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Studying the spatial pattern and interspecific associations of plant species may provide valuable insights into processes and mechanisms that maintain species coexistence. Point pattern analysis was used to analyze the spatial distribution patterns of twenty dominant tree species, their interspecific spatial associations and changes across life stages in a 20-ha permanent plot of seasonal tropical rainforest in Xishuangbanna, China, to test mechanisms maintaining species coexistence. Torus-translation tests were used to quantify positive or negative associations of the species to topographic habitats. The results showed: (1) fourteen of the twenty tree species were negatively (or positively) associated with one or two of the topographic variables, which evidences that the niche contributes to the spatial pattern of these species. (2) Most saplings of the study species showed a significantly clumped distribution at small scales (0–10 m) which was lost at larger scales (10–30 m). (3) The degree of spatial clumping deceases from saplings, to poles, to adults indicates that density-dependent mortality of the offspring is ubiquitous in species. (4) It is notable that a high number of positive small-scale interactions were found among the twenty species. For saplings, 42.6% of all combinations of species pairs showed positive associations at neighborhood scales up to five meters, but only 38.4% were negative. For poles and adults, positive associations at these distances still made up 45.5% and 29.5%, respectively. In conclusion, there is considerable evidence for the presence of positive interactions among the tree species, which suggests that species herd protection may occur in our plot. In addition, niche assembly and limited dispersal (likely) contribute to the spatial patterns of tree species in the tropical seasonal rain forest in Xishuangbanna, China.

Habitat heterogeneity and dispersal limitation are widely considered to be the two major mechanisms in determining tree species distributions. However, few studies have quantifi ed the relative importance of these two mechanisms at diff erent life stages of trees. Moreover, rigorous quantifi cation of the eff ects of dominant tree species in determining species distributions has seldom been explored. In the present study, we tested the hypothesis that the distribution of tree species is regulated by diff erent mechanisms at diff erent life history stages. In particular, we hypothesised that dispersal limitation regulates the distribution of trees at early life stages and that environmental factors control the distribution of trees as they grow, because of niche diff erentiation resulting from environmental fi ltering. To test this, trees in 400-m 2 quadrats in a 20-ha plot in Xishuangbanna, southwest China were grouped into four classes on the basis of the diameter at breast height (DBH) that roughly represent diff erent stages in the life history of trees. A neighbourhood index was computed to represent a neutral spatial autocorrelation eff ect. We used both biotic (dominant species) and abiotic (topography and soil) predictor variables to model the distribution of each target species while controlling for spatial autocorrelation within each of the DBH classes. To determine which factors played the largest role in regulating target species distribution, the simulated annealing method was used in model selection based on Akaike information criterion (AIC) values. Th e results showed that the relative importance of neutral and niche processes in regulating species distribution varied across life stages. Th e neutral neighbourhood index played the most important role in determining the distributions of small trees (1 cm Յ DBH Յ 10 cm), and dominant species, as biotic environmental predictor variables, were the next most important regulators for trees of this size. Environmental predictor variables played the most important role in determining the distributions of large trees (10 cm Յ DBH). Th is fi nding builds on previous research into the relative importance of neutral and niche processes in determining species distributions regardless of life stages or DBH classes.

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Spatial dispersion patterns of trees at different life stages are an important aspect to investigate in understanding the mechanisms that facilitate species coexistence. In this paper, Ripley's univariate L(r) and bivariate L 12 (r) functions were used to analyze spatial distribution patterns and spatial associations across different life stages of 131 tree species in a 20-ha plot of a tropical rainforest in Xishuangbanna, southwest China. Our results show that: (1) Saplings of 109 (83.2%) species have a significant clumped distribution, which confirms the ubiquity of clumped spatial distributions among tropical tree species.(2) Adults of 126 (96.2%) species have a random distribution suggesting that density-dependent mortality can make the spatial pattern of tropical trees more regular with time.(3) At small scales (0-10 m), 95 (72.5%) species have a neutral or negative sapling-adult association, implying that there is recruitment limitation within the vicinity of their conspecific adults. The reduction in spatial clumping in going from younger to older life stages and the neutral or negative sapling-adult association imply densitydependent mortality in the vicinity of adult trees. In accordance with the Janzen-Connell hypothesis, such density-dependent mortality can free up space for other species to colonize, contributing to the maintenance of species diversity.

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The integrity of blood–brain‐barrier (BBB) is essential for normal brain functions, synaptic remodeling, and angiogenesis. BBB disruption is a common pathology during Parkinson's disease (PD), and has been hypothesized to contribute to the progression of PD. However, the molecular mechanism of BBB disruption in PD needs further investigation. Here, A53T PD mouse and a 3‐cell type in vitro BBB model were used to study the roles of α‐synuclein (α‐syn) in BBB disruption with the key results confirmed in the brains of PD patients obtained at autopsy. The A53T PD mouse studies showed that the expression of tight junction‐related proteins decreased, along with increased vascular permeability and accumulation of oligomeric α‐syn in activated astrocytes in the brain. The in vitro BBB model studies demonstrated that treatment with oligomeric α‐syn, but not monomeric or fibrillar α‐syn, resulted in significant disruption of BBB integrity. This process involved the expression and release of vascular endothelial growth factor A (VEGFA) and nitric oxide (NO) from oligomeric α‐syn treated astrocytes. Increased levels of VEGFA and iNOS were also observed in the brain of PD patients. Blocking the VEGFA signaling pathway in the in vitro BBB model effectively protected the barrier against the harmful effects of oligomeric α‐syn. Finally, the protective effects on BBB integrity associated with inhibition of VEGFA signaling pathway was also confirmed in PD mice. Taken together, our study concluded that oligomeric α‐syn is critically involved in PD‐associated BBB disruption, in a process that is mediated by astrocyte‐derived VEGFA.