Chinese Subtropical Forest Research Articles

Canopy structural heterogeneity drives α and β species–genetic diversity correlations in a Chinese subtropical forest

Patterns and drivers of species–genetic diversity correlations (SGDCs) have been broadly examined across taxa and ecosystems and greatly deepen our understanding of how biodiversity is maintained. However, few studies have examined the role of canopy structural heterogeneity, which is a defining feature of forests, in shaping SGDCs. Here, we determine what factors contribute to α- and β-species–genetic diversity correlations (i.e., α- and β-SGDCs) in a Chinese subtropical forest. For this purpose, we used neutral molecular markers to assess genetic variation in almost all adult individuals of the dominant tree species, Lithocarpus xylocarpus, across plots in the Ailaoshan National Natural Reserve. We also quantified microhabitat variation by quantifying canopy structure heterogeneity with airborne laser scanning on 20 1-ha subtropical forest plots. We found that species α-diversity was negatively correlated with genetic α-diversity. Canopy structural heterogeneity was positively correlated with species α-diversity but negatively correlated with genetic α-diversity. These contrasting effects contributed to the formation of a negative α-SGDC. Further, we found that canopy structural heterogeneity increases species α-diversity and decreases genetic α-diversity by reducing the population size of target species. Species β-diversity, in contrast, was positively correlated with genetic β-diversity. Differences in canopy structural heterogeneity between plots had non-linear parallel effects on the two levels of β-diversity, while geographic distance had a relatively weak effect on β-SGDC. Our study indicates that canopy structural heterogeneity simultaneously affects plot-level community species diversity and population genetic diversity, and species and genetic turnover across plots, thus driving α- and β-SGDCs.

Soil water availability strongly drives the differentiation of acquisitive and conservative strategies for coexisting woody species in a Chinese subtropical evergreen forest

AbstractQuestionsTrait–environment relationships hold great promise for elucidating the mechanisms that drive community assembly. We asked to what extent does functional trait coordination underlie the differentiation of acquisitive–conservative strategies along environmental gradients? Also, to what extent does soil water availability determine trait combinations and subsequently shape microhabitat preferences for species with distinct ecological strategies?LocationYunnan Province, southwest China.MethodsPearson correlation and trait network analyses were used to quantify trait associations. A combined RLQ–fourth‐corner analysis and a spatial autoregressive error model were employed to examine trait–environment relationships and explain species distribution along environmental gradients.ResultsLeaf and stem traits were tightly coordinated along the first RLQ axis. Herein, traits representing construction cost exhibited trade‐offs against traits representing resource acquisition rate, underlying an acquisitive–conservative strategy differentiation along the environmental gradients. Significant overall and bivariate trait–environment relationships were found. In addition, the topographic wetness index contributed substantially to the environmental variation, and significantly influenced most traits alone. These results supported the hypothesis that soil water availability was the key environmental factor in selecting trait combinations and driving the differentiation of acquisitive and conservative strategies for coexisting species compared with other environmental factors. Finally, deciduous and a proportion of the evergreen species were acquisitive, whereas the remaining evergreen species were conservative, preferring wetter and drier microhabitats, respectively.ConclusionsOur findings indicate that soil water availability plays a central role in shaping both the trait combinations and microhabitat preferences of species with different ecological strategies rather than different leaf habits. These findings also highlight the fundamental significance of functional traits in facilitating the differentiation between acquisitive and conservative strategies along environmental gradients.

Multidimensional beta-diversity across local and regional scales in a Chinese subtropical forest: The role of forest structure.

Beta-diversity, or the spatio-temporal variation in community composition, can be partitioned into turnover and nestedness components in a multidimensional framework. Forest structure, including comprehensive characteristics of vertical and horizontal complexity, strongly affects species composition and its spatial variation. However, the effects of forest structure on beta-diversity patterns in multidimensional and multiple-scale contexts are poorly understood. Here, we assessed beta-diversity at local (a 20-ha forest dynamics plot) and regional (a plot network composed of 19 1-ha plots) scales in a Chinese subtropical evergreen broad-leaved forest. We then evaluated the relative importance of forest structure, topography, and spatial structure on beta-diversity and its turnover and nestedness components in taxonomic, functional, and phylogenetic dimensions at local and regional scales. We derived forest structural parameters from both unmanned aerial vehicle light detection and ranging (UAV LiDAR) data and plot inventory data. Turnover component dominated total beta-diversity for all dimensions at the two scales. With the exception of some components (taxonomic and functional turnover at the local scale; functional nestedness at the regional scale), environmental factors (i.e., topography and forest structure) contributed more than pure spatial variation. Explanations of forest structure for beta-diversity and its component patterns at the local scale were higher than those at the regional scale. The joint effects of spatial structure and forest structure influenced component patterns in all dimensions (except for functional turnover) to some extent at the local scale, while pure forest structure influenced taxonomic and phylogenetic nestedness patterns to some extent at the regional scale. Our results highlight the importance and scale dependence of forest structure in shaping multidimensional beta-diversity and its component patterns. Clearly, further studies need to link forest structure directly to ecological processes (e.g., asymmetric light competition and disturbance dynamics) and explore its roles in biodiversity maintenance.

Effects of the Dominate Plant Families on Elevation Gradient Pattern of Community Structure in a Subtropical Forest

Dominant groups and rare groups often coexist in forests. Dominate plant families generally have more species diversity and occupy most of the biomass. However, their effects on community construction and species diversity maintenance remain to be studied. This study aims to investigate this question by using the two dominant plant families in a Chinese subtropical forest (Lauraceae and Fagaceae). In this study, 22 forest dynamic plots were established in the forest distribution area of Daming Mountain Nature Reserve at an elevation of 300–1400 m. We explored the elevational change in the community structure and its relationship with Lauraceae and Fagaceae using generalized additive models (GAM) and a regression analysis. Then we used a correlation analysis to describe the relationship between environmental factors and the community structure. Our results showed the species diversity index showed a hump-shape trend along the elevation, and the total basal area showed no significant change along the elevation. The richness of Lauraceae and Fagaceae showed a positive correlation with the species diversity, but only the richness of Fagaceae showed a positive correlated with the total basal area. Species diversity was positively correlated with soil total nitrogen and soil organic matter. Species diversity and the total basal area were negatively correlated with the soil pH. In summary, Lauraceae and Fagaceae are correlated for community structure, and this correlation is not affected by elevation. Fagaceae has played a more important role than Lauranceae. This study highlights the impact of dominant plant families on the forest community assembly along the elevational transect.

Potentially differential impacts on niche overlap between Chinese endangered Zelkova schneideriana and its associated tree species under climate change

Climate change has a significant impact on the potential distribution for endangered trees. However, to date, little is known about how the endangered trees and main associated ones in different types of subtropical forests respond to climate change. Here, we first selected the endangered Zelkova schneideriana endemic to China and its associated trees as focus species from two subtropical forest communities (i.e. deciduous broad-leaf forest, bamboo and broad-leaf mixed forest) in China, and divided them into two species pairs: Z. schneideriana vs. Celtis sinensis, and Z. schneideriana vs. Phyllostachys edulis. Then, we simulated the three species’ suitable areas under current and future climate scenarios using Maxent based on the occurrence records and environmental variables, and further measured niche overlap between each species pair over time. Our Maxent showed: (1) Temperature-related factors have greater influence on Z. schneideriana than the other factors. The most important factor influencing its population distribution was Min temperature of coldest month (Bio6), followed by Mean diurnal range (Bio2), with the total contribution of 78.9%. (2) Currently, the suitable area of Z. schneideriana was predicted to be 106.50 × 104 km2, mostly located in the subtropical region of China, especially in Anhui, Hubei, Hunan, Jiangsu, and Zhejiang provinces. (3) Its suitable area would decrease and its average migration distance was 19.72 km under 16 future climate scenarios, with the core distribution migrating northeast. (4) There is an asynchrony of potential niche overlap between species pairs. One species pair with C. sinensis will rise in terms of Schoener’s D and I values whereas the other one with P. edulis will decline in the future. This asynchrony can be ascribed to the different future suitable ranges of these focus species as well as their ecological characteristics. Our study provides a new perspective on the conservation for endangered trees and surrounding neighbors in Chinese subtropical forests.

Germination of shrub species from Chinese subtropical forests: implications for restoration.

Incorporating native shrubs into restoration projects can improve biodiversity conservation and enhance the sustainability of ecosystem functions. Shrubs grow under different forest canopy structures, having varied microclimatic conditions according to forest type and composition. Currently, there is a lack of information on propagation from seed and planting material availability for the utilization of shrubs in forest restoration. In the present study, we evaluated the effects of temperature and light on germination of ten shrub species (Ardisia japonica, Callicarpa cathayana, Callicarpa giraldii var. subcanescens, Deutzia schneideriana, Fraxinus sieboldiana, Hydrangea chinensis, Maesa japonica, Rhododendron simsii, Spiraea japonica var. fortunei and Weigela japonica var. sinica) occurring in subtropical forests in China. No seeds of any species germinated in the coolest thermal regime (5/10 °C), while optimal temperature requirements varied from 10/20 °C to 25/35 °C. Seeds of small-seeded species had higher germination percentages in the light treatments, while larger seeds were not photoblastic. There was no relationship between germination in the light and the seed shape index. Our results may assist in identification of seed traits and suitable shrub species for restoration in specific forest types, thus aiding native forest recovery of structure and composition. Successful recovery leads to enhanced biodiversity, reestablishment of microhabitats and ecological interactions in the forest understorey.

Species diversity pattern and its drivers of the understory herbaceous plants in a Chinese subtropical forest

Understory herbaceous plants are an important component of forest ecosystems, playing important roles in species diversity and forest dynamics in forests. However, the current understanding of the biodiversity of forest communities is mostly from woody plants, and knowledge of community structure and species diversity for understory herbaceous plants remains scarce. In a subtropical forest in China, we investigated understory vascular herbaceous diversity from 300 plots (5 × 5 m) in the main growing season. In this study, we analyzed the community structure and diversity pattern of the understory herbaceous community and linked the species diversity pattern to both abiotic and biotic environments. We found a rich diversity of understory herbaceous communities in this forest (81 species belonging to 55 genera), and floristic elements at the genus level were dominated by tropical elements, followed by temperate elements. The diversity pattern of the understory herbaceous showed a significant habitat preference, with the highest diversity in the lowland valleys and then followed by in middle slopes. In addition, herbaceous diversity was significantly affected by both abiotic factors (such as terrain convexity) and biotic factors (such as the diversity of surrounding woody plants). Our study indicated that species diversity of understory herbaceous showed a remarkable habitat preference, such as lowland valleys, and highlighted the importance of both abiotic and biotic environments in driving herbaceous diversity patterns in the subtropical forest understory.

中国南亚热带不同造林模式碳汇林土壤碳、氮、磷的积累及化学计量特征

PDF HTML阅读 XML下载 导出引用 引用提醒 中国南亚热带不同造林模式碳汇林土壤碳、氮、磷的积累及化学计量特征 DOI: 10.5846/stxb202112283676 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金青年项目（31100402）；广东省自然科学基金项目（S2011040001125）；广东省林业科技创新专项资金项目（2015KJCX028）；广州市林业科技项目（201806） Characteristics of soil carbon, nitrogen, phosphorus accumulation and their ecological stoichiometry in Chinese subtropical forest under different afforestation patterns Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:碳汇林的固碳效益十分显著，是实现"碳中和"的主要碳增汇途径之一，在研究南亚热带地区不同造林模式碳汇林土壤碳汇的同时，研究碳汇林土壤氮、磷储量及土壤碳、氮、磷的生态化学计量特征，能够为不同造林模式碳汇林的土壤碳、氮、磷储量的评估提供科学依据。采取单因素随机区组设计，选择立地条件基本一致的研究区和4种造林模式（新造林、封山育林、补植套种、更新改造）的碳汇林，分析了不同造林模式土壤有机碳（SOC）、全氮（TN）、全磷（TP）的含量、储量、生态化学计量比，以及微生物生物量碳（MBC）的变化特征。结果表明，不同造林模式碳汇林的土壤理化性质，碳氮磷储量、化学计量比及微生物生物量碳含量差异显著，且土壤碳氮磷储量随土壤深度增加而减少，表现出明显的垂直变化特征。4种造林模式下碳氮比（C/N）介于11.11-17.86，碳磷比（C/P）介于17.00-242.59，氮磷比（N/P）介于1.18-15.99之间，在0-60 cm土层上均以更新改造模式林最低，封山育林模式下C/P和N/P均显著高于其他造林模式。土壤SOC与TN、N/P呈极显著正相关关系，土壤TN与C/P呈极显著正相关关系。综合来看，封山育林模式土壤的碳、氮含量和储量最高，而磷含量最低，说明封山育林模式更有利于土壤碳、氮的累积，而后期表现出土壤磷的缺乏，可能会限制封山育林模式碳汇林后期的碳汇功能。更新改造模式碳汇林的土壤养分限制少，后期碳汇潜力较大。 Abstract:Carbon sink forests is one of the effective ways to achieve "carbon neutrality". Besides soil carbon (C) sink, soil nitrogen (N), phosphorus (P) stocks and stoichiometry were investigated under different afforestation patterns, which provides the theoretical basis for estimating soil nutrition stocks. A single-factor randomized block design was used to investigate C, N, and P contents, stocks, their stoichiometric characteristics and soil microbial biomass carbon in soils with different soil depths under four afforestation patterns including new afforestation, closed mountain afforestation, replanting, and regeneration. The results showed that the storage of soil C, N, and P decreased with soil depth. The range of soil C/N, C/P and N/P under the four afforestation patterns were 11.11-17.86, 17.00-242.59 and 1.18-15.99, respectively. Soil C/P and N/P under closed mountain afforestation were significantly higher than the other afforestation patterns, while regeneration pattern showed the lowest. The soil C was positively correlated to soil N and N/P, soil N was positively correlated with C/P. In summary, the content and stock of soil C and N were the highest under closed mountain afforestation, while phosphorus content was the lowest, indicating that the closed mountain afforestation is an effective afforestation to promote the accumulation of soil carbon and nitrogen. However, the capacity of soil carbon sink under closed mountain afforestation may be weakened by soil phosphorus limitation. Regeneration pattern has less nutrition limitation, which has great potential to sequestration carbon. 参考文献 相似文献 引证文献

Liana and vine germination requirements in a subtropical forest zone

Climbing plants may be classified as lianas (woody) or vines (herbaceous). This grouping is not taxonomic but is useful for functional ecology. We evaluated seed germination of three liana and four vine species from the Chinese subtropical forest zone. Seeds were collected from different environments (abandoned agricultural land, broadleaf forest, coniferous mixed forest) and seed traits characterized (seed fresh mass, seed shape index, water gain). Germination tests were conducted under different temperature regimes: 5/10, 10/20, 20/30, 25/35, 35/40 °C under light and dark conditions. Additionally, we tested the relationships of germination responses (germination percentage, mean germination times, and relative light-germination index) with seed traits. The two species from abandoned agricultural land (Glycine max subsp. soja and Ipomoea triloba) had impermeable seed coats and reached only intermediate values of germination percentage, from 20 to 40% in the tested temperatures, with little effect from light regimes. In the broadleaf forest, Ipomoea nil also had impermeable seeds, but reached 91% of germination under 25/35 °C. Seeds of I. nil were the largest ones (4.8 g per 100 seeds), and there was a positive relationship of seed mass with G%. Seeds of Kadsura longipedunculata (liana) had low G% under all tested temperatures, probably related to some dormancy type, which requires further investigation. Species from coniferous mixed forest were greatly affected by temperature regimes, and Coptosapelta diffusa (the smallest seeds) had also the most light-dependent germination. Light-germination index had thus a significant/negative relationship with seeds mass. Environmental factors may help to explain the distribution of seed traits in our study, but further research should examine a larger set of species to better understand regeneration ecology of climbing plants in contrasting ecosystems.

Tree species with conservative foliar nutrient status and strong phosphorus homeostasis are regionally abundant in subtropical forests

Abstract Foliar nitrogen (N) or phosphorus (P) status and their stoichiometric homeostasis are integral parts of the plant nutrient economy that determines the success of plant species in environments where N or P limits plant growth. Despite growing evidence for higher predictability of stoichiometric homeostasis of N (HN) than that of P (HP) on plant species abundance in temperate grasslands, no previous studies explicitly examined how foliar N and P status modulate the relationships between stoichiometric homeostasis and species distribution (regional species abundance) of woody plants, especially in P‐limited (sub)tropical ecosystems. We hypothesized that species with a conservative foliar nutrient status but a higher HP (but not HN) would be regional abundant in P‐limited forest. We measured foliar N (LNC) and P (LPC) contents of 54 woody species, community composition and soil N and P contents across 94 forest plots in Chinese subtropical forests. Then we evaluated the species' levels of N and P stoichiometric homeostasis and their regional abundance to test our hypotheses. HN and HP significantly increased with decreasing LNC and LPC. Foliar nutrient status positively correlated with the minimum values of both soil N and P contents, but only negatively associated with the maximum value of soil P content, indicating that conservative species can occupy a wider range of soil P‐ than N‐based nutrient niche. Meanwhile, species abundance negatively correlated with LNC and LPC, and positively correlated with HN and HP. However, the structure equation model analysis showed that species abundance increased with decline of LNC but not yet with increased HN. In contrast, species abundance enhanced with increased HP and decreased LPC via HP, rather than directly with a decline of LPC. Synthesis. This study provides empirical evidence that species with conservative foliar nutrient status are more stable in terms of N and P stoichiometric homeostasis, and foliar N and P economy modulate species abundance distribution in different ways. Our results suggest that maintaining strong stoichiometric homeostasis of leaf P, while maintaining conservative economy of N, is a key physiochemical mechanism for shaping species abundance distribution in P‐limited forests.

Changes in the stoichiometry of Castanopsis fargesii along an elevation gradient in a Chinese subtropical forest.

Elevation is important for determining the nutrient biogeochemical cycle in forest ecosystems. Changes in the ecological stoichiometry of nutrients along an elevation gradient can be used to predict how an element cycle responds in the midst of global climate change. We investigated changes in concentrations of and relationships between nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) in the leaves and roots of the dominant tree species, Castanopsis fargesii, along an elevation gradient (from 500 to 1,000 m above mean sea level) in a subtropical natural forest in China. We analyzed correlations between C. fargesii’s above-ground biomass and stoichiometry with environmental factors. We also analyzed the soil and plant stoichiometry of this C. fargesii population. Our results showed that leaf N decreased while leaf K and Ca increased at higher elevations. Meanwhile, leaf P showed no relationship with elevation. The leaf N:P indicated that C. fargesii was limited by N. Elevation gradients contributed 46.40% of the total variance of ecological stoichiometry when assessing environmental factors. Our research may provide a theoretical basis for the biogeochemical cycle along with better forest management and fertilization for this C. fargesii population.

How evergreen and deciduous trees coexist during secondary forest succession: Insights into forest restoration mechanisms in Chinese subtropical forest

Deciduous broadleaved forests and evergreen broadleaved forests constitute the majority of global subtropical forests. Subtropical vegetation has faced strong anthropogenic pressures, and many natural forests destroyed throughout history have been restored to secondary forests due to various government conservation policies. In early forest restoration, deciduous and evergreen species with significantly different functional traits play different roles and jointly determine secondary forest successional trends, questioning how coexistence and competition between evergreen and deciduous trees profoundly impact community assembly and forest restoration. Although subtropical forest has been seriously threatened, scientific information on how these important ecological processes influence secondary forest regrowth is lacking. We established a 6-ha plot in a subtropical evergreen broadleaved forests in Guanshan National Nature Reserve, southeastern China. All trees individual were stem-mapped, identified to species and measured for diameter at breast height (DBH) and height. Pair-correlation and marked correlation functions indicated that deciduous species are comparatively more vulnerable to density-dependent regulation and size reduction in adjacent individuals during secondary forest succession. The interspecific association between deciduous and evergreen trees in the same DBH class gradually changes from significantly positive to negative as DBH increases, indicating seedling coexistence and competition between large late successional trees at a fine scale. Our results indicate that self-thinning caused by conspecific density-dependent regulation in deciduous trees and interspecific repulsion between evergreen and deciduous trees lead to forest gaps where clustered offspring naturally regenerate to coexist, embodying the important coexistence mechanisms facilitating secondary forest recovery to the zonal climax vegetation types dominated by evergreen species in this region. These findings improve our understanding of the self-recovery mechanisms of subtropical forests in secondary succession and provide important theoretical support to accelerate the future recovery of secondary forests.

Machine learning and geostatistical approaches for estimating aboveground biomass in Chinese subtropical forests

BackgroundAboveground biomass (AGB) is a fundamental indicator of forest ecosystem productivity and health and hence plays an essential role in evaluating forest carbon reserves and supporting the development of targeted forest management plans.MethodsHere, we proposed a random forest/co-kriging framework that integrates the strengths of machine learning and geostatistical approaches to improve the mapping accuracies of AGB in northern Guangdong Province of China. We used Landsat time-series observations, Advanced Land Observing Satellite (ALOS) Phased Array L-band Synthetic Aperture Radar (PALSAR) data, and National Forest Inventory (NFI) plot measurements, to generate the forest AGB maps at three time points (1992, 2002 and 2010) showing the spatio-temporal dynamics of AGB in the subtropical forests in Guangdong, China.ResultsThe proposed model was capable of mapping forest AGB using spectral, textural, topographical variables and the radar backscatter coefficients in an effective and reliable manner. The root mean square error of the plot-level AGB validation was between 15.62 and 53.78 t∙ha− 1, the mean absolute error ranged from 6.54 to 32.32 t∙ha− 1, the bias ranged from − 2.14 to 1.07 t∙ha− 1, and the relative improvement over the random forest algorithm was between 3.8% and 17.7%. The largest coefficient of determination (0.81) and the smallest mean absolute error (6.54 t∙ha− 1) were observed in the 1992 AGB map. The spectral saturation effect was minimized by adding the PALSAR data to the modeling variable set in 2010. By adding elevation as a covariable, the co-kriging outperformed the ordinary kriging method for the prediction of the AGB residuals, because co-kriging resulted in better interpolation results in the valleys and plains of the study area.ConclusionsValidation of the three AGB maps with an independent dataset indicated that the random forest/co-kriging performed best for AGB prediction, followed by random forest coupled with ordinary kriging (random forest/ordinary kriging), and the random forest model. The proposed random forest/co-kriging framework provides an accurate and reliable method for AGB mapping in subtropical forest regions with complex topography. The resulting AGB maps are suitable for the targeted development of forest management actions to promote carbon sequestration and sustainable forest management in the context of climate change.

The Impact of Water-Soluble Inorganic Ions in Particulate Matter (PM2.5) on Litter Decomposition in Chinese Subtropical Forests

Although numerous studies have demonstrated the toxic effects of fine particulates less than 2.5 µm (PM2.5) on the health of humans, little information is available on the ecotoxicity of PM2.5. Water-soluble inorganic ions (WSII, including Na+, NH4+, K+, Mg2+, Ca2+, Cl−, NO3−, and SO42−) can compose more than 60% of PM2.5. To better understand the possible impacts of WSII-PM2.5 on leaf litter decomposition, we conducted an experiment in which two leaf litters from oak (Quercus variabilis) and pine (Pinus massoniana) dominant forests in subtropical China were incubated in microcosms containing their respective forest soils and treated with WSII-PM2.5. Our results showed that, after six-months of decomposition, the WSII-PM2.5 treatments inhibited leaf litter decomposition rates, carbon and nitrogen loss, microbial biomass, and enzyme activities in the two forests. In addition, higher WSII-PM2.5 concentration led to stronger negative effects. Comparative analysis showed that the negative effects of WSII-PM2.5 on oak forest were greater than on pine forest, relating to the higher susceptibility to changes of soil microenvironment in oak forests. WSII-PM2.5 may influence decomposition through soil acidification and salinization, which could also cause a sub-lethal depression in soil isopod activity. However, in the first month of decomposition, mass loss of the oak and pine leaf litters under the low concentration WSII-PM2.5 were 21.63% and 35.64% higher than that under the control, respectively. This suggests that transitory low concentrations of WSII-PM2.5 have a promoting effect on decomposition. Long-term PM2.5 exposure, therefore, may have profound ecosystem consequences by altering the balance of ecosystem carbon flux, nutrient cycling, and humus formation in the future.

Mercury fluxes, budgets, and pools in forest ecosystems of China: A review

Mercury (Hg) retention in forest ecosystem plays a key role in global biogeochemical cycling. We present a comprehensive review of the research status of forest Hg in China to characterize the Hg budgets and pools. Averaged total Hg (THg) inputs at remote forests and rural and suburban forests in China are about 2–4-fold and 2.5–5-fold higher than the observed values in Europe and North America, respectively. The highly elevated THg inputs are mainly derived from the elevated atmospheric Hg concentrations and litterfall biomass. THg outputs from the forest ecosystems are much lower than the atmospheric depositions. The annual THg retentions range from 26.1 to 60.4 µg m−2 at subtropical forests and from 12.4 to 26.2 µg m−2 at temperate forests. Given the large areal coverage, THg retention in forest is ∼69 t year−1 in China and is much high than that in global scale estimated by models. The much higher THg retention has elevated the THg pools in Chinese subtropical forests. This study has implication for the role of China forests in the global Hg biogeochemical cycle and the optimization of atmospheric Hg transport and deposition models.

Effect of drought and season on arbuscular mycorrhizal fungi in a subtropical secondary forest

Arbuscular mycorrhizal (AM) fungi in both soil and roots were examined in May (summer) and December (winter) under a 4-y drought experiment in a Chinese subtropical secondary forest. Drought significantly decreased AM fungal extra-radical hyphal density, spore density, and root colonization rate in both seasons. These AM parameters were significantly higher in summer than in winter in the control treatment, but only AM fungal extra-radical hyphal density exhibited the same seasonal trend in the drought treatment. In total, 45 AM fungal operational taxonomic units (OTUs) were obtained at a 97% sequence similarity level using Illumina sequencing of 18S rDNA. Drought and season had no significant effects on AM fungal OTU richness in soil and roots. AM fungal community composition in soil and roots was significantly affected by season but not by drought. This finding enhances our understanding of the response of AM fungi to global climate change in subtropical forest ecosystems. • AM fungal abundance was decreased by drought in both seasons. • AM fungal richness was not affected by drought or season. • AM fungal community composition was influenced by season but not by drought.

Variation in the functional traits of fine roots is linked to phylogenetics in the common tree species of Chinese subtropical forests

The phylogenetic variations of fine root traits, which are related to plant growth and development as well as to physiological and ecological processes, are not fully understood. This study aimed to: (1) examine how tree species and sampling methodology affect the anatomical, morphological and nutrient traits of fine roots; and (2) determine whether phylogenetic signals affect fine root trait relationships and influence comparison of root traits between the branch order-based and diameter-based cut-off sampling categories. Fine root samples of 16 subtropical forest tree species were obtained and their anatomical, morphological and nutrient traits were studied. The phylogenetic signals of trait variations were calculated to determine trait relationships. Tree species and sampling methodology significantly affected fine root traits (p < 0.05). Mean root diameters, root tissue density (RTD) and carbon-to-nitrogen ratio were the lowest in the first-order category and highest in the ≤2 mm category. The reverse pattern was found for specific root length, specific root area and nitrogen concentration. Morphological traits showed significant phylogenetic signals; however, nutrient traits did not reflect phylogenetic conservatism. Phylogenetic factors influenced correlations between traits for the first-order root economics spectrum. Root traits were multidimensional and RTD was loaded on a novel phylogenetic principal component analysis. Functional traits of fine roots are multidimensional for subtropical tree species and closely linked to phylogeny. Morphological traits of first order roots showed a much stronger phylogenetic signal than those of roots ≤2 mm (traditionally defined fine roots). The findings improve understanding of root trait strategies in response to environmental changes.

Tree species identity surpasses richness in affecting soil microbial richness and community composition in subtropical forests

Plant interactions and feedbacks with soil microorganisms play an important role in sustaining the functions and stability of terrestrial ecosystems, yet the effects of tree species diversity on soil microbial community in forest ecosystems are still not well understood. Here, we examined the effects of tree species richness (1–12 species) and the presence of certain influential tree species (sampling effect) on soil bacterial and fungal communities in Chinese subtropical forests, using high-throughput Illumina sequencing for microbial identification. We observed that beta rather than alpha diversities of tree species and soil microorganisms were strong coupled. Multivariate regression and redundancy analyses revealed that the effects of tree species identity dominated over tree species richness on the diversity and composition of bacterial and fungal communities in both organic and top mineral soil horizons. Soil pH, nutrients and topography were always identified as significant predictors in the best multivariate models. Tree species have stronger effect on fungi than bacteria in organic soil, and on ectomycorrhizal fungi than saprotrophic fungi in mineral topsoil. Concluding, tree species identity, along with abiotic soil and topographical conditions, were more important factors determining the soil microbial communities in subtropical forests than tree diversity per se.

Spatial distribution of LAI and its relationship with throughfall kinetic energy of common tree species in a Chinese subtropical forest plantation

The hilly red soil region in southern China is still facing serious soil erosion, even after long-term afforestation projects. This might result from structural shortcomings of the tree species chosen for afforestation. Within the Biodiversity and Ecosystem Functioning China project (BEF China), we used point cloud data from terrestrial laser scanners (TLS) and splash cups to analyze spatial leaf area index (LAI) and to predict the potential of splash erosion in subtropical forests. High LAI of Lithocarpus glaber and Schima superba was measured mainly at the middle and lower parts of the trees while for Sapindus saponaria it was found at the upper parts. LAI was decreasing from the tree stems to the edges of the canopy. Lognormal and exponential linear models were suitable to describe the vertical and horizontal LAI distribution of selected tree species, respectively. Sapindus saponaria generally had the highest values of throughfall kinetic energy (TKE) among the analyzed tree species and measured rainfall events. In the radial direction, higher LAI tended to produce lower TKE, whereas in the vertical direction, higher skewness of LAI distribution had higher TKE. LAI and its spatial distribution both were important for TKE. These findings can help to understand mechanisms of splash erosion in forest plantations related to unsuitable spatial LAI of tree species planted. It might further improve our knowledge how tree diversity may influence splash erosion by enriching the canopy layers in an early successional stage of subtropical forest plantations.

Intra-annual wood formation of subtropical Chinese red pine shows better growth in dry season than wet season

China's subtropical forests play a vital role in sequestering global carbon; therefore, it is critical to conduct a precise investigation of intra-annual wood formation in these ecosystems to clarify the mechanisms behind this. Two field experiments were established in Chinese subtropical forests to monitor weekly the intra-annual xylem formation of Pinus massoniana Lamb. from January to December 2015, using the recently developed micro-sampling approach. The effects of climate on wood formation were also assessed using linear or mixed models. Results indicate that there is an inactive period that might be semi-dormancy in subtropical pine ecosystems in January compared with the complete dormancy in temperate and boreal ecosystems and the fully active or short-term dormancy in tropical ecosystems. The duration of xylem formation of Chinese red pine in subtropical China in 2015 was 4-6 months longer than temperate and boreal forests. Moreover, trees were found to grow better during the dry season than the wet season, indicating that the Chinese red pine ecosystem is more strongly regulated by net energy than by environmental factors. Our findings indicate that China's subtropical pine forests may benefit from the expected longer dry seasons, possibly leading to better forest growth and improved carbon sequestration under continued climate warming.