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The Greater Mekong Subregion (GMS) is one the global biodiversity hotspots. However, the diversity has been seriously threatened due to environmental degradation and deforestation, especially by expansion of rubber plantations. Yet, little is known about the impact of expansion of rubber plantations on regional plant diversity as well as the drivers for plant diversity of rubber plantations in this region. In this study, we analyzed plant diversity patterns of rubber plantations in the GMS based on a ground survey of a large number of samples. We found that diversity varied across countries due to varying agricultural intensities. Laos had the highest diversity, followed China, Myanmar, and Cambodia. Plant species richness of Laos was about 1.5 times that of Vietnam. We uncovered latitudinal gradients in plant diversity across these artificial forests of rubber plantations and these gradients caused by environmental variables such as temperature. Results of redundancy analysis (RDA), multiple regression, and random forest demonstrated that latitude and temperature were the two most important drivers for the composition and diversity of rubber plantations in the GMS. Meanwhile, we also found that higher dominance of some exotic species (such as Chromolaena odorata and Mimosa pudica) was associated with a loss of plant diversity within rubber plantations; however, not all exotic plants cause the loss of plant diversity in rubber plantations. In conclusion, not only environmental factors (temperature), but also exotic species were the main factors affecting plant diversity of these artificial stands. Much more effort should be made to balance agricultural production with conservation goals in this region, particularly to minimize the diversity loss in Vietnam and Cambodia.

Soil microbial communities play a crucial role in ecosystem functioning. Past research has examined the effects of forest conversion on soil microbial composition and diversity, but it remains unknown how networks within these communities respond to forest conversion, including when tropical rainforests are replaced with rubber plantations. Microbial networks are viewed as critical indicators of soil health and quality. They consist of two parts: nodes and edges. In this study, we used data from Illumina sequencing and shotgun metagenome sequencing to analyze bacterial and fungal community network structure in a large number of soil samples from tropical rainforests and rubber plantation sites on Hainan Island, China. Our results showed that only 5 %–10 % of shared network edges (i.e., links between species A and B existing in both rubber plantations and rainforests) were observed in both bacterial and fungal communities, which indicates that forest conversion altered the soil microbial network structure. The identity of keystone operational taxonomic units (OTUs) differed entirely between rubber plantation and rainforest sites, further underscoring the altered network structure. More edges and more negative correlations within the soil bacterial–fungal networks were observed at rubber plantation sites (dry season: 4284 total edges, 844 negative; rainy season: 7257 total edges, 1744 negative) than at rainforest sites (dry season: 3650 total edges, 149 negative; rainy season: 6018 total edges, 489 negative), demonstrating that soil bacterial–fungal network structure was more complex and stable in rubber plantations than in rainforests. For bacteria, a larger number of network edges were observed among bacterial networks in samples from tropical rainforest than in samples from rubber plantations, indicating that rainforest bacterial networks were more complex than those from rubber plantations. However, soil fungal networks from rubber plantations showed more links, suggesting that forest conversion increased fungal network complexity. More edges of network and more links between species and functions were observed in the rainy season than in the dry season, indicating that seasonal changes had a strong effect on network structure and function. Further analysis shows that soil pH, potassium (AK), and total nitrogen (TN) had more links with species of some phyla. In conclusion, forest conversion results in an increase in soil pH as well as a decrease in AK and TN, and these changes as well as seasonal variations had a great impact on soil microbial composition, network structure and function.

Although presynaptic loss measured by cerebrospinal fluid (CSF) growth-associated protein-43 (GAP-43) is significantly involved in Alzheimer's disease (AD), the sequential association between CSF GAP-43 and AD-typical neurodegeneration is poorly understood.We compared baseline CSF GAP-43 levels (n = 730) and longitudinal CSF GAP-43 changes (n = 327) in various biological stages of AD, and investigated their relationships with cross-sectional and longitudinal measures of residual hippocampal volume,Elevated CSF GAP43 levels were significantly associated with faster rates of hippocampal atrophy, AD-signature hypometabolism and cortical thinning, and middle temporal gray matter atrophy-related and AD-signature hypometabolism-related cognitive decline. In contrast, baseline levels of all these neurodegeneration biomarkers did not predict longitudinal CSF GAP-43 increases.These findings suggest that presynaptic loss may occur prior to neurodegeneration, highlighting the importance of lowing tau aggregation and tau-related synaptic dysfunction in elderly adults and AD patients.

Recent and rapid expansion of rubber [Hevea brasiliensis (Willd. ex A. Juss.) Müll. Arg.] plantations requires understanding their effects on soil physicochemical properties and soil quality. An ideal testbed for analyzing such land-use change and its impacts is Hainan Island, the largest tropical island in China, which in recent decades has seen a dramatic expansion in the rubber industry. Based on 14 soil physicochemical properties at two soil depths (0-20 and 20-40 cm), a comprehensive assessment index was established using principal component analysis to assess soil qualities under rubber plantations (RPs; monoculture and intercropping) and five additional land-use types (areca palm [Areca L.], eucalyptus [Eucalyptus loxophleba Benth.] and banana [Musa L.] plantations, secondary forest, and tropical rainforest [TR]). The following results were obtained: (a) total porosity, ammoniacal N, total P, available P, and soil organic matter were vital soil physicochemical properties contributing to the comprehensive assessment index; (b) the comprehensive assessment indices of RPs were significantly lower than those of TR and areca palm plantation; (c) intercropping improves most soil physicochemical properties in RPs comparing monoculture and intercropped RPs; and (d) redundancy analysis demonstrated that land-use type interacted with climatic, geographical, and edaphic factors and collectively explained about half of the variation in the soil physicochemical properties across the study area. Deteriorating soil quality by converting TR to RPs and other land-use types provides another reason to protect TRs, especially on area-limited islands like Hainan.

Increased presynaptic dysfunction measured by cerebrospinal fluid (CSF) growth-associated protein-43 (GAP43) may be observed in Alzheimer's disease (AD), but how CSF GAP43 increases relate to AD-core pathologies, neurodegeneration, and cognitive decline in AD requires further investigation.We analyzed 731 older adults with baseline β-amyloid (Aβ) positron emission tomography (PET), CSF GAP43, CSF phosphorylated tau181 (p-TauRegardless of Aβ positivity and clinical diagnosis, CSF p-TauTau aggregations rather than Aβ plaques primarily drive presynaptic dysfunction measured by CSF GAP43, which may lead to sequential neurodegeneration and cognitive impairment in AD or neurodegenerative diseases. ANN NEUROL 2022;92:1001-1015.

Phyllosphere microbiomes play an essential role in maintaining host health and productivity. Still, the diversity patterns and the drivers for the phyllosphere microbial community of the tropical cash crop Rubber tree (Hevea brasiliensis) - are poorly understood. We sampled the phyllosphere of field-grown rubber trees in South China. We examined the phyllosphere bacterial and fungal composition, diversity and main drivers of these microbes using the Illumina® sequencing and assembly. Fungal communities were distinctly different in different climatic regions (i.e. Xishuangbanna and Hainan Island) and climatic factors, especially mean annual temperature, and they were the main driving factors of foliar fungal communities, indicating fungal communities showed a geographical pattern. Significant differences of phyllosphere bacterial communities were detected in different habitats (i.e. endophytic and epiphytic). Most of the differences in taxa composition came from Firmicutes spp., which have been assigned as nitrogen-fixing bacteria. Since these bacteria cannot penetrate the cuticle like fungi, the abundant epiphytic Firmicutes spp. may supplement the deficiency of nitrogen acquisition. And the main factor influencing endophytic bacteria were internal factors, such as total nitrogen, total phosphorus and water content of leaves. External factors (i.e. climate) were the main driving force for epiphytic bacteria community assembly. Our work provides empirical evidence that the assembly of phyllosphere bacterial and fungal differed, which creates a precedent for preventing and controlling rubber tree diseases and pests and rubber tree yield improvement.

Land-use changes can alter soil properties and thus affect soil quality. Our understanding of how forest conversion (from tropical rainforest to rubber plantations) affects soil properties and soil quality is limited. An ideal testing ground for analyzing such land-use change and its impacts is Hainan Island, the largest tropical island in China. Based on 21 soil physicochemical and biological properties, a soil quality index (SQI) employed principal component analysis to assess soil quality changes from the conversion of tropical rainforests to rubber plantations. The results showed that (i) soil available potassium, available phosphorus, microbial biomass carbon, cellulose decomposition, acid phosphatase, and urease were vital soil properties for soil quality assessment on Hainan Island. (ii) The SQI of rubber plantations decreased by 26.48 % compared to tropical rainforests, while four investigated soil properties (soil pH, total phosphorus, cellulose decomposition, and actinomyces) increased. (iii) The SQI of both the tropical rainforests and rubber plantations showed significant spatial differences, which, under tropical rainforests, was more sensitive to seasonal changes than those under rubber plantations. (iv) Structural equation modeling suggested that forest conversion directly impacted soil quality and, indirectly impacted soil qualities' spatial variation by their interaction with soil types and geographical positions. Overall, though the conversion of tropical rainforest to rubber plantation did not decrease all soil properties, the tropical rainforest with its high soil quality should be protected.

Soil microbial communities play a crucial role in ecosystem functioning. Past research has examined the effects of forest conversion on soil microbial composition and diversity, but it remains unknown how networks within these communities respond to forest conversion such as when tropical rainforest are replaced with rubber plantations. In this study, we used Illumina sequencing and metagenome shotgun sequencing to analyze bacterial and fungal community network structure in a large number of soil samples from tropical rainforest and rubber plantation sites in Hainan Island, China. Our results showed only a few shared network edges were observed in both bacterial and fungal communities, which indicates that forest conversion altered soil microbial network structure. We found a greater degree of network structure and a larger number of network edges among bacterial networks in samples from tropical rainforest compared to samples from rubber plantations. The difference was especially pronounced during the rainy season and indicates that rainforest bacterial networks were more complex than rubber plantation bacterial networks. However, rubber plantations soil fungal networks showed more higher links and higher network degree, suggesting that forest conversion does not reduce fungal network complexity. We found that some groups of Acidobacteria were keystone taxa in our tropical rainforest soils, while Actinobacteria were keystone taxa in rubber plantation soils. In addition, seasonal change had a strong effect on network degree, the complexity of soil bacterial and fungal network structure. In conclusion, forest conversion changed soil pH and other soil properties, such as available potassium (AK) and total nitrogen (TN), which resulted in changes in bacterial and fungal network composition and structure.

Soil microbial communities play a crucial role in ecosystem functioning. Past research has examined the effects of forest conversion on soil microbial composition and diversity, but it remains unknown how networks within these communities respond to forest conversion such as when tropical rainforest are replaced with rubber plantations. In this study, we used Illumina sequencing and metagenome shotgun sequencing to analyze bacterial and fungal community network structure in a large number of soil samples from tropical rainforest and rubber plantation sites in Hainan Island, China. Our results showed only a few shared network edges were observed in both bacterial and fungal communities, which indicates that forest conversion altered soil microbial network structure. We found a greater degree of network structure and a larger number of network edges among bacterial networks in samples from tropical rainforest compared to samples from rubber plantations. The difference was especially pronounced during the rainy season and indicates that rainforest bacterial networks were more complex than rubber plantation bacterial networks. However, rubber plantations soil fungal networks showed more higher links and higher network degree, suggesting that forest conversion does not reduce fungal network complexity. We found that some groups of Acidobacteria were keystone taxa in our tropical rainforest soils, while Actinobacteria were keystone taxa in rubber plantation soils. In addition, seasonal change had a strong effect on network degree, the complexity of soil bacterial and fungal network structure. In conclusion, forest conversion changed soil pH and other soil properties, such as available potassium (AK) and total nitrogen (TN), which resulted in changes in bacterial and fungal composition and network structure.

Rubber plantations have expanded rapidly over the past 20 years in tropical Asia and their impacts on regional ecosystems have garnered much concern. While much attention has been given to the negative impacts on aboveground diversity and function, the belowground bacterial soil community has received much less attention. Here, we investigated the community composition and diversity of soil bacteria of rubber plantations on Hainan Island in south China. The goals of the study were to describe changes in bacterial compositions and diversity across seasons. We found that seasonality defined by differences in rainfall amount strongly influenced bacterial communities. At both the Phylum and Family levels, we found significant differences in the total number of taxa, as well as the composition of the community as a function of season. Diversity of soil samples in the dry-rainy season was highest of three seasons, suggesting that bacterial structure was more sensitive in alternate periods of season. Diversity in the rainy season was substantial lower than in dry season. Results from a redundancy analysis showed that seasonal changes explained the largest part (31.9%) of the total variance of bacterial community composition. In conclusion, seasonal change had the greatest influence on bacterial communities, which overshadowed the effects of soil nutrient as well as other factors, and controls the bacterial communities in soils of RP in tropical region of Hainan.

In Xishuangbanna, a China's tropical region, monoculture rubber plantations have expanded dramatically over the past two decades. However, the effect of conversion of tropical forests to rubber plantations on the soil microbial compositions and diversity remains poorly understood. This study investigated the bacterial diversity and biomass in rubber plantations, secondary tropical forest and tropical seasonal rainforest in Xishuangbanna, southwest China, based on an analyses of PCR-amplified 16S rRNA genes. Our findings revealed that soil of rubber plantation was different from secondary tropical forest and tropical seasonal rainforest in bacterial compositions at phylum, family as well as OTU (Operational Taxonomic Units) levels. The most abundant phylum in rubber plantations was Firmicutes, however, in secondary tropical forest and tropical seasonal rainforest it was Proteobacteria. In both dry season and rainy season, diversity indices of rubber plantations were significantly higher than those of secondary tropical forest and tropical seasonal rainforest, indicating that bacterial diversity increased after conversion. However, the total microbial biomass carbon (MBC) decreased after conversion. RDA results showed that soil pH and total phosphorus maybe the two most important factors controlling bacterial communities, and explained 49.2% and 38.1% of total variance of phylum compositions, respectively. Management practices, especially the application of fertilizers with phosphorous would thus probably influence the taxonomic composition and diversity of bacterial communities in this region.

Due to rapid losses of mangrove forests caused by anthropogenic disturbances and climate change, accurate and contemporary maps of mangrove forests are needed to understand how mangrove ecosystems are changing and establish plans for sustainable management. In this study, a new classification algorithm was developed using the biophysical characteristics of mangrove forests in China. More specifically, these forests were mapped by identifying: (1) greenness, canopy coverage, and tidal inundation from time series Landsat data, and (2) elevation, slope, and intersection-with-sea criterion. The annual mean Normalized Difference Vegetation Index (NDVI) was found to be a key variable in determining the classification thresholds of greenness, canopy coverage, and tidal inundation of mangrove forests, which are greatly affected by tide dynamics. In addition, the integration of Sentinel-1A VH band and modified Normalized Difference Water Index (mNDWI) shows great potential in identifying yearlong tidal and fresh water bodies, which is related to mangrove forests. This algorithm was developed using 6 typical Regions of Interest (ROIs) as algorithm training and was run on the Google Earth Engine (GEE) cloud computing platform to process 1941 Landsat images (25 Path/Row) and 586 Sentinel-1A images circa 2015. The resultant mangrove forest map of China at 30 m spatial resolution has an overall/users/producer’s accuracy greater than 95% when validated with ground reference data. In 2015, China’s mangrove forests had a total area of 20,303 ha, about 92% of which was in the Guangxi Zhuang Autonomous Region, Guangdong, and Hainan Provinces. This study has demonstrated the potential of using the GEE platform, time series Landsat and Sentine-1A SAR images to identify and map mangrove forests along the coastal zones. The resultant mangrove forest maps are likely to be useful for the sustainable management and ecological assessments of mangrove forests in China.

The ratio of intercellular to ambient CO2 concentrations (c i/c a) plays a key role in ecophysiology, micrometeorology, and global climatic change. However, systematic investigation on c i/c a variation and its determinants are rare. Here, the c i/c a was derived from measuring ecosystem fluxes in an even-aged monoculture of rubber trees (Hevea brasiliensis). We tested whether c i/c a is constant across environmental gradients and if not, which dominant factors control c i/c a variations. Evidence indicates that c i/c a is not a constant. The c i/c a exhibits a clear "V"-shaped diurnal pattern and varies across the environmental gradient. Water vapor pressure deficit (D) is the dominant factor controls over the c i/c a variations. c i/c a consistently decreases with increasing D. c i/c a decreases with square root of D as predicted by the optimal stomatal model. The D-driving single-variable model could simulate c i/c a as well as that of sophisticated model. Many variables function on longer timescales than a daily cycle, such as soil water content, could improve c i/c a model prediction ability. Ecosystem flux can be effectively used to calculate c i/c a and use it to better understand various natural cycles.

Updated and accurate maps of tropical forests and industrial plantations, like rubber plantations, are essential for understanding carbon cycle and optimal forest management practices, but existing optical-imagery-based efforts are greatly limited by frequent cloud cover. Here we explored the potential utility of integrating 25-m cloud-free Phased Array type L-band Synthetic Aperture Radar (PALSAR) mosaic product and multi-temporal Landsat images to map forests and rubber plantations in Hainan Island, China. Based on structure information detected by PALSAR and yearly maximum Normalized Difference Vegetation Index (NDVI), we first identified and mapped forests with a producer accuracy (PA) of 96% and user accuracy (UA) of 98%. The resultant forest map showed reasonable spatial and areal agreements with the optical-based forest maps of Fine Resolution Observation and Monitoring Global Land Clover (FROM-GLC) and GlobalLand30. We then extracted rubber plantations from the forest map according to their deciduous features (using minimum Land Surface Water Index, LSWI) and rapid changes in canopies during Rubber Defoliation and Foliation (RDF) period (using standard deviation of LSWI) and dense canopy in growing season (using maximum NDVI). The rubber plantation map yielded a high accuracy when validated by ground truth-based data (PA/UA > 86%) and evaluated with three farm-scale rubber plantation maps (PA/UA >88%). It is estimated that in 2010, Hainan Island had 2.11 × 10 6 ha of forest and 5.15 × 10 5 ha of rubber plantations. This study has demonstrated the potential of integrating 25-m PALSAR-based structure information, and Landsat-based spectral and phenology information for mapping tropical forests and rubber plantations.

In tropical plant communities with diverse species, many congeners are found to coexist. Do environment or biotic interactions structure the coexistence of congeners in tropical forest communities? In this paper, we aimed to disentangle the effect of environment (first-order effects) and species interactions (second-order effects) on the spatial distributions of tree species. We used a classification scheme and torus-translation to test the first-order interaction of 48 species from 17 genera in a fully mapped 20-ha dipterocarp tropical seasonal rain-forest plot in Xishuangbanna, south-west China. Then we used heterogeneous Poisson null models to reveal significant uni- and bivariate second-order interactions. The results demonstrated that (1) 34 of the 48 studied species showed a significant relation with at least one topographic variable. This confirmed that topographical heterogeneity is important for distribution of these congeners. Spatial segregation (36.6%) and partial overlap (34.8%) were the most common bivariate association types in Xishuangbanna plot, which indicated first-order effects (environment) were strong. (2) For small-scale associations, 51% saplings (1 to ≤ 5 cm) (68.8% for large trees with dbh > 5 cm) of the species showed non-significant associations. For large-scale associations, 61.6% saplings (81.2% for large trees) of the species showed non-significant associations. Lack of significant species interactions provides evidence for the unified neutral theory. In conclusion, both environment and biotic interactions structure congeneric species' coexistence in tropical seasonal rain forest in this region.

Fungi play an important role in maintaining tropical forest diversity. However, little research to date has examined the effects of forest conversion (from rainforest to rubber plantation) on fungal community composition and diversity. In this study, we used Illumina MiSeq sequencing to assess soil fungal community composition and diversity for both rubber plantations and rainforest. We found: (1) Distinct differences in soil fungal community composition between rainforest and rubber plantation sites, indicating an important effect of forest conversion. The Ascomycota (36.71%) was the most abundant phylum in the rubber plantations, followed by the Basidiomycota (12.43%); these two phyla reversed in abundance in the tropical rainforest. (2) The OTU richness and Faith’s phylogenetic diversity were comparable in the plantations and rainforest, suggesting that conversion did not result in a reduction in α diversity. However, conversion resulted in a reduction in β diversity and a net loss of total diversity. (3) Community similarity of rubber plantations and rainforests declined with the distance among sample points at the fine scale, suggesting that dispersal limitation (a neutral process) was the main mechanism of fungal community assembly. Spatial variables (i.e., site location and elevation) accounted for 28% of the total variance in diversity, and therefore habitat filtering (a niche process) may drive fungal community assembly at the regional scale. In conclusion, the conversion of tropical rainforest to rubber plantations altered the composition of soil fungal communities, and reduced β diversity which resulted in a net loss of landscape level diversity. However, the underling mechanisms of community assembly remain unchanged. Neutral processes may explain fungal community assembly at fine scales, whereas niche processes likely drive patterns of assembly at the regional scale.

Hainan Island is the largest tropical island in China, which is vulnerable to intensifying human activities, such as deforestation and urbanization affecting the quality of the eco-environment on regional scale. To quantify the temporal and spatial changes of the environment, multispectral Landsat data with 30-m spatial resolution was used to establish an Eco-environmental Quality Index (EQI) model for evaluating the ecological status of Hainan Island from 1990 to 2015. According to the EQI value, the study area was divided into different regions marked by five eco-environmental quality levels: excellent, good, normal, poor, and very poor. Finally, temporal and spatial analysis was incorporated into the eco-environmental quality assessment, and the effect of land-use changes on eco-environmental quality was discussed. The results indicated that the overall eco-environmental quality of Hainan Island belonged to the good class but with obvious spatial variation: the eco-environmental quality was excellent in the central mountainous area, good in the flat plane surrounding the mountains, whereas the quality was normal in the coastal areas. From a temporal perspective, the eco-environmental quality of Hainan Island has reduced slightly from 1990 to 2015, while the regions gaining better quality grades have exceeded those becoming worse. Land-use changes dominated eco-environmental quality variations during the study period in Hainan Island: areas of poor eco-environmental quality have expanded in residential regions and eco-environmental quality of rubber plantations has increased since its expanding area has been converted from land of worse eco-environmental quality grades. The EQI results are expected to provide a quantitative foundation for planning sustainable development and the rational use of resources in Hainan Island.

Monodominance in the tropics is often seen as an unusual phenomenon due to the normally high diversity in tropical rain forests. Here we studied Parashorea chinensis H. Wang (Dipterocarpaceae) in a seasonal tropical forest in south-west China, to elucidate the mechanisms behind its monodominance. Twenty-eight 20 × 20-m plots were established in monodominant and mixed forest in Xishuangbanna, Yunnan province. All individuals ≥1 cm stem diameter and 16 soil variables were measured. Parashorea chinensis forest had a significantly higher mean tree dbh compared with mixed forest. Diversity did not differ significantly between the two forest types. However, within monodominant patches, all diversity indices decreased with an increase in P. chinensis dominance. Floristic composition of P. chinensis forest did differ significantly from the mixed forest. These differences were associated with more fertile soils (significantly higher pH, Mn, K and lower carbon pools and C:N ratio) in the P. chinensis forest than the mixed forest. In contrast to current paradigms, this monodominant species is not associated with infertile, but with fertile soils. Parashorea chinensis seems to be especially associated with high manganese concentrations which it can tolerate, and with edaphic conditions (water, K) that allow this tall and exposed emergent species to maintain its water balance. This is in contrast with most previous studies on monodominance in the tropics that found either no effect of soil properties, or predict associations with nutrient-poor soils.

Aim: Our objective was to quantify four components and study effect factors of soil respiration in rubber plantation ecosystems. Providing the basic data support for the establishment of the trade of rubber plantation ecosystem carbon source/sink. Methods: We used Li-6400 (IRGA, Li-COR) to quantitate four components of soil respiration in rubber plantation ecosystems at different ages. Soil respiration can be separated as four components: heterotrophic respiration (Rh), Respiration of roots (Rr), respiration of litter layer (Rl) and respiration of mineral soil (Rm). Important findings: The soil respiration rate (Rs) showed significant seasonal variation. The maximum soil respiration rate of the whole year appeared in August and the minimum in November or December. The components of soil respiration rate order showed as: heterotrophic respiration>respiration of roots>respiration of litter layer> respiration of mineral soil. The soil respiration rate was highly significant correlation (p

This study aimed to compare monoculture and mixed rubber plantations in terms of their soil bacterial and fungal composition. An Illumina MiSeq sequencing analysis was performed to investigate the composition and diversity of the soil bacterial and fungal communities among three different rubber ( Hevea brasiliensis) plantations: monoculture, Mixed 1 ( Hevea brasiliensis and Mytilaria laosensis), and Mixed 2 ( Hevea brasiliensis and Michelia macclurei) in Hainan. The results showed that the bacterial composition of the three rubber plantations was basically similar. However, there was a significant difference in fungal communities among the three rubber plantations at both the phylum and operational taxonomic unit level. The species richness, Chao, and Shannon diversity of bacterial communities of monoculture rubber plantations were higher than the Mixed 1 and Mixed 2 rubber plantations, whereas all diversity indexes of fungal communities were relatively equal for the monoculture and mixed rubber plantations. Soil nutrition (such as total nitrogen and total potassium) and soil pH are the main drivers of the bacterial composition ( p < .001). However, soil pH and water content are the main drivers of the fungal composition ( p < .001), and to some extent, soil pH can increase soil bacteria diversity. We suggest that alkaline fertilizers should be applied in mixed rubber plantations to improve the soil pH and, consequently, to increase the total diversity of the rubber plantation.

Having been introduced to the northern edge of Asian tropics, the rubber tree (Hevea brasiliensis) has become deciduous in this climate with seasonal drought and cold stresses. To determine its internal nutrient strategy during leaf senescence and deciduous periods, we investigated mature leaf and senescent leaf nutrients, water-soluble soil nutrients and characteristics of soil microbiota in nine different ages of monoculture rubber plantations. Rubber trees demonstrate complicated retranslocation of N, P, and K during foliar turnover. Approximately 50.26% of leaf nutrients and 21.47% of soil nutrients were redistributed to the rubber tree body during the leaf senescence and withering stages. However, no significant changes in the structure- or function-related properties of soil microbes were detected. These nutrient retranslocation strategy may be important stress responses. In the nutrient retranslocation process, soil plays a dual role as nutrient supplier and nutrient “bank.” Soil received the nutrients from abscised leaves, and also supplied nutrients to trees in the non-growth stage. Nutrient absorption and accumulation began before the leaves started to wither and fall.

We censused all free-standing trees ≥1 cm diameter at breast height (dbh) in a 20-ha plot established in a tropical seasonal rainforest in Xishuangbanna National Nature Reserve, southwest China. A total of 95,834 free-standing trees ≥1 cm dbh were recorded, and 95,498 individuals (accounting for 99.65% of the total), including 468 morphospecies in 213 genera and 70 families, were identified. Thirteen of 468 species (2.78%) had more than 1,000 individual ≥1 cm dbh, which represented 56.36% individuals of the total. On the other hand, 230 of 468 species (49.14%) had a mean density of ≤1 tree per ha, and 69 of 468 species (14.74%) were singletons in the 20-ha plot. The mean species richness, density and basal area per ha were 216.50 species, 4,791.70 stems and 42.34 m2, respectively. Pittosporopsis kerrii (20,918 stems, ≥1 cm dbh) of Icacinaceae and Parashorea chinensis (7,919 stems, ≥1 cm dbh) of Dipterocarpaceae were the two most abundant species dominating the emergent layer and treelet layer, respectively. Compared with other 50-ha plots established in other equatorial regions, tree species richness per ha and tree abundance per ha of the plot were at the moderate level.

Habitat heterogeneity and dispersal limitation are widely considered to be the two major mechanisms in determining tree species distributions. However, few studies have quantified the relative importance of these two mechanisms at different life stages of trees. Moreover, rigorous quantification of the effects 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 different mechanisms at different 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 differentiation resulting from environmental filtering. 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 different stages in the life history of trees. A neighbourhood index was computed to represent a neutral spatial autocorrelation effect. 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. The results showed that the relative importance of neutral and niche processes in regulating species distribution varied across life stages. The neutral neighbourhood index played the most important role in determining the distributions of small trees (1 cm  DBH  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  DBH). This finding builds on previous research into the relative importance of neutral and niche processes in determining species distributions regardless of life stages or DBH classes.