Subtropical Evergreen Broadleaf Forest Research Articles

Detection of Typical Forest Degradation Patterns: Characteristics and Drivers of Forest Degradation in Northeast China

The accurate identification of forest degradation and its driving factors is a prerequisite for implementing high-quality forest management. However, distinguishing degradation patterns is often neglected in large-scale forest quality assessments. The indicators were constructed to identify typical forest degradation patterns using remote sensing indexes, followed by an analysis of the spatiotemporal dynamics of forest degradation and quantification of the contributions from various driving factors. The results indicated that the constructed indicators could effectively distinguish typical forest degradation patterns, with a fire degradation identification accuracy of 90.0% and a fitting accuracy of drought and pest degradation higher than 0.7. The cold temperate conifer forest zone had the largest proportion of fire degradation, accounting for 67.7% of the area, and totals of 99.0% of the subtropical evergreen broadleaf forest zone and 92.8% of the temperate conifer and broadleaf mixed forest zone were moderately to severely affected by drought, with long-term stability. Additionally, 0.1% of the temperate grassland region and 0.1% of the cold temperate conifer forest zone underwent severe pest infestations, with a long-term stable trend. Meteorological factors were the primary contributors to all typical degradation patterns, accounting for 81.35%, 58.70%, and 82.29%, respectively. The research developed an index for assessing forest degradation and explained the importance of natural and anthropogenic factors in forest degradation. The results are beneficial for the scientific management of forest degradation and for improving forest management efficiency.

2018年浙江古田山24公顷亚热带常绿阔叶林动态监测样地林冠结构和地形数据集

The canopy structure and topographic variables jointly influence the spatiotemporal variation of habitats for important species. However, there remains a considerable scarcity of precise forest canopy structure and topographic data obtained from large-scale forest dynamic plots. LiDAR data, acquired via near-surface remote sensing platforms, facilitates precise computation of both canopy structure and topographic variables. This study selected a 24-hectare dynamic plot in the subtropical evergreen broadleaf forest, located in the core conservation area of Qianjiangyuan National Park, as its focal point for survey. We conducted an analysis and quality control process on the point cloud data in 2018 using a near-surface remote sensing platform for this area, aiming to acquire corresponding canopy structure and topography data. Ground data collected through Real-Time Kinematic (RTK) surveying were used to verify and validate the accuracy of the digital elevation model (DEM). The analysis revealed a mean square error of elevation of 0.07 meters, suggesting a high level of accuracy of the dataset. The 24-hectare subtropical evergreen broad-leaved forest dynamic plot represents a typical low-altitude evergreen broad-leaved forest in the central subtropical region. This dataset can provide essential data support for the monitoring and research of biodiversity in subtropical broadleaved evergreen forests.

Resource-acquisitive species have greater plasticity in leaf functional traits than resource-conservative species in response to nitrogen addition in subtropical China

The evergreen broad-leaf forest is subtropical zonal vegetation in China, and its species diversity and stability are crucial for maintaining forest ecosystem functions. The region is generally affected by global changes such as high levels of nitrogen deposition. Therefore, it is critical to determine the adaptation strategies of subtropical dominant species under nitrogen addition. Here, we conducted two-year field experiments with nitrogen addition levels as 0 kg N ha−1 yr−1 (CK), 50 kg N ha−1 yr−1 (LN) and 100 kg N ha−1 yr−1 (HN). We investigated the effects of nitrogen addition on leaf functional traits (including nutrition, structural and physiological characteristics) of five dominant species in subtropical evergreen broad-leaf forest. Results suggested that the effect of nitrogen addition on leaf functional traits was species-specific. Contrary to Rhododendron delavayi and Eurya muricata, Quercus glauca, Schima superba and Castanopsis eyrei all responded more to the HN treatment than LN treatment. Compared to other leaf functional traits, leaf anatomical structure traits had the highest average plasticity (0.246), and the relative effect of leaf photosynthetic property was highest (7.785) under N addition. Among the five species, S. superba was highest in terms of the index of plasticity for leaf functional traits under nitrogen addition, followed by Q. glauca, E. muricata, C. eyrei and R. delavayi. The major leaf functional traits representing the economic spectrum of leaves (LES) showed resource acquisitive strategy (high SLA, LNC, LPC, Pn) and conservative strategy (high LTD, LDMC, C/N) clustering on the opposite ends of the PCA axis. The PCA analysis indicated that species with high leaf plasticity adopt resource acquisitive strategy (S. superba and Q. glauca), whereas species with low leaf plasticity adopt resource conservative strategy (E. muricata, C. eyrei and R. delavayi). In aggregate, resource-acquisitive species benefit from nitrogen addition more than resource-conservative species, suggesting that S. superba and Q. glauca will occupy the dominant position in community succession under persistently elevated nitrogen deposition.

How Climate Change Impacts the Distribution of Lithocarpus hancei (Fagaceae), a Dominant Tree in East Asian Montane Cloud Forests

Subtropical montane moist evergreen broadleaf forests (MMEBFs) have a unique environmental envelope harboring a high percentage of endemic biota. This ecosystem is highly vulnerable, and little is known about its possible response to future climate change. In this study, we used Lithocarpus hancei (Fagaceae), a dominant tree in East Asian subtropical MMEBFs, as a proxy to investigate MMEBF distribution dynamics and key distribution restriction factors. In total, 607 L. hancei occurrence points were obtained after being gathered and filtered. The random forest model was selected as the best model. Eight environmental variables were selected to simulate the potential suitable areas of L. hancei under the Last Glacial Maximum (LGM), present, and future (2041–2060, 2061–2080, 2081–2100) climate change scenarios, including four shared socioeconomic pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP5-8.5). Our results showed that precipitation of the warmest quarter, the precipitation of the driest month, the mean diurnal range, and temperature seasonality are the key factors affecting the spatial range of L. hancei with 11.2%, 10.9%, 8.1%, and 7.6% contributions, respectively. The current distribution of L. hancei is mainly within East and South China, with a scattered distribution in North Indo-China and the Southeast Himalayas. The highly suitable area only accounts for 9.7% of the total distribution area. The distribution area of the current suitability area is the smallest compared to that under LGM and future scenarios. In all the future climatic scenarios, the highly suitable areas of L. hancei would decrease or even disappear, whereas the medium- and low-suitability areas might increase with the centroid of the total suitable area northern. Its distribution in Central China, the Southern Himalayas, and Northern Indo-China will be lost in the future. Overall, our study predicted a prominent degradation of East Asian MMEBFs in the future. In situ and ex situ conservation on East Asian MMEBFs should be prioritized and enforced.

Six new species of zombie-ant fungi from Yunnan in China

Some Ophiocordyceps species infecting ants are able to manipulate the host behavior. The hosts are manipulated in order to move to location that are advantageous for fungal spore transmission. Ophiocordyceps species that are able to manipulate the ant's behavior are called "zombie-ant fungi". They are widespread within tropical forests worldwide, with relatively few reports from subtropical monsoon evergreen broad-leaf forest. Zombie-ant fungi have been described and reported in different countries worldwide. However, there were a few reports from China. This study proposed six new species of zombie-ant fungi from China based on multi-gene (SSU, LSU, TEF, RPB1 and RPB2) phylogenetic analyses and morphological characteristics. Six novel species of Ophiocordyceps from China were identified as the Ophiocordyceps unilateralis core clade, forming a separate lineage with other species. Six novel species of Ophiocordyceps with hirsutella-like asexual morphs exclusively infecting ants were presented herein, namely, Ophiocordyceps acroasca, Ophiocordyceps bifertilis, Ophiocordyceps subtiliphialida, Ophiocordyceps basiasca, Ophiocordyceps nuozhaduensis and Ophiocordyceps contiispora. Descriptions and illustrations for six taxon were provided. Five of these species were collected from the subtropical monsoon evergreen broad-leaf forest, and one was collected from the rainforest and subtropical monsoon evergreen broad-leaf forest. This work proposes that the same host of Camponotus can be infected by multiple ant pathogenic fungi, while multiple ants of Polyrhachis can be infected by the same pathogenic fungi at the same time. This study contributes towards a better understanding of the evolutionary relationship between hosts and fungi, and provides novel insights into the morphology, distribution, parasitism, and ecology of Ophiocordyceps unilateralis sensu lato. We have provided a method for obtaining living cultures of Ophiocordyceps unilateralis complex species and their asexual morphs based on the living cultures, which is of significant value for further studies of Ophiocordyceps unilateralis complex species in the future.

Embolism resistance explains mortality and recovery of five subtropical evergreen broadleaf trees to persistent drought.

Subtropical evergreen broadleaf forests (SEBF) are experiencing and expected to suffer more frequent and severe drought events. However, how the hydraulic traits directly link to the mortality and recovery of SEBF trees remains unclear. In this study, we conducted a drought-rewatering experiment on tree seedlings of five dominant species to investigate how the hydraulic traits were related to tree mortality and the resistance and recovery of photosynthesis (A) and transpiration (E) under different drought severities. Species with greater embolism resistance (P50 ) survived longer than those with a weaker P50 . However, there was no general hydraulic threshold associated with tree mortality, with the lethal hydraulic failure varying from 64% to 93% loss of conductance. The photosynthesis and transpiration of tree species with a greater P50 were more resistant to and recovered faster from drought than those with lower P50 . Other plant traits could not explain the interspecific variation in tree mortality and drought resistance and recovery. These results highlight the unique importance of embolism resistance in driving carbon and water processes under persistent drought across different trees in SEBFs. The absence of multiple efficient drought strategies in SEBF seedlings implies the difficulty of natural seedling regeneration under future droughts, which often occurs after destructive disturbances (e.g., extreme drought events and typhoon), suggesting that this biome may be highly vulnerable to co-occurring climate extremes.

Original karst tiankeng with underground virgin forest as an inaccessible refugia originated from a degraded surface flora in Yunnan, China

Karst tiankengs are rare natural habitats, having a local microclimate different from surrounding regions. A contrast study on plant communities at the inside and outside of the primitive tiankeng was carried out by performing the qualitative analysis of species compositions of arborous and shrub layers. We found that plant communities in the primitive tiankeng belong to the subtropical moist evergreen broad-leaf forest, whereas those outside the tiankeng belong to subtropical semi-moist needle-broadleaved mixed forest. Trapped habitat of primitive karst tiankeng protects the plant communities significantly different from those in external karst ecosystems, so karst tiankeng has the great value in the plant species protection. Although the trapped habitat decreases plant species abundance inside tiankeng to some extent, highly diverse shrub species are present in the inside-tiankeng plant communities, and the primitive karst tiankeng plays an important role in the plant diversity protection. The primitive karst tiankeng is an important refugia for plant not only as a plant species protection library, but a plant diversity protection library. When implementing measures for the reduction of damages to biodiversity due to global climate changes and human activities, more attention should be paid to the primitive karst tiankeng as a small ecological refugia and biodiversity protection library.

Effects of microtopography on soil fungal community diversity, composition, and assembly in a subtropical monsoon evergreen broadleaf forest of Southwest China

Topography is a critical factor that determines the patterns and assemblages of species diversity in forest communities. Small-scale topography changes are defined as microtopography. Our understanding of the effects of microtopography on soil fungal communities is limited. The objective of this study was to explore the impact of microtopography on soil fungal community diversity, community composition, functional guild, and community assembly. The diversity, composition, and relative abundance of functional guilds were measured using Illumina high-throughput sequencing techniques in five microtopography types (steep slope, less-steep slope, valley, high plateau, and ridge). OTU and Chao1 indexes were the highest in the high plateau and were the lowest in the ridge. The Shannon index was the lowest in the valley, and the highest in the ridge. In the valley, soil fungal networks were more interconnected than they were in any of the other microtopography unit. At the class level, the relative abundance of Agaricomycetes, Eurotionmycetes, and Leotiomycetes were higher in the ridge unit. The relative abundance of ectomycorrhiza, the dominant functional guild, in the ecosystem was as follows: less-steep slope (71.31%), ridge (66.55%), valley (65.83%), steep slope (63.91%), and high plateau (60.09%). Microtopography influenced fungal community diversity and composition more than any other factor and had, varying degrees of influence on the different functional guilds. In every microtopography type, dispersal limitations and drift shaped fungal communities more than other ecological processes. Our findings suggest that microtopography affects soil fungal community diversity and composition, functional guilds, and the relative contributions of ecological processes to soil fungal communities in subtropical monsoon evergreen broadleaf forests.

自然增温对南亚热带森林土壤微生物群落与有机碳代谢功能基因的影响

基于海拔高度下降气温上升的关系，将模拟的生态系统（含植物和土壤）从高海拔整体移位至低海拔地区，实现自然增温的效果。通过对自然增温条件下土壤环境因子及其相关理化性质的动态监测，结合磷脂脂肪酸分析与宏基因组学方法，测定土壤微生物群落结构以及与土壤有机碳分解相关基因丰度，探究自然增温对鼎湖山南亚热带山地常绿阔叶林土壤有机碳代谢的影响及其微生物学机制。结果表明：（1）增温处理显著改变了0-10 cm土壤温度与湿度：2016-2018年间土壤温度显著上升2.48℃，湿度显著下降23.93%。（2）增温处理下，干季土壤有机碳含量与湿季土壤硝态氮含量显著降低，其他土壤理化因子无显著变化。（3）增温处理下，干季和湿季土壤微生物群落结构发生改变，且湿季变化显著。土壤湿度是影响干季和湿季土壤微生物群落结构变化的主要因子，解释了干季50.2%的变异度与湿季79.2%的变异度。（4）宏基因组结果表示：增温抑制了干季山地常绿阔叶林土壤有机碳代谢基因丰度，增强了湿季山地常绿阔叶林土壤有机碳代谢基因丰度。以上结果表明，增温通过改变土壤微生物生物量和群落结构以及有机碳代谢相关功能基因的丰度，最终影响南亚热带山地常绿阔叶林土壤有机碳代谢过程。;Global warming is one of the environmental problems that cannot be ignored in the world today, which has significant impacts on soil carbon dynamics and soil microorganisms. Soil microorganisms affect the soil nutrient cycling process and play a leading role in the net carbon balance of terrestrial ecosystems. Further exploration of microbial responses to climate warming is important for predicting the feedback and carbon budget of terrestrial ecosystems in warmer climates. In this study, soil organic carbon content and microorganisms were investigated in a natural warming experiment using a whole ecosystem transplanting approach. Plots of plants and soil in a subtropical mountain evergreen broadleaf forest at high altitude (600 m asl) were moved to low altitude (30 m asl) in Dinghu Mountain to experience natural warming treatment. Plots with similar plants and soil at the high altitude were used as the control. To explore the effect of natural warming on soil organic carbon metabolism and its microbiological mechanism, soil physical and chemical properties, soil organic carbon, and soil microbial community structure were measured for the control and warmed plots. Meta-genomic method was used for soil microbial community structure and abundance of genes involved in soil organic carbon decomposition. Results showed that:(1) warming treatment significantly changed soil temperature and moisture of the 0-10 cm soil layer. From 2016 to 2018, the soil temperature was significantly increased by 2.48℃, and the soil moisture was significantly decreased by 23.93% (relative change). (2) Warming treatment significantly reduced soil organic carbon content in the dry season and the soil nitrate nitrogen content in the wet season, while other soil physical and chemical factors had no significant changes under warming treatment. (3) Warming treatment changed the soil microbial community structure in both dry and wet seasons, and the change was significant in wet season. The results of principal component analysis showed that soil moisture was the main factor affecting the variation of soil microbial community structure in dry season and wet season, which explained the variation degree of 50.2% in dry season and 79.2% in wet season. (4) The metagenomic analysis showed that increasing temperature inhibited the abundance of soil organic carbon metabolism genes of mountain evergreen broadleaf forest in the dry season, and enhanced them in the wet season. Our results showed that although warming treatment did not significantly change soil microbial biomass in the subtropical mountain evergreen broadleaf forest, it reduced soil organic carbon by changing soil microbial community structure and abundance, and significantly affected soil organic carbon metabolic processes and ecosystem carbon cycling.

Nitrogen addition slows litter decomposition accompanied by accelerated manganese release: A five-year experiment in a subtropical evergreen broadleaf forest

Litter decomposition is a key process for mediating ecosystem carbon (C) and nutrient cycling and is sensitive to increasing atmospheric nitrogen (N) deposition. However, the response of long-term litter decomposition to elevated N input remains unclear, especially in (sub) tropical regions with high ambient N deposition. We conducted a five-year field experiment in an evergreen broadleaf forest in the West China Rain Zone, an area with one of the highest levels of atmospheric N deposition. Three levels of N addition: control (ambient N deposition), low-N (+50 kg N ha−1 year−1), and high-N (+50 kg N ha−1 year−1); were applied monthly from April 2013. The decomposition process of two leaf litters, Schima sinensis and Lithocarpus hancei, was then investigated from November 2013 to December 2018. For both litters, we observed two distinct decomposition stages: an early stage of rapid decomposition (the first 2 years); and a late stage of slow decomposition (the following 3 years). Nitrogen addition did not affect the early-stage but significantly inhibited late-stage mass loss, thus decreasing decomposition rate or extent. Cellulose degradation was accelerated in the early stage but suppressed in the late stage, while lignin degradation was inhibited throughout the decomposition process. Nitrogen addition affected the concentration and/or release of nutrients during decomposition, and especially significantly accelerated manganese (Mn) release, with a significant decrease in Mn concentration and Mn/lignin ratio. The reduction in Mn concentration may explain the inhibition of litter decomposition after N addition, because it retards the degradation of lignin through reducing ligninolytic enzyme activity. The slowed litter decomposition in this forest is predicted to decrease soil CO2 emissions and increase soil C content.

Estimations of forest water retention across China from an observation site-scale to a national-scale

Hydrological models or remote sensing methods are often used to estimate water retention amounts and to analyze large spatial variations in water retention across large scales. However, the accuracies of hydrological models and remote sensing data are far lower than those of site-observational data. Here, we used 1045 observational sites of forest ecosystem across China and a random forest (RF) model for more accurate spatial prediction of canopy interception amount (CIA), litter maximum water-holding amount (LWHA), soil water storage amount (SSA), and forest water retention amount (WRA) of China. We found that the total forest WRA in China was 2325.87 × 108 m3, and the CIA, LWHA and SSA contributed 21.24%, 5.37% and 73.39%, respectively, to the WRA. The forest WRA in the basins of southern China including the Yangtze River Basin (YTRB), Songhua River Basin (SHRB), Southwest Rivers Basin (SWRB) and Pearl River Basin (PRB) accounted for 78.33% of the total forest WRA. The WRAs of subtropical needleleaf forest, temperate deciduous broadleaf forest, subtropical and tropical mountainous needleleaf forest, subtropical evergreen broadleaf forest, and cold and temperate mountainous needleleaf forest accounted for 50.61% of the total WRA in Chinese forests. The WRAs of cold and temperate forest types were relatively higher in the SHRB, Liao River Basin (LRB), Northwest Rivers Basin (NWRB), Hai River Basin (HRB) and Yellow River Basin (YRB) than in other basins, and the WRAs of subtropical and tropical forests types were relatively higher in the YTRB, Southeast Rivers Basin (SERB), SWRB, and PRB than in the other basins. This observation-based assessment can provide insight into spatial variations in different layers of the WRA including canopy, litter, and soil which can be used for forest resource management in China.

High-throughput sequencing-based microsatellite genotyping for polyploids to resolve allele dosage uncertainty and improve analyses of genetic diversity, structure and differentiation: A case study of the hexaploid Camellia oleifera.

Conventional microsatellite (simple sequence repeat, SSR) genotyping methods cannot accurately identify polyploid genotypes leading to allele dosage uncertainty, introducing biases in population genetic analysis. Here, a new SSR genotyping method was developed to directly infer accurate polyploid genotypes. The frequency distribution of SSR sequences was obtained based on deep-coverage high-throughput sequencing data. Corrections were performed accounting for the "stutter peak" and amplification efficiency of SSR sequences. Perl scripts and an online SSR genotyping tool "SSRSeq" were provided to process the sequencing data and output genotypes with corrected allele dosages. Hexaploid Camellia oleifera is the dominant woody oilseed crop in China. Understanding the geographical pattern of genetic variation in wild C. oleifera is essential for the conservation and utilization of genetic resources. Six wild C. oleifera populations were sampled across geographical ranges in subtropical evergreen broadleaf forests of China. Using 35 SSR markers, the high-throughput sequencing-based SSRSeq method was applied to obtain accurate hexaploid genotypes of wild C. oleifera. The results demonstrated that the new method could resolve allele dosage uncertainty and considerably improve genetic diversity, structure and differentiation analyses for polyploids. The genetic variation patterns of wild C. oleifera across geographical ranges agree with the "central-marginal hypothesis", stating that genetic diversity is high in the central population and declines from the central to the peripheral populations, and genetic differentiation increases from the centre to the periphery. This method and findings can facilitate the utilization of wild C. oleifera genetic resources for the breeding of cultivated C. oleifera.

Phytogeographic History of the Tea Family Inferred Through High-Resolution Phylogeny and Fossils.

The tea family (Theaceae) has a highly unusual amphi-Pacific disjunct distribution: most extant species in the family are restricted to subtropical evergreen broadleaf forests in East Asia, while a handful of species occur exclusively in the subtropical and tropical Americas. Here, we used an approach that integrates the rich fossil evidence of this group with phylogenies in biogeographic analysis to study the processes behind this distribution pattern. We first combined genome-skimming sequencing with existing molecular data to build a robust species-level phylogeny for c.130 Theaceae species, resolving most important unclarified relationships. We then developed an empirical Bayesian method to incorporate distribution evidence from fossil specimens into historical biogeographic analyses and used this method to account for the spatiotemporal history of Theaceae fossils. We compared our method with an alternative Bayesian approach and show that it provides consistent results while significantly reduces computational demands which allows analyses of much larger data sets. Our analyses revealed a circumboreal distribution of the family from the early Cenozoic to the Miocene and inferred repeated expansions and retractions of the modeled distribution in the Northern Hemisphere, suggesting that the current Theaceae distribution could be the remnant of a larger continuous distribution associated with the boreotropical forest that has been hypothesized to occupy most of the northern latitudes in the early Cenozoic. These results contradict with studies that only considered current species distributions and showcase the necessity of integrating fossil and molecular data in phylogeny-based parametric biogeographic models to improve the reliability of inferred biogeographical events. [Biogeography; genome skimming; phylogenomics; plastid genome; Theaceae.].

Forest Land Quality Evaluation and the Protection Zoning of Subtropical Humid Evergreen Broadleaf Forest Region Based on the PSO-TOPSIS Model and the Local Indicator of Spatial Association: A Case Study of Hefeng County, Hubei Province, China

Forest land is the carrier for growing forests. It is of great significance to evaluate the forest land quality scientifically and delineate forestland protection zones reasonably for realizing better forest land management, promoting ecological civilization construction, and coping with global climate change. In this study, taking Hefeng County, Hubei Province, a subtropical humid evergreen broad-leaved forest region in China, as the study area, 14 indicators were selected from four dimensions—climatic conditions, terrain, soil conditions, and socioeconomics—to construct a forest land quality evaluation index system. Based on the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) model, we introduced the Particle Swarm Optimization (PSO) algorithm to design the evaluation model to evaluate the forest land quality and analyze the distribution of forest land quality in Hefeng. Further, we used the Local Indicator of Spatial Association (LISA) to explore the spatial distribution of forest land quality and delineate the forest land protection zones. The results showed the following: (1) the overall quality of forest land was high, with some variability between regions. The range of Forest Land Quality Index (FLQI) in Hefeng was 0.4091–0.8601, with a mean value of 0.6337. The forest land quality grades were mainly first and second grade, with the higher-grade forest land mainly distributed in the central and southeastern low mountain regions of Zouma, Wuli, and Yanzi. The lower-grade forest land was mainly distributed in the northwestern middle and high mountain regions of Zhongying, Taiping, and Rongmei. (2) The global spatial autocorrelation index of forest land quality in Hefeng County was 0.7562, indicating that the forest land quality in the county had a strong spatial similarity. The spatial distribution of similarity types high-high (HH) and low-low (LL) was more clustered, while the spatial distribution of dissimilarity types high-low (HL) and low-high (LH) was generally dispersed. (3) Based on the LISA of forest land quality, forest land protection zones were divided into three types: key protection zones (KPZs), active protection zones (APZs), and general protection zones (GPZs). The forest land protection zoning basically coincided with the forest land quality. Combining the characteristics of self-correlated types in different forestland protection zones, corresponding management and protection measures were proposed. This showed that the PSO-TOPSIS model can be effectively used for forest land quality evaluation. At the same time, the spatial attributes of forest land were incorporated into the development of forest land protection zoning scheme, which expands the method of forest land protection zoning, and can provide a scientific basis and methodological reference for the reasonable formulation of forest land use planning in Hefeng County, while also serving as a reference for similar regions and countries.

浙西南常绿阔叶林凋落物空间分布及其对土壤养分的影响

凋落物作为养分的有效载体对于森林养分循环具有重要作用。近年来学者们对凋落物开展了大量研究，但森林内部凋落物的空间分布及其对土壤养分空间异质性的作用一直没有得到充分重视。利用位于浙江西南部的百山祖亚热带中山常绿阔叶林5 hm²森林动态监测样地中50个凋落物收集器2009-2017年收集到的凋落物产量数据和相关的地形和土壤养分数据，运用同步自相关回归分析地形和群落结构因素对凋落物及其组分产量的影响，利用多元线性混合效应模型研究了地形和年均凋落物输入量与土壤养分含量的关系。结果表明（1）地形因子中仅凸度与总凋落物以及叶凋落物产量呈显著正相关；（2）邻域内的大树和中树（胸径≥ 5 cm）的平均胸径显著提高总凋落物、叶凋落物和小枝及树皮凋落物产量，而邻域物种数显著提高总凋落物、叶凋落物和繁殖器官凋落物产量；（3）地形凸度与表层（深0-10 cm）土壤有机质含量显著正相关，但与近表层（深10-20 cm）土壤总氮、有效磷和速效钾含量显著负相关；（4）凋落物输入量显著提高表层（深0-10 cm）土壤的碱解氮和有机质含量。总体来看，百山祖常绿阔叶林凋落物产量的空间变异受到地形和森林群落结构因素的制约，并影响林内土壤养分的空间异质性。;Litter, as an effective carrier of nutrients, plays an important role in forest nutrient cycles. In recent years, several studies have been carried out on litter; however, the spatial distribution of litter yield in a forest and its effects on the spatial heterogeneity of soil nutrients have not been thoroughly investigated. In August 2008, 50 litter traps with horizontal areas of 0.71 m×0.71 m along a path in the 5hm² forest dynamics plot in the Baishanzu midmountain subtropical evergreen broadleaf forest were set and all the litter falling into the traps was collected twice a month during a period from 2009 to 2017. The litter was dried in an oven at 80℃ at a constant weight and was weighed. In July 2014, soil samples were collected using an earth drill from three 1m×1m quadrats with an adjacent distance of 2 m for each of the 50 litter traps (50×3 samples). Each soil sample was divided into surface and near-surface layers (i.e., 0-10 cm and 10-20 cm, respectively). Total nitrogen, alkaline nitrogen, available phosphorus, available potassium, and organic matter contents of these soil samples were determined by common soil chemistry approaches. The elevation, convexity, and slope of each 10m×10m quadrat were assigned to the trap located in the quadrat. Simultaneous auto-regression was used to analyze effects of topography and community structure on the litter and its component yields. The community structure was described according to the total basal area, mean diameter at breast height (DBH), and species richness of trees with DBH ≥ 5 cm in the 5 or 10 m radius neighborhood of the traps. Further, the impact of the topography and annual litter input on the soil nutrient concentration was studied using a linear mixed effect model. The results showed that (1) only convexity was positively correlated with the total and leaf litter yields; (2) the mean DBH of the trees in the neighborhood significantly increased the total, leaf, twig and bark litter yields, while the number of species in the neighborhood significantly increased the total, leaf, and reproductive organ litter yields; (3) the terrain convexity significantly increased the soil organic matter content in the surface layer (0-10 cm deep) but significantly reduced the total nitrogen, available phosphorus, and available potassium contents in the near-surface layer (10-20 cm deep); and (4) the total litter input significantly increased the alkaline nitrogen and organic matter contents in the surface soil. Our results suggest that the spatial variation in the litter yield in the Baishanzu evergreen broadleaf forest is determined by the topography and forest structure and affects the spatial heterogeneity of soil nutrients in the forest.

Litterfall mercury reduction on a subtropical evergreen broadleaf forest floor revealed by multi-element isotopes.

Litterfall mercury (Hg) deposition is the dominant source of soil Hg in forests. Identifying reduction processes and tracking the fate of legacy Hg on forest floor are challenging tasks. Interplays between isotopes of carbon (C) and nitrogen (N) may shed some lights on Hg biogeochemical processes because their biogeochemical cycling closely links with organic matters. Isotope measurements at the evergreen broadleaf forest floor at Mt. Ailao (Mountain Ailao) display that δ202Hg and Δ199Hg both significantly correlate with δ13C and δ15N in soil profiles. Data analysis results show that microbial reduction is the dominant process for the distinct δ202Hg shift (up to ∼1.0‰) between Oi and 0-10cm surface mineral soil, and dark abiotic organic matter reduction is the main cause for the Δ199Hg shift (∼-0.18‰). Higher N in foliage leads to greater Hg concentration, and Hg0 re-emission via microbial reduction on forest floor is likely linked to N release and immobilization on forest floor. We thus suggest that the enhanced N deposition in global forest ecosystems can potentially influence Hg uptake by vegetation and litter Hg sequestration on forest floor.

Decomposition of roots of different diameters in response to different drought periods in a subtropical evergreen broad-leaf forest in Ailao Mountain

Root decomposition plays an essential role in the terrestrial carbon cycle, but the responses of roots of different diameters to varying drought periods are unclear. Therefore, artificial drought experiments were conducted to investigate the effects of different drought periods on the decomposition rates of roots with different diameters. In this study, <2 mm, 2–5 mm and 5–10 mm root decomposition experiments were conducted in control (CK), short-term drought (STD, drought during one rainy season), and long-term drought (LTD, two-year drought) treatments. The drought treatments significantly reduced the soil water content by 52.5% and 83.6%, respectively, but had little effect on the soil temperature. According to the decomposition coefficient (k value), the root decomposition rate decreased significantly with increasing diameter, and STD and LTD significantly reduced root decomposition of different diameters by 30.4–41.7% and 47.8–70.8%, respectively. The analysis of covariance (ANCOVA) revealed that diameter, treatments and their interaction had significant effects on the k values. The linear regression showed that the dependence of the stage root decomposition rate (SRDR) and k value of fine roots on the soil water content were higher than that of coarse roots. Therefore, fine roots were more sensitive to the soil water content and drought, thereby affecting the nutrient cycle and health of forest ecosystems under drought. This study provides new insight into the impact of climate change on root decomposition and the carbon cycle.

Stable Mercury Isotope Transition during Postdepositional Decomposition of Biomass in a Forest Ecosystem over Five Centuries.

Organic soil is an important transient reservoir of mercury (Hg) in terrestrial ecosystems, but the fate of deposited Hg in organic forest soil is poorly understood. To understand the dynamic changes of deposited Hg on forest floor, the composition of stable Hg and carbon (C) isotopes in decomposing litters and organic soil layer was measured to construct the 500 year history of postdepositional Hg transformation in a subtropical evergreen broad-leaf forest in Southwest China. Using the observational data and a multiprocess isotope model, the contributions of microbial reduction, photoreduction, and dark reduction mediated by organic matter to the isotopic transition were estimated. Microbial reduction and photoreduction play a dominant role in the initial litter decomposition during first 2 years. Dark redox reactions mediated by organic matter become the predominant process in the subsequent 420 years. After that, the values of Hg mass dependent fractionation (MDF), mass independent fractionation (MIF), and Δ199Hg/Δ201Hg ratio do not change significantly, indicating sequestration and immobilization of Hg in soil. The linear correlations between the isotopic signatures of Hg and C suggest that postdepositional transformation of Hg is closely linked to the fate of natural organic matter (NOM). Our findings are consistent with the abiotic dark reduction driven by nuclear volume effect reported in boreal and tropical forests. We recommend that the dark reduction process be incorporated in future model assessment of the global Hg biogeochemical cycle.

The roles of initial litter traits in regulating litter decomposition: a “common plot” experiment in a subtropical evergreen broadleaf forest

Leaf litter decomposition constitutes a valuable ecosystem service by recycling nutrients, transferring energy, and sequestrating carbon (C) in terrestrial ecosystems. However, variations in microclimate and decomposers among studies have impeded a mechanistic understanding of what controls litter decomposition. We conducted a “common plot” experiment leveling-off impacts of environmental and biotic factors to explore roles of the initial leaf litter traits in decomposition. The experiment was carried out in an evergreen broadleaf forest in the Wuyi Mountains from May 2017 to August 2018. Leaf litter was collected from Populus deltoids in a nitrogen (N) addition experiment at Dongtai Forest Farm, Liriodendron chinense × tulipifera and Platanus acerifolia on campus Nanjing Forestry University, and Quercus variabilis and Pinus taeda at Xiashu Forest Farm. Our results showed that N addition significantly increased leaf litter decomposition of P. deltoids. This may result from N induced increases in litter quality, indicated by a high N content and a low C:N ratio of leaf litter. Across the five tree species (with no litter from P. deltoids under N addition treatment), we found litter decomposition of the coniferous tree (P. taeda) was much lower than that of the other four broadleaf trees due to its lower litter quality. Multiple regression analyses indicated that the initial C:N ratio and sodium (Na) content that was critical for decomposers dominated decomposition. Our results confirmed the importance of the initial litter quality, such as C:N ratio, in regulating leaf litter decomposition. In addition, our results suggest the Na control of the decomposition of leaf litter in subtropical ecosystems through Na limiting the activity and abundance of decomposers in brown food webs.

Topography and plant community structure contribute to spatial heterogeneity of soil respiration in a subtropical forest

Soil respiration is the largest carbon (C) flux from terrestrial ecosystems into the atmosphere. Accurate estimates of the magnitude and distribution of soil respiration are critically important to models of global C cycling and predictions of future climate change. One of the greatest challenges to accurate large-scale estimation of soil respiration is its great spatial heterogeneity at the site level. Our study explored how soil respiration varies in space and the drivers that lead to this variance in a natural subtropical evergreen broadleaf forest in Southern China. We conducted a two-year soil respiration measurement for 168 randomly selected sampling points in a 4 ha plot. We measured the spatial variance of soil respiration and tested its correlation to a variety of abiotic and biotic factors including topography, aboveground plant community structure, soil environmental factors, soil organic matter, and microbial community structure. We found that soil respiration was highly varied across the study plot, with a spatial variation coefficient (CV) of 32.75%. The structural equation modeling (SEM) analysis showed that elevation influenced tree species diversity, productivity, and soil water content, which in turn affected soil respiration via soil C content, clay content, fungal:bacterial ratio, annual litterfall, and fine root biomass. 31% of the total spatial variation of soil respiration was accounted for in the SEM, mostly by elevation, soil C content, annual litterfall biomass, tree species diversity as estimated by the Simpson's index, and soil water content, with standardized total effects of 0.31, −0.31, 0.29, 0.19, and −0.18, respectively. Our data demonstrated that soil respiration was highly spatially varied at the fine scale, and was primarily regulated by factors of topography and plant community structure. More studies investigating the spatial variation of soil respiration are therefore needed to better understand and assess terrestrial ecosystem C cycling.