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Large‐diameter, tall‐stature, and big‐crown trees are the main stand structures of forests, generally contributing a large fraction of aboveground biomass, and hence play an important role in climate change mitigation strategies. Here, we hypothesized that the effects of large‐diameter, tall‐stature, and big‐crown trees overrule the effects of species richness and remaining trees attributes on aboveground biomass in tropical forests (i.e., we term the “big‐sized trees hypothesis”). Specifically, we assessed the importance of: (a) the “top 1% big‐sized trees effect” relative to species richness; (b) the “99% remaining trees effect” relative to species richness; and (c) the “top 1% big‐sized trees effect” relative to the “99% remaining trees effect” and species richness on aboveground biomass. Using environmental factor and forest inventory datasets from 712 tropical forest plots in Hainan Island of southern China, we tested several structural equation models for disentangling the relative effects of big‐sized trees, remaining trees attributes, and species richness on aboveground biomass, while considering for the full (indirect effects only) and partial (direct and indirect effects) mediation effects of climatic and soil conditions, as well as interactions between species richness and trees attributes. We found that top 1% big‐sized trees attributes strongly increased aboveground biomass (i.e., explained 55%–70% of the accounted variation) compared to species richness (2%–18%) and 99% remaining trees attributes (6%–10%). In addition, species richness increased aboveground biomass indirectly via increasing big‐sized trees but via decreasing remaining trees. Hence, we show that the “big‐sized trees effect” overrides the effects of remaining trees attributes and species richness on aboveground biomass in tropical forests. This study also indicates that big‐sized trees may be more susceptible to atmospheric drought. We argue that the effects of big‐sized trees on species richness and aboveground biomass should be tested for better understanding of the ecological mechanisms underlying forest functioning.

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Aim and hypothesis This study aims to disentangle the direct and indirect roles of space, climate, edaphic, and biodiversity effects on aboveground biomass in natural tropical forests, thereby evaluating the relative effects and contribution of abiotic and biotic factors on aboveground biomass. We hypothesized that the space effect drives the longitudinal, latitudinal, and elevational patterns in climatic and edaphic factors, thereby directly and indirectly determining the relationships between biodiversity and aboveground biomass in natural tropical forests. Methods We used structural equation modelling for linking spatial, climatic, edaphic, and biotic factors of aboveground biomass, using data from 247,691 trees across 907 tropical forest plots (total sampling area of 145.23 ha) of Hainan Island in Southern China. Results Aboveground biomass increased directly with functional dominance, individual tree size inequality, and climatic water availability but decreased directly with space and edaphic effects. However, space effect increased aboveground biomass indirectly via simultaneously differential direct changes (positive, negative, and non‐significant) in climatic, edaphic, and biotic factors. As such, indirect effects of mean annual temperature and climatic water availability decreased aboveground biomass through differential direct changes in biotic factors, but opposite was true for soil fertility. Conclusions We argue that, despite the high relative contribution of biodiversity to aboveground biomass, the direct and indirect roles of space, climatic, and edaphic effects are also important for explaining biotic factors and aboveground biomass under the predictions of several abiotic‐based hypotheses. Hence, conserving biodiversity across space is important for forest management and land development under climate change.

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Ovarian carcinoma is one of the most common malignant cancers in women. Previous research has shown that Smad4 participates in the progression of multiple biological reactions as a crucial regulator. Nevertheless, studies on the role of Smad4 in ovarian carcinoma have been extremely limited. The study aim was to explore the mechanism underlying Smad4 regulation of HO-8910 and SKOV3 cell viability and autophagy. We observed that Smad4 gene expression in ovarian carcinoma tissues and cell lines was downregulated, and Smad4 overexpression resulted in decreased proliferation and increased autophagy in HO-8910 and SKOV3 cells (ovarian carcinoma cells). We also found that Smad4 overexpression induced apoptosis of ovarian carcinoma cells. A co-immunoprecipitation assay also revealed that Smad4 interacted with the P85 subunit of PI3K and caused its degradation and dephosphorylation. Subsequently, expression of mTOR was inhibited. Accordingly, these findings showed that further investigation of the biological mechanisms underlying ovarian carcinoma occurrence and progression is warranted, which may lead to new ovarian carcinoma treatment strategies. Impact statement This study investigated the effect and mechanism of Smad4 in ovarian carcinoma (OC) cell viability and demonstrated that Smad4 acted as a tumor suppressor in OC, which may contribute to the understanding of molecular mechanisms underlying OC occurrence and progression. Smad4 expression was decreased in the OC specimens, but Smad4 recovery in the OC cell lines impaired the survival and viability of OC cells by increasing autophagy and apoptosis. Further investigation showed that Smad4 interacted with the P85 subunit of PI3K and caused deactivation of the PI3K/mTOR pathway. Therefore, Smad4 could be considered as a target in cancer therapy due to its regulatory effect in OC carcinogenesis.