Neighborhood diversity structure and neighborhood species richness effects differ across life stages in a subtropical natural secondary forest

Natural secondary forest has a strong capacity to regrow naturally and recover biodiversity rapidly on abandoned lands. However, at the neighborhood scale, which can truly reflect the facilitative or competitive interactions among individual plants, the local diversity spatial structure in secondary forest and the feedback effects of neighborhood diversity on natural regeneration remain unclear, and this may be the key to properly understand the mechanisms of natural secondary forest species diversity recovery. To this end, this study established a dynamic plot in a rehabilitated secondary forest after disturbance and conducted a comprehensive survey of 68,336 individual plants with repeated measurements at 5-year interval to assess the characteristics of neighborhood diversity structure across life history stages and link the neighborhood species richness (NSR) effect translated by species interactions at species diversity structure with individual trees recruitment/mortality in secondary forest regeneration. The results showed that, compared with tropical and temperate natural forests, a higher proportion of diversity accumulators and a lower proportion of repellers in subtropical secondary forests resulted in neighborhood diversity structures characterized by heterospecific or high-diversity patches, which are beneficial to the maintenance or restoration of biodiversity. As an important supplement to the research on the relationship between diversity and productivity, our findings show a positive diversity–survival relationship in subtropical secondary forests. Importantly, we observed that the neighborhood diversity structure exhibited a trend of accumulator-dominated to neutral-dominant changes with life stage from sapling to adult, which, in turn, determined the direction and strength of NSR effects on recruitment/mortality. Specifically, diverse local neighborhoods at a later successional stage characterized with ‘neutral’ species–species interactions can act as a ‘welfare net’ by offering favorable microhabitats for the most vulnerable recruitments or saplings, i.e., the NSR effects that promoted individual recruitment/survival in our study. These results not only enrich our understanding of the biodiversity–productivity–survival relationship but also highlight the importance of retaining late-successional species of native trees in intensive forest production or in situ conservation policies.