Search

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. • We used long-term tree census data from secondary forest succession. • Analyzing functional diversity change over time using long-term and multiple chronosequence resampling techniques. • Finding the decrease in functional diversity as forests advance through the stem exclusion stage. • Highlights the crucial role of stem exclusion phase in shaping plant trait diversity. [ABSTRACT FROM AUTHOR]

Species coexistence is a result of biotic interactions, environmental and historical conditions. The Janzen-Connell hypothesis assumes that conspecific negative density dependence (CNDD) is one of the local processes maintaining high species diversity by decreasing population growth rates at high densities. However, the contribution of CNDD to species richness variation across environmental gradients remains unclear. In 32 large forest plots all over the Japanese archipelago covering > 40,000 individual trees of > 300 species and based on size distributions, we analysed the strength of CNDD of individual species and its contribution to species number and diversity across altitude, mean annual temperature, mean annual precipitation and maximum snow depth gradients. The strength of CNDD was increasing towards low altitudes and high tree species number and diversity. The effect of CNDD on species number was changing across altitude, temperature and snow depth gradients and their combined effects contributed 11–18% of the overall explained variance. Our results suggest that CNDD can work as a mechanism structuring forest communities in the Japanese archipelago. Strong CNDD was observed to be connected with high species diversity under low environmental limitations where local biotic interactions are expected to be stronger than in niche-based community assemblies under high environmental filtering. [ABSTRACT FROM AUTHOR]