Decline in functional diversity during the stem exclusion phase: Long-term tree census of secondary succession in a cool-temperate forest, central Japan

Although successional changes in the functional diversity (FD) of plant communities have been reported in earlier studies, the processes driving these changes remain unclear. Based on the datasets of a long-term tree census conducted during a secondary succession, we comprehensively investigated the FD and its rate of change during a secondary succession in cool, temperate secondary forests in Chichibu, Central Japan. Data from more than 30-years of observations in plots of secondary forest stands with a wide range of stand ages were used to test changes in species diversity, community-weighted means (CWMs) of each trait, FD, and their change rates. Although species diversity decreased constantly with stand age, the FD (Rao's Q) of all traits decreased significantly at the mid-successional stage, corresponding to the stem-exclusion stage. Changes in CWMs of traits indicated the directional shifts of species composition from resource-acquisitive to resource-conservative along succession, especially at the stem exclusion stage. Although the FD of each trait changed differently with succession, that of several resource-conserving traits decreased significantly at the stem exclusion stage. Based on long-term multiple chronosequence resampling, we found that stem exclusion significantly decreased the FD along a secondary succession in cool temperate forests. This sheds light on the critical role of the stem exclusion stage in shaping plant community traits, indicating the importance of this stage in forest management strategies.