Big-sized trees overrule remaining trees' attributes and species richness as determinants of aboveground biomass in tropical forests.

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.

Plant diversity accurately predicts insect diversity in two tropical landscapes.

Plant diversity surely determines arthropod diversity, but only moderate correlations between arthropod and plant species richness had been observed until Basset et al. (Science, 338, 2012 and 1481) finally undertook an unprecedentedly comprehensive sampling of a tropical forest and demonstrated that plant species richness could indeed accurately predict arthropod species richness. We now require a high-throughput pipeline to operationalize this result so that we can (i) test competing explanations for tropical arthropod megadiversity, (ii) improve estimates of global eukaryotic species diversity, and (iii) use plant and arthropod communities as efficient proxies for each other, thus improving the efficiency of conservation planning and of detecting forest degradation and recovery. We therefore applied metabarcoding to Malaise-trap samples across two tropical landscapes in China. We demonstrate that plant species richness can accurately predict arthropod (mostly insect) species richness and that plant and insect community compositions are highly correlated, even in landscapes that are large, heterogeneous and anthropogenically modified. Finally, we review how metabarcoding makes feasible highly replicated tests of the major competing explanations for tropical megadiversity.

Predicting current and future potential distributions of the greater bandicoot rat (Bandicota indica) under climate change conditions.

BACKGROUND: Rodent infestation is a global problem. Rodents cause huge harm to agriculture, forestry, and animal husbandry around the world and spread various zoonoses. In this study, we simulated the potentially suitable habitats of Bandicota indica and predicted the impact of future climate change on its distribution under different socio-economic pathway scenarios of CMIP6 using a parameter-optimized maximum entropy (MaxEnt) model. RESULTS: The average area under the receiver operating characteristic curve (AUC) value (0.958 ± 0.006) after ten repetitions proved the high accuracy of the MaxEnt model. Model results show that the annual mean temperature (≥ 15.93 °C), isothermality (28.52-80.49%), annual precipitation (780.13-3863.13 mm), precipitation of the warmest quarter (≥ 204.37 mm), and nighttime light (≥ 3.38) were important limiting environmental variables for the distribution of B. indica. Under current climate conditions, the projected potential suitable habitats for B. indica were mainly in India, China, Myanmar, Thailand, and Vietnam, which cover a total area of 301.70 × 104 km2 . The potentially suitable areas of B. indica in the world will expand under different future climate change scenarios by 1.61-17.65%. CONCLUSIONS: These results validate the potential influence of climate change on the distribution of B. indica and aid in understanding the linkages between B. indica niches and the relevant environment, thereby identifying urgent management areas where interventions may be necessary to develop feasible early warning and prevention strategies to protect against this rodent's spread. © 2023 Society of Chemical Industry.

Temperature and historical land connectivity jointly shape the floristic relationship between Hainan Island and the neighbouring landmasses.

Present-day biodiversity in insular biota results from the interplay among geographical barriers, environmental filtering, and historical biogeography, but how these factors interact on insular biodiversity patterns is poorly understood. Here, we analysed the geographical patterns of beta diversity of seed plants between Hainan Island and the neighbouring landmasses in relation to space and the environmental factors to assess the relative effects of historical processes and ecological gradients on community assembly. We assessed beta diversity patterns by quantifying the turnover and nestedness components and used clustering and ordination to investigate the relationships between local floras from Hainan and the neighbouring landmasses. Utilising simple linear regression and linear mixed effect models, we evaluated the importance of historical processes and environmental gradients in shaping these beta diversity patterns. Our results show that the contributions of nestedness and turnover components to the total beta diversity vary across space. The flora of Hainan predominantly nests with the flora of Vietnam but shows larger species turnover with Guangdong, Guangxi, and Taiwan. Clustering and ordination analyses indicate that Hainan is first merged with Vietnam, after which it is grouped with mainland China and finally with Taiwan and the Philippines. The results of the linear mixed effect models consistently reveal that temperature, followed by the historical land connectivity, has the most important role in shaping the floristic dissimilarity. We conclude that the flora of Hainan is of continental origin and has the highest floristic affinity with Vietnam. The periodic emergence of a land bridge during Quaternary glacial cycles determines the origin of Hainan's flora, and temperature shapes the floristic dissimilarities via environmental filtering. Our study highlights the critical roles of historical sea level change and current environmental limitation in structuring the plant communities on Southeast Asian islands.

Evolutionary history of zoogeographical regions surrounding the Tibetan Plateau.

The Tibetan Plateau (TP) and surrounding regions have one of the most complex biotas on Earth. However, the evolutionary history of these regions in deep time is poorly understood. Here, we quantify the temporal changes in beta dissimilarities among zoogeographical regions during the Cenozoic using 4,966 extant terrestrial vertebrates and 1,278 extinct mammal genera. We identify ten present-day zoogeographical regions and find that they underwent a striking change over time. Specifically, the fauna on the TP was close to the Oriental realm in deep time but became more similar to the Palearctic realms more recently. The present-day zoogeographical regions generally emerged during the Miocene/Pliocene boundary (ca. 5 Ma). These results indicate that geological events such as the Indo-Asian Collision, the TP uplift, and the aridification of the Asian interior underpinned the evolutionary history of the zoogeographical regions surrounding the TP over different time periods.

Determinants of the beta diversity of tree species in tropical forests: Implications for biodiversity conservation.

The mapping of earth's biodiversity has advanced our theoretical and empirical understanding of biodiversity and has thus guided conservation efforts. Yet, early biodiversity maps often relied on alpha diversity indices, while beta diversity has rarely been used for practical conservation actions. We used generalized dissimilarity modelling (GDM) and variance partitioning to map beta diversity patterns of Hainan Island, China, and explore its underlying factors based on a large dataset of 248,538 individual trees belonging to 1,016 species in 902 forest plots. We used principal component analysis and hierarchical clustering to visualize community similarity, and spatial overlap analysis to assess the ability of the current protected areas (PAs) to encompass beta diversity. The GDMs explained 27.65% and 26.58% of the variation in beta diversity at the genus and species levels, respectively. The community composition of tree species in Hainan presented a general east-to-west gradient, and three floristic regions were delineated. This biogeographical pattern is predominantly structured by mean annual precipitation. Environmental variables, rather than geographical distance, were the most important factors determining present beta diversity patterns. Currently, PAs of Hainan Island are concentrated on mountain forest areas, while the lowland forest has largely been ignored. Thus, we suggest that biodiversity mapping based only on alpha diversity is not enough to identify conservation gaps, and the inclusion of beta diversity in such maps constitutes a promising tool to maximize the biodiversity coverage of PAs. Our study provides empirical evidence that a spatially explicit analysis of beta diversity in a specific region can be used for conservation planning.

Climatic water availability is the main limiting factor of biotic attributes across large-scale elevational gradients in tropical forests.

Climatic water availability is a key spatial driver of species distribution patterns in natural forests. Yet, we do not fully understand the importance of climatic water availability relative to temperature, and climate relative to edaphic factors for multiple biotic attributes across large-scale elevational gradients in natural forests. Here, we modelled multiple abiotic factors (elevation, climate, and edaphic factors) with each of the taxonomic-related (Shannon's species diversity, species richness, species evenness, and Simpson's dominance) and tree size or biomass-related (individual tree size variation, functional dominance and divergence, and aboveground biomass) biotic attributes through boosted regression trees (BRT) models, using biophysical data from 247,691 trees across 907 plots in tropical forests in Hainan Island of Southern China. The tested multiple abiotic factors explained simultaneously 43, 50, 36, 45, 37, 50, 17 and 46%, respectively, of the variations in Shannon's species diversity, species richness, species evenness, Simpson's dominance, individual tree size variation, functional dominance, functional divergence and aboveground biomass. After the large influences of elevation (i.e. 30.43 to 62.83%), climatic water availability accounted for most (i.e. 15.52 to 25.30%) of the variations in all biotic attributes. Beside the increasing trend with elevational gradients, taxonomic diversity increased strongly with climatic water availability whereas tree size or biomass-related biotic attributes showed strong decreasing and increasing trends. Tree size or biomass-related rather than taxonomic-related biotic attributes also decreased apparently with mean annual temperature. Most of the biotic attributes monotonically increased with soil fertility but decreased with soil pH, whereas soil textural properties had mostly negligible influences. This study strongly reveals that future climate change (i.e. a decrease in climatic water availability with an increase in mean annual temperature) is thus likely to have a substantial influence on the biotic attributes in the studied tropical forests across large-scale elevational gradients.

Tree crown complementarity links positive functional diversity and aboveground biomass along large-scale ecological gradients in tropical forests.

Most of the previous studies have shown that the relationship between functional diversity and aboveground biomass is unpredictable in natural tropical forests, and hence also contrary to the predictions of niche complementarity effect. However, the direct and indirect effects of functional diversity on aboveground biomass via tree crown complementarity in natural forests remain unclear, and this potential ecological mechanism is yet to be understood across large-scale ecological gradients. Here, we hypothesized that tree crown complementarity would link positive functional diversity and aboveground biomass due to increasing species coexistence through efficient capture and use of available resources in natural tropical forests along large-scale ecological gradients. We quantified individual tree crown variation, functional divergence of tree maximum height, and aboveground biomass using data from 187,748 trees, in addition to the quantifications of climatic water availability and soil fertility across 712 tropical forests plots in Hainan Island of Southern China. We used structural equation modeling to test the tree crown complementarity hypothesis. Aboveground biomass increased directly with increasing functional diversity, individual tree crown variation and climatic water availability. As such, functional diversity enhanced individual tree crown variation, thereby increased aboveground biomass indirectly via individual tree crown variation. Additional positive effects of climatic water availability and soil fertility on aboveground biomass were accounted indirectly via increasing individual tree crown variation and/or functional diversity. This study shows that tree crown complementarity mediates the positive effect of functional diversity on aboveground biomass through light capture and use along large-scale ecological gradients in natural forests. This study also mechanistically shows that tree crown complementarity increases species coexistence through maintenance of functional diversity, which in turn enhances aboveground biomass in natural tropical forests. Hence, managing natural forests with the aim of increasing tree crown complementarity holds promise for enhancing carbon storage while conserving biodiversity in functionally-diverse communities.

Changes in the spatial and temporal pattern of natural forest cover on Hainan Island from the 1950s to the 2010s: implications for natural forest conservation and management.

The study of the past, present, and future state and dynamics of the tropical natural forest cover (NFC) might help to better understand the pattern of deforestation and fragmentation as well as the influence of social and natural processes. The obtained information will support the development of effective conservation policies and strategies. In the present study, we used historical data of the road network, topography, and climatic productivity to reconstruct NFC maps of Hainan Island, China, from the 1950s to the 2010s, using the random forest algorithm. We investigated the spatial and temporal patterns of NFC change from the 1950s to the 2010s and found that it was highly dynamic in both space and time. Our data showed that grid cells with low NFC were more vulnerable to NFC decrease, suggesting that conservation actions regarding natural forests need to focus on regions with low NFC and high ecological value. We also identified the hot spots of NFC change, which provides insights into the dynamic changes of natural forests over time.