged P-3. We examined how species diversity of trees changed through succession for three size classes: large (diameter at breast height, dbh ≥ 4.5 cm), medium (4.5 cm > dbh ≥ 1.0 cm), and small (dbh < 1.0 cm). Before the typhoons occurred, species diversity and evenness generally increased over time in all size classes in both plots, except for medium trees in P-1. After the typhoons, species diversity increased over time without exception, and this usually involved increases in both species evenness and number. We found that recruitment always contributed to increases in species diversity but mortality did not. Recruitment increased species diversity because the recruitment rate of rare species was higher than that of abundant species. This reflected delayed recruitment of late successional species before the typhoons and the mass recruitment of early successional species after the typhoons. Abundance of the most dominant tree species of the large class in P-1 was greatly reduced due to mortality caused by the typhoon, resulting in an increase in species diversity in the large tree community. Gap formation by the typhoons also caused mass recruitment into the small and medium size classes, including many rare species, leading to increased species diversity in the small and medium tree communities. Through these processes, the typhoons enhanced species diversity in this Japanese warm-temperate forest. Synthesis: This is the first study on the dynamics of species diversity based on long-term monitoring of a single site in a Japanese warm-temperate forest. This direct monitoring revealed the species dynamics plus the significant impact of rare events, typhoons, on the species diversity.