Subtropical Forests In Southern China Research Articles

Effects of Tree Diversity, Functional Composition, and Large Trees on the Aboveground Biomass of an Old-Growth Subtropical Forest in Southern China

Forest aboveground biomass (AGB) plays an important role in regulating the global carbon cycle and is thus an essential component of ecosystem functioning. In the relationships between biodiversity and ecosystem functioning (BEF), studies have shown that many biotic factors (e.g., species, functional traits, and large trees) and abiotic factors have significant impacts on AGB. However, the relative strength of these affecting factors remains unclear. In this study, we analyzed woody plants (diameter at breast height [DBH] ≥ 1 cm) within a 1.6 ha plot in an old-growth subtropical natural forest in southern China. We used structural equation models to test the effects of tree diversity (species, phylogenetic, functional, and size inequality), functional composition, large trees, and environmental factors (topography, soil nutrients, and understory light) on AGB. Our results indicated that size inequality, the community-weighted mean of maximum DBH (CWM_MDBH), and large trees had significant, positive effects on AGB (p < 0.001), while lower soil phosphorus content was found to promote an increase in AGB. Furthermore, large trees, which were mostly composed of dominant tree species, were the main driver of AGB, and the effect of functional composition (e.g., CWM_MDBH) on AGB was substantially reduced by large trees. We argue that the selection effect plays a key role in regulating BEF relationships in subtropical natural forests and conclude that retaining large-diameter trees and dominant species, along with sustaining a complex stand structure, are key measures for improving productivity.

Effect of Glomus intraradices on root morphology, biomass production and phosphorous use efficiency of Chinese fir seedlings under low phosphorus stress.

Available phosphorus (P) scarcity in the highly weathered soils of the subtropical forests in southern China is a serious concern. To ensure whether inoculation of arbuscular mycorrhizal fungi (AMF) with Chinese fir (Cunninghamia lanceolata) under low P stress conditions could promote its growth and P utilization capacity, an indoor pot simulation experiment was carried out with the different P supply treatments and Chinese fir seedlings as the tested material. The experiment had two P supply treatments, no P supply (P0, 0 mmol·L-1 KH2PO4) and normal P supply (P1, 1.0 mmol·L-1 KH2PO4). The seedling in each P supply treatment was inoculated with Glomus intraradices (Gi), a widespread species of AMF in the natural environment, and with no AMF inoculation as a control treatment (CK). The Gi infection rate in the root system, root cortex tissue dissolution rate, root morphological indexes and biomass, whole plant P use efficiency, and root P use efficiency of Chinese fir were determined under different treatment conditions. The results showed that P0 treatment significantly increased the Gi infection rate (p< 0.05). After inoculating AMF with different P supply treatments, the root cortex tissue dissolution rate was considerably enhanced. In contrast, the Chinese fir's root length and surface area were reduced; however, the root volume did not change significantly. The average root diameter in the P0 treatment and inoculated with AMF was significantly more prominent than in the uninoculated treatment (p< 0.05). The root biomass and root-to-shoot ratio at different P supply treatments were significantly higher in the Gi infection treatment than in the CK group. Under different P supply treatments, root inoculation with Gi promoted root P use efficiency and whole plant P use efficiency. In conclusion, low P stress condition promoted the colonization of AMF in the root system, increased the dissolution of root cortex tissue, root volume, and the average diameter, and promoted root biomass accumulation and P use efficiency.

Effects of Exotic and Native Earthworms on Soil Micro-Decomposers in a Subtropical Forest: A Field Mesocosm Experiment

Biological invasion is one of the most important factors affecting global biodiversity change, which can adversely affect ecosystem function. However, little is known about the effects of belowground biological invasions on soil ecosystems. In this study, we conducted a field-based mesocosm experiment to observe the effects of exotic and native earthworms (Pontoscolex corethrurus and Amynthas corticis, respectively) on soil nutrients and micro-decomposers (i.e., soil microorganisms and nematodes) in a subtropical forest in southern China. We found that exotic and native earthworms had different effects on nutrient availability and nematode communities in the soil. Specifically, exotic earthworms significantly decreased the content of nitrates in the soil and tended to decrease the total nematode abundance compared with native earthworms. Furthermore, nematode species richness and Shannon–Wiener index were lowest in the treatment with exotic earthworms and were the highest in the treatment with native earthworms. However, neither native nor exotic earthworms significantly affected soil microbial community composition. The results of redundancy analysis indicated that available phosphorus was positively correlated with nematode community. Our results demonstrated that exotic earthworms had adverse effects on the available nutrient content in the soil, and had a potential negative effect on the abundance of soil microfauna. These findings will be helpful in understanding the influence of exotic earthworms on soil micro-decomposers and the ecological consequences of earthworm invasion.

Tree Diversity, Structure and Functional Trait Identity Promote Stand Biomass Along Elevational Gradients in Subtropical Forests of Southern China

AbstractSubtropical forests play an important role in regulating global carbon storage, a substantial portion of these forest carbon pools are stored in biomass stocks. Yet, we do not fully understand how tree diversity, stand structure and functional trait identity influence biomass stocks along elevational gradients. Here we used forest inventory data from nine 1‐ha plots across different elevational gradients in subtropical forests of southern China. We analyzed the effects of tree diversity, structure and functional trait identity on biomass along elevational gradients, and tested the complementarity effect, selection effect, mass‐ratio hypothesis and specie‐energy hypothesis. We found that multiple metrics of diversity and structural attributes significantly promoted biomass. Forest biomass was improved by traits with greater maximum height, larger seed mass (SM) and lower wood density (WD). Specifically, large‐diameter trees were the strongest independent predictor of biomass relative to other single variables. A significant increase in tree diversity, density and biomass was observed with increasing elevational gradients; additionally, trees dominated by traits related to greater maximum tree height, larger SM and lower WD also increased. Elevation indirectly increased biomass by increasing diversity and community‐weighted mean traits, but indirectly decreased it by reducing structural attributes. Our results support the complementarity effect, selection effect and mass‐ratio hypothesis simultaneously, but not the species‐energy hypothesis. The selection effect had greater effects on stand biomass than the complementarity effect and mass‐ratio hypothesis. Our findings suggest that developing natural tree diversity and structural complexity as well as dominant resource‐acquisitive species is beneficial to maintain and enhance carbon storage capacity in species‐rich subtropical forests along elevational gradients.

Assessing spatial patterns and drivers of burn severity in subtropical forests in Southern China based on Landsat 8

Burn severity assessment is vital for the development of effective fire management strategies. Subtropical forests in Southern China support rich forest resources but are also disturbed by frequent forest fires, resulting in varying burn severity areas. However, the lack of accuracy assessment of burn severity based on spectral indices has limited our understanding of spatial patterns and drivers of burn severity in this region and the implementation of effective forest fire management. In this study, we compared the accuracy of different spectral indices from Landsat 8 Operational Land Imager (OLI) in assessing burn severity. Moreover, the optimal spectral index was selected to map burn severity and analyze its spatial patterns and drivers by using the random forest model. The results showed that (1) dNBR, RdNBR, and RBR outperformed the others in accuracy of fitting burn severity, and dNBR was slightly higher in accuracy classification; (2) vegetation (pre-fire NDVI) and human activity (distance to the nearest road or settlement) were the most important drivers affecting burn severity patterns; (3) the proportion of moderate and low burn severity was higher, and most burn areas were concentrated near settlements and roads, but areas with higher burn severity tended to be distributed in uphill, sunny slopes, and high altitudes. In view of this, we recommend targeted forest fire management according to the distribution pattern and drivers of burn severity in this region, including setting up fire belts near residential areas and roads, and prioritizing fuel removal plans in areas prone to develop higher burn severity.

The Effects of Species Abundance, Spatial Distribution, and Phylogeny on a Plant-Ectomycorrhizal Fungal Network.

Plant and root fungal interactions are among the most important belowground ecological interactions, however, the mechanisms underlying pairwise interactions and network patterns of rhizosphere fungi and host plants remain unknown. We tested whether neutral process or spatial constraints individually or jointly best explained quantitative plant–ectomycorrhizal fungal network assembly in a subtropical forest in southern China. Results showed that the observed plant–ectomycorrhizal fungal network had low connectivity, high interaction evenness, and an intermediate level of specialization, with nestedness and modularity both greater than random expectation. Incorporating information on the relative abundance and spatial overlap of plants and fungi well predicted network nestedness and connectance, but not necessarily explained other network metrics such as specificity. Spatial overlap better predicted pairwise species interactions of plants and ectomycorrhizal fungi than species abundance or a combination of species abundance and spatial overlap. There was a significant phylogenetic signal on species degree and interaction strength for ectomycorrhizal fungal but not for plant species. Our study suggests that neutral processes (species abundance matching) and niche/dispersal-related processes (implied by spatial overlap and phylogeny) jointly drive the shaping of a plant-ectomycorrhizal fungal network.

Variations in leaf and stem traits across two elevations in subtropical forests.

Understanding the variations in plant traits across elevations may provide valuable insights into the species structure and function of forests and their responses to climate change. To explore the patterns of trait variation across elevations, we analysed 14 leaf and stem traits associated with resource acquisition and stress tolerance in Schima superba Gardner & Champion, Castanopsis chinensis (Sprengel) Hance, and Pinus massoniana Lambert trees at two elevations in a subtropical forest in southern China. Wood density increased, whereas crown width, leaf water potential at 0700 hours (Ψ L-0700 ), and leaf δ 18 O decreased in high-elevation plants. Vessel diameter, daily maximum sap flux density, leaf δ 13 C, and leaf C and N concentrations per unit mass were comparable across elevations. We found species-specific variations in specific leaf area, midday leaf water potential, and leaf P concentration across elevations. Decreasing crown width with increasing elevation was associated with decreasing leaf δ 18 O and Ψ L-0700 , suggesting that higher stomatal conductance may moderate the loss of carbon assimilation. We elucidated the adaptive strategies of plants in response to environmental change, and showed that physiological traits varied in coordination with structural traits. Future studies incorporating multi-dimensional trait analyses can improve our understanding of the responses of forest ecosystems to climate change and global warming.

Abiotic Drivers of Seedling Bank Diversity in Subtropical Forests of Southern China

Abiotic factors are important to shape plant community composition and diversity through processes described as environmental filtering. Most studies on plant diversity in forests focus on adult trees, while the abiotic drivers of forest seedling community characteristics are less understood. Here, we studied seedling banks’ composition, richness, diversity, and abundance, and investigated their relationships with microsite abiotic conditions along a wide elevational gradient. We sampled seedling communities in 312 1-m2 quadrats, distributed in 13 one-ha plots in four subtropical forests in south China, covering an elevation gradient of 1500 m, for 2 years. We measured light availability, slope, and 11 soil nutrients for each seedling quadrat. We used analysis of similarities and multivariate analysis of variance to compare the composition and abiotic drivers of the four forests’ seedling communities. We then used mixed models and structural equation modeling to test the direct and indirect effects of abiotic factors on seedling species richness, diversity, and abundance. The differences in seedling community composition among these forests were mostly explained by differences in elevations and soil nutrients. Seedling diversity as Shannon and Simpson diversity index decreased with increasing elevation and increased with increasing slope, but seedling abundance and species richness did not. Elevation had an indirect effect on Simpson’s diversity index through modulating the direct effects of soil properties. Our findings show that soil properties play a prominent role in favoring differentiation in species composition among the four forests we studied and provide additional evidence to decreasing species diversity with elevation. However, this was reflected in decreasing Shannon and Simpson indices rather than species richness, which is more commonly studied. Whether and to what extent future environmental changes in climate and soil acidification will alter future forest composition and diversity needs to be investigated.

Mixing overstory tree- and understory fern-derived dissolved organic carbon produces non-additive effects on biodegradation in subtropical forests of southern China

Mixing overstory tree- and understory fern-derived dissolved organic carbon produces non-additive effects on biodegradation in subtropical forests of southern China

Tree mycorrhizal effect on litter-leached DOC amounts and biodegradation is highly dependent on leaf habits in subtropical forests of southern China

Tree mycorrhizal effect on litter-leached DOC amounts and biodegradation is highly dependent on leaf habits in subtropical forests of southern China

Seedling emergence and environmental filters determine Ficus recruitment in a subtropical landscape

Early regeneration, involving the transition from a seed into a successfully recruited seedling, is a complex process characterized by high mortality rates and the strong association with environmental filters. However, we have limited understanding of the factors and transitional regeneration stages that lead to these broad mortality events, even in species that are key for ecosystem function and comprise the bulk of species in diversity assessments, such as Ficus species. Using eight, 1-ha long term monitoring plots, established along an elevation gradient in tropical to subtropical forests in southern China, and partitioning the early plant regeneration process into four stages (seed germination, seedling emergence, establishment and recruitment), we conducted a seed addition experiment to assess the factors driving the early regeneration process. We used four hemi-epiphytic and four non-hemiepiphytic Ficus species, specifically asking whether elevation, microsite conditions and growth form affect the success of regeneration outcomes. We observed that Ficus early regeneration was highly limited throughout the subtropical landscape with similar seed germination probabilities along all elevations, but greater seedling emergence at higher elevations. Growth form and elevation interacted significantly during seed germination and seedling emergence stages. The strongest filtering effect was observed during seedling emergence, with greater canopy openness facilitating, but drier and cooler soils being less conducive to seedling emergence, especially for hemiepiphytic species. Once emerged, Ficus seedlings were released from the environmental filters assessed here, yet, continued to face successively greater mortality during establishment and recruitment. Greater seed germination and seedling emergence on the ground observed in hemiepiphytes indicate that they may benefit from evolutionary traits associated with canopy regeneration under water and temperature stress, but only at the early regeneration stages, as seedling establishment and recruitment were similar in both groups. In general, our results demonstrate that even when seeds are available, successful recruitment is strongly limited by microsite conditions and growth form, through filtering mechanisms associated with the transition from seedling emergence to seedling establishment. The identification of environmental filters that operate during the complex and vulnerable early regeneration stages provides a baseline to develop management strategies for ex situ conservation, as well as in situ population reestablishment for restoration.

Bedrock and climate jointly control the phosphorus status of subtropical forests along two elevational gradients

Terrestrial carbon sequestration is constrained by the supply of soil phosphorus (P). Climate and bedrock are two key independent drivers of soil P supply. Their potential interactions with soil P supply remains poorly understood. To address this issue, we examined the P status of subtropical forests in southern China along two adjacent elevational gradients with contrasting bedrock types (granite vs. slate). The results show that the P concentration of granite was significantly lower than that of slate. Accordingly, the P concentration of mineral soil, litter, and fine-root P on the granite was significantly lower than that on the slate. In contrast, their ratios to the bedrock P concentration were higher in the granite transect than in the slate transect. Moreover, we found a stronger nutrient uplift effect on P in the P-poor transects. Although bedrock P concentrations were constant along elevation, topsoil total P, soil labile inorganic P (Pi), soil labile organic P (Po), and soil moderately labile Pi concentrations showed a significant increasing trend with elevation on both transects. Multivariate linear regression models revealed that bedrock P explained more variation of plant and soil P than did climate. Soil moderately labile Po concentration showed contrasting elevational patterns on different bedrock types, which indicated an interactive effect between bedrock P concentration and climate on the soil moderately labile Po concentration. The Pearson correlation analysis indicated that plant and soil P measures were more tightly coupled in the P-poor transect than in the P-rich transect. These results indicate that the P status of subtropical forests is determined predominantly by the bedrock P concentration and by its interaction with climate. Our results suggest that predictions of ecosystem P status and its responses to climate change can be improved significantly by incorporating local-scale parent material properties into the modeling frameworks.

Habitat loss-biodiversity relationships are influenced by assembly processes and the spatial configuration of area loss

Habitat loss greatly threatens biodiversity worldwide. However, how different facets of biodiversity (taxonomic, phylogenetic and functional diversity) decline with habitat loss is currently poorly understood. Habitat loss-biodiversity relationships, while important to understand in their own right, have the potential to reveal the underlying mechanisms that naturally structure plant communities and can further illuminate how spatial configurations of habitat loss affect biodiversity in a given amount of habitat. In this study, we used four spatial point process models to assess the relative importance of the processes of random placement of individuals, habitat filtering, dispersal limitation and the combined effects of habitat filtering and dispersal limitation in producing habitat loss-biodiversity relationships across different biodiversity facets for plant species in a subtropical forest. We assessed this using a 50 ha fully mapped subtropical forest in southern China by applying two simulated habitat destruction patterns, namely random and aggregated habitat removal to both real and simulated forest communities. We found that the combined effects of habitat filtering and dispersal limitation provided the best fit among the four competing assembly mechanisms, to the simulated habitat loss-biodiversity relationships applied to real forest data across all three biodiversity metrics. We also found that phylogenetic and functional diversity were less sensitive to the loss of habitat area than species richness and biodiversity declined more gradually under random habitat removal than for aggregated removal for both empirical forests and the simulated communities where assembly mechanisms generated intraspecific aggregation. Overall, our study examined how different assembly processes and the spatial configuration of habitat loss result in differing susceptibilities to diversity loss from habitat destruction and identified the potential mechanisms underlying the spatial patterns of diversity. This study also provides guidelines for the prioritization of the conservation of different facets of biodiversity under different spatial configurations of habitat loss.

Plant‐caterpillar food web: Integrating leaf stoichiometry and phylogeny

1. Trophic interactions and plant‐herbivore network structure may be regulated by ecological and evolutionary factors. The allocation of resources to physical and chemical traits may influence plant nutritional values and defence against insect herbivores. However, the elemental consistency of plant leaves is rarely considered a functional trait, despite leaf chemical traits being critical for understanding trophic interactions and network modularity.2. We constructed a high‐resolution network of a plant‐caterpillar food web in a subtropical forest in southern China using DNA barcoding. We analysed whether abundance, phylogeny, and functional traits (including leaf chemical elements) significantly differed among modules and conducted linear regressions to explore how these traits could explain the intensity and likelihood of trophic interactions between plants and caterpillars.3. Caterpillar feeding intensity increased with increasing specific leaf area and leaf nitrogen to phosphorus (N:P) ratio. The intensity and likelihood of feeding decreased with increasing calcium and magnesium levels. Leaf dry matter content and leaf concentrations of potassium, magnesium, copper, and manganese varied among modules. Caterpillar feeding likelihood and intensity were constrained by plant and caterpillar phylogenies, and module phylogenetic clustering was apparent. Species abundance of plants and caterpillars constrained the feeding intensity but not the likelihood of feeding and network module composition.4. We highlighted the combined roles of functional traits and phylogeny in shaping trophic interactions and modularity in plant‐insect herbivore networks and illustrated the importance of leaf stoichiometry in establishing dietary associations.

Species abundance is jointly determined by functional traits and negative density dependence in a subtropical forest in southern China

Abstract Aims The factors affecting species abundance are a subject of ongoing debates in community ecology. Empirical studies have demonstrated that tree abundance is affected by plant functional traits and negative density dependence (NDD). However, few studies have focused on the combined effects of NDD and plant functional traits on species abundance. Methods In this study, we used tree functional traits and two census data from a 50-ha forest dynamic plot in the Heishiding (HSD) Nature Reserve to explore the combined effects of functional traits and NDD on species abundance. Using hierarchical Bayesian models, we analyzed how neighbor densities affected the survival of saplings from 130 species and extracted posterior means of the coefficients to represent NDD. The structural equation modeling (SEM) analysis was then applied to investigate the causal relationships among species functional traits, NDD and species abundance. Important Findings SEM showed that tree functional traits, including specific leaf area (SLA), leaf area (LA), leaf dry matter content (LDMC), leaf N content (LNC), maximum electron transport rate (ETRmax) and conspecific adult negative density dependence (CNDDadult), together explained 20% of the total variation in tree abundance. Specifically, SLA affected tree abundance both directly and indirectly via CNDDadult, with a totally negative influence on abundance. LDMC and LNC had only indirect effects mediated by CNDDadult on tree abundance. ETRmax and LA had directly negative effects on abundance, but their direct connections with CNDDadult were not observed. In addition, CNDDadult was negatively correlated with species abundance, indicating that abundant species are under stronger NDD. Among these investigated traits, SLA contributed the most to the variation in CNDDadult and abundance. We argued that our findings of trait–CNDDadult–abundance relationships can improve our understanding of the determinants of species commonness and rarity in forests.

氮磷添加和栽植密度对大叶相思林土壤微生物群落功能多样性的影响

土壤微生物群落是土壤质量潜在的生态指标，在维持生态系统功能和服务中起着关键的作用。采用Biolog-ECO微平板法，探讨氮磷添加与不同栽植密度交互对大叶相思林（Acacia auriculiformis）土壤微生物功能多样性的影响，以便为建立合理的林分密度和氮磷施肥模式，提高土壤质量提供理论参考。以大叶相思林为研究对象，选择氯化铵（NH₄Cl）作为氮肥模拟大气氮沉降，用二水合磷酸二氢钠（NaH₂PO₄·2H₂O）进行磷添加。氮磷处理设置4个水平，即CK、施N、施P和施N+P。种植密度设置4个水平，即1667、2500、4444、10000株/hm²（分别以低密度、中密度、较高密度、高密度表示）。研究结果表明，施P显著提高了4个密度大叶相思林土壤微生物的AWCD值、碳源利用丰富度指数、McIntosh指数、Shannon指数和Simpson指数，而施N和N+P则相反。随着林分密度的减小，各处理的土壤微生物AWCD值趋于减少。通常低密度林分的土壤微生物的丰富度指数、Shannon-Wiener指数、McIntosh指数和Simpson指数较小。;Climate change affects the physiological activities of soil microbes, which causes changes of soil microbial community structure and functional diversity and cascades the effects on biogeochemical cycling and climate-ecosystem feedbacks. Among them, nitrogen (N) and phosphorus (P) are considered as the limiting factors that influences the plant biodiversity and primary productivity in forest ecosystems. The external N addition or atmospheric N deposition has been found to affect the aboveground biology processes and the underground biochemistry of soil, both directly and indirectly. Chronic elevated N input has been shown to lead to many adverse impacts, including soil acidification, nutrients imbalance, and the increased greenhouse gas emissions. Forest soils of southern China are P-limited, because they are highly weathered and leached, and little P is released even from the weathering of primary P-bearing minerals. As a result, P fertilization is widely used in the subtropical forests of southern China. As an important component in regulating belowground ecological processes, the soil microbes are primary mediators of organic matter decomposition and nutrient cycling, and thus play a key role in maintaining function and sustainability of terrestrial ecosystems. Additionally, changes in soil microbial function and community composition may trigger a series of responses, such as impacting litter and organic matter decomposition rates, humus formation nutrient transformation and cycling, and then alter the interaction between soil microbes and plant communities. In this study, Biolog-ECO microplate method was used to investigate the interactive effects of nitrogen and phosphorus additions and different stand densities on soil microbial functional diversity of Acacia auriculiformis stands, which can provide a basis for establishing reasonable stand density, fertilization method, and improve soil quality. NH₄Cl and NaH₂PO₄·2H₂O were selected to simulate atmospheric nitrogen and phosphorus additions with 4 treatments (CK, N, P and N+P) and stand density was set at 4 levels (1667, 2500, 4444, and 10000 seedlings/hm²). The results showed that P treatment promoted the average well color development (AWCD) values, richness index, McIntosh index, Shannon index, and Simpson index of soil microbes in the four density stands, while the N and N+P treatments were opposite. With the decrease of forest density, the AWCD values of soil microbes in all treatments decreased. Generally, soil microbial richness index, Shannon-Wiener index, McIntosh index and Simpson index were small in the low density stands.

Biotic and abiotic properties most closely associated with subtropical forest soil respiration differ in wet and dry seasons: A 10-year in situ study

Forest ecosystems greatly affect the global carbon (C) budget. Because subtropical regions typically have wet and dry seasons that are likely to differ in their effects on soil respiration, our estimates of carbon dioxide (CO2) emissions from subtropical soil would be improved by consideration of both wet and dry seasons rather than wet seasons alone. In this 10-year study, we determined soil respiration rates (SRRs) and soil properties in a typical subtropical forest in southern China, and we quantified the relationships between SRRs and soil abiotic and biotic properties during the dry and wet seasons. During the 10 years, soil organic carbon contents, microbial biomass, and fine root biomass were generally higher in the wet season than in the dry season. Microbial biomass C and fine root biomass were positively associated with precipitation in dry seasons but negatively associated with precipitation in wet seasons. The SRRs of wet season were consistently and significantly higher than the counterparts of dry season, with the average SRR being 5.94 ± 0.32 μmol CO2 m−2 s−1 in the wet season and 1.56 ± 0.41 μmol CO2 m−2 s−1 in the dry season. In the wet and dry seasons, cumulative CO2 emissions were positively correlated with microbial biomass C and fine root biomass, but the abiotic and biotic factors explaining variation in SRR differed in the wet and dry seasons. Most of the variation in SRRs was explained by fine roots in the wet season but by fine roots, microorganisms, and soil pH in the dry season. The results revealed that the precipitation generated significantly positive effects on the cumulative CO2 emissions in the dry seasons and that the fine root was the crucial factor contributing to the SRRs in wet and dry seasons.

Effects of ice storm on species richness and abundance of resident birds in subtropical forest in southern China.

Between January and February 2008, a severe ice storm occurred in southern China. The storm damaged nearly 21 million hectares of forest lands with broken branch, trunk breakage, and stem bending of many trees. This natural disturbance provides a unique opportunity to study impacts of extreme climatic event on forest birds. Using hierarchical multi-species occupancy model (species richness; 27 species) and N-mixture model (abundance; 3 species), we investigated how the storm affected resident bird species in subtropical forest in southern China. We sampled birds at Chebaling National Nature Reserve in Guangdong Province, monthly for 1 year each before and after the storm by mist nets. To take into account variations in temperature between years and between months, we incorporated monthly mean temperature into the models as an additional covariate. Observed species richness was greater before the storm. Estimated species richness (from the hierarchical model) also tended to be greater before than after the storm although 95% Bayesian credible intervals of these estimates were overlapped. Temperature showed a negative effect on estimated species richness, indicating greater species richness during winter. Among three species captured the most, abundance of Chestnut Bulbul (Hemixos castanonotus) significantly decreased after the storm. Other two species, Grey Treepie (Dendrocitta formosae) and Huet's Fulvetta (Alcippe hueti) showed insignificant responses to the storm. Our study indicates that the 2008 ice storm may not have a significant impact on species richness of resident birds in subtropical forest; however, the effect can be species-specific and negative to some species' population.

Nitrogen addition to the canopy of Castanopsis chinensis (Sprengel) Hance promoted xylem formation in a subtropical forest in China

Nitrogen addition during 4 years to the canopy of Castanopsis chinensis (Sprengel) Hance promoted xylem formation, mainly by increasing radial growth during the early growth season. Increased atmospheric nitrogen (N) deposition caused by intensive anthropogenic activities resulted in inconsistent effects on the xylem formation of trees, in N-limited boreal and temperate forests. However, the effects of N deposition on xylem formation of trees in subtropical forests are still poorly understood. The aims of the study were to (i) characterize the dynamics of xylem formation in C. chinensis in a subtropical forest in China and (ii) determine the effect on xylem formation of adding N to the canopy. From 2013 to 2016, 50 kg N ha−1 year−1 was applied over the canopy of C. chinensis at the end of each month (April to October) in a subtropical forest in southern China. Then, the dynamics of xylem formation were monitored during 2015 and 2016 using a microcore sampling approach. Xylem formation of C. chinensis lasted from February to December and the growth rate peaked during April–May. Adding N to the canopy promoted xylem width, by 21% in 2015 and 20% in 2016, mainly by increasing the growth rate during the early growth season (February to May in 2015 and February to April in 2016). Our study suggests that in a subtropical forest, canopy N addition could significantly promote the xylem ring width of C. chinensis. However, the response of tree growth to N addition showed significant difference between the early and late growth season. Therefore, further studies are needed to more comprehensively address the effect of N deposition on tree growth under global climate change.

Whole-plant water hydraulic integrity to predict drought-induced Eucalyptus urophylla mortality under drought stress

Drought-driven tree mortality has been occurring globally and been investigated by many researchers. Yet, while subtropical forests of southern China are predicted to experience an increased frequency and intensity of climate change‐induced drought events in the near future, there still a big knowledge gap in our understanding of the physiological vulnerability of Eucalyptus urophylla S.T. Blake under drought stress. In this study, E. urophylla was selected both for field and greenhouse experiments under controlled soil water conditions. We investigated whole-plant hydraulic conductivity (Kplant), sap flow density (Js), leaf photosynthetic characteristics and non-structural carbohydrates (NSC) to explore how E. urophylla species responded to drought from the perspective of hydraulic and carbon dynamics. Results showed that the percentage loss of stem xylem hydraulic conductance (PLC) caused by drought was up to 84.4%. Both Kplant and Js significantly decreased with the decline of the soil water availability. Also, the interaction of hydraulic conductivity to stomatal conductance was more sensitive under drought. In addition, drought-induced decline of total NSC content in leaf and stem tissue was not distinct. A conversion of starch to soluble sugar was found under drought stress. Our findings suggested that hydraulic failure could be a major threat to the survival of E. urophylla species under drought stress due to a high xylem embolism vulnerability, which improves prediction of the response of E. urophylla species to future climate changes.