Typical Subtropical Forests Research Articles

Estimation of biogenic volatile organic compound (BVOC) emissions in forest ecosystems using drone-based lidar, photogrammetry, and image recognition technologies

Abstract. Biogenic volatile organic compounds (BVOCs), as a crucial component that impacts atmospheric chemistry and ecological interactions with various organisms, play a significant role in the atmosphere–ecosystem relationship. However, traditional field observation methods are challenging for accurately estimating BVOC emissions in forest ecosystems with high biodiversity, leading to significant uncertainty in quantifying these compounds. To address this issue, this research proposes a workflow utilizing drone-mounted lidar and photogrammetry technologies for identifying plant species to obtain accurate BVOC emission data. By applying this workflow to a typical subtropical forest plot, the following findings were made: the drone-mounted lidar and photogrammetry modules effectively segmented trees and acquired single wood structures and images of each tree. Image recognition technology enabled relatively accurate identification of tree species, with the highest-frequency family being Euphorbiaceae. The largest cumulative isoprene emissions in the study plot were from the Myrtaceae family, while those of monoterpenes were from the Rubiaceae family. To fully leverage the estimation results of BVOC emissions directly from individual tree levels, it may be necessary for communities to establish more comprehensive tree species emission databases and models.

Elevated atmospheric CO2 concentration and vegetation structural changes contributed to gross primary productivity increase more than climate and forest cover changes in subtropical forests of China

Abstract. The subtropical forests of China play a pivotal role in the global carbon cycle and in regulating the global climate. Quantifying the individual and combined effects of forest cover change (FCC), vegetation structural change (e.g. leaf area index (LAI)), CO2 fertilisation, and climate change (CC) on the annual gross primary productivity (GPP) dynamics of different subtropical forest types are essential for mitigating carbon emissions and predicting future climate changes, but these impacts remain unclear. In this study, we used a processed-based model to comprehensively investigate the impacts of these factors on GPP variations with a series of model experiments in China's subtropical forests from 2001 to 2018. Simulated GPP showed a significant increasing trend (20.67 gCm-2yr-1, p<0.001) under the interaction effects of FCC, LAI change, rising CO2, and CC. The CO2 fertilisation (6.84 gCm-2yr-1, p<0.001) and LAI change (3.79 gCm-2yr-1, p=0.004) were the two dominant drivers of total subtropical forest GPP increase, followed by the effects of FCC (0.52 gCm-2yr-1, p<0.001) and CC (0.92 gCm-2yr-1, p=0.080). We observed different responses to drivers depending on forest types. The evergreen broad-leaved forests showed the maximum carbon sequestration rate due to the positive effects of all drivers. Both the FCC (0.19 gCm-2yr-1, p<0.05) and CC (1.22 gCm-2yr-1, p<0.05) significantly decreased evergreen needle-leaved forest GPP, while their negative effects were almost offset by the positive impact of LAI changes. Our results indicated that LAI outweighed FCC in promoting GPP, which is an essential driver that needs to be accounted for in studies and ecological and management programmes. Overall, our study offers a novel perspective on different drivers of subtropical forest GPP changes and provides valuable information for policy makers to better manage subtropical forests to mitigate climate change risks.

Improving the Simulation Accuracy of the Net Ecosystem Productivity of Subtropical Forests in China: Sensitivity Analysis and Parameter Calibration Based on the BIOME-BGC Model

Subtropical forests have strong carbon sequestration potential; however, the spatiotemporal patterns of their carbon sink are unclear. The BIOME-BGC model is a powerful tool for forest carbon sink estimation while the numerous parameters, as well as the localization, limit their application. This study takes three typical subtropical forests (evergreen broadleaf forest, EBF; evergreen needleleaf forest, ENF; and bamboo forest, BF) in China as examples, assesses the sensitivity of 43 ecophysiological parameters in the BIOME-BGC model both by the Morris method and the extended Fourier amplitude sensitivity test (EFAST), and then evaluates the net ecosystem productivity (NEP) estimation accuracy based on the dataset of the fiveFi long-term carbon flux sites of those three typical forests from 2000 to 2015. The results showed that (1) both sensitivity analysis methods can effectively screen out important parameters affecting NEP simulation while the Morris method is more computationally efficient and the EFAST is better in the quantitative evaluation of sensitivity. (2) The highly sensitive parameters obtained using the two methods are basically the same; however, their importance varies across sites and vegetation types, e.g., the most sensitive parameters are k for the EBF and ENF and Ract25 for the BF, respectively. (3) The optimized parameters successfully improved the NEP simulation accuracy in subtropical forests, with average correlation coefficients increased by 25.19% and normalized root mean square error reduced by 21.74% compared with those simulated by original parameters. This study provides a theoretical basis for the optimization of process model parameters and important technical support for accurate NEP simulations of subtropical forest ecosystems.

Changes in Relationship between Forest Biomass Productivity and Biodiversity of Different Type Subtropical Forests in Southern China

Forest productivity is influenced by various factors, including biodiversity, environmental factors, functional traits, and forest types. However, the relative importance of these factors in determining the productivity of subtropical forests in southern China remains controversial. In this study, we analyzed a dataset of 24 forest plots from four subtropical forest types in the Nanling Mountains with the main goal of identifying and quantifying the relative contribution of the main driving factors of forest productivity in these forests. Generalized linear regression and structural equation modeling were used to examine the relationship between forest biomass productivity (aboveground, belowground and total), biodiversity (taxonomic diversity, phylogenetic diversity and functional diversity), and environmental variables (i.e., physiography and climate). The results indicated that both environmental factors and biodiversity played pivotal roles in explaining the biomass productivity of the Nanling subtropical forests. Environmental factors had the greatest influence on total productivity, while the impacts of different types of biodiversity on various productivity components (aboveground and belowground) varied notably. Taxonomic diversity showed the strongest positive effect on the aboveground and belowground biomass productivity. However, phylogenetic and functional diversity had negative effects on productivity. Furthermore, these relationships also exhibited variations when considering different altitude gradients, with low altitudes generally leading to negative biodiversity–productivity correlations. We contextualized our results regarding the three state-of-the-art theories about biodiversity–productivity relationships (selection probability, niche complementarity, and biomass ratio) and concluded that both selection probability and niche complementarity are the driving mechanisms of productivity in the subtropical forests of the Nanling Mountains. This study offers valuable insights into the functioning and biodiversity mechanisms of subtropical forest ecosystems in southern China.

Lack of P limitation on litter decomposition: a case study from long-term addition of phosphorus and nitrogen in three subtropical forests in southern China

ABSTRACT Contrary to the widely accepted belief that elevated soil respiration after phosphorus (P) fertilization indicates P limitations for soil microorganisms in (sub)tropical forests, our previous studies have proposed that the elevated respiration results from the abiotically elevated availability of carbon (C), originating from competitive desorption from sorption sites on the surface of soil particles following P fertilization. In this study, we tested the impact of P fertilization (150 kg NaH2PO4-P ha−1 yr−1) on the decomposition of tea bags (Lipton Rooibos and Green teas) in three types of subtropical forests (primary, secondary, and planted forests) at the Dinghushan Biosphere Reserve, where previous findings reported increased soil respiration following P fertilization. If the traditional view holds true, early-stage litter decomposition should be stimulated by P fertilization, similar to the observed stimulation in soil respiration. Conversely, if this is not the case, it strongly reinforces our new hypothesis. Tea bags were employed to serve a twofold purpose: to assess the effects of P fertilization on litter decomposition within the soil, rather than on the soil surface, and to concentrate on the decomposition of early-stage litter. Our results demonstrated that P fertilization did not accelerate the decomposition of tea bags, indicating the exclusive enhancement of soil respiration without simultaneous stimulation of litter decomposition. This contradicts expectations if P were limiting microbial activity, reinforcing our hypothesis. In conclusion, our study reaffirmed the hypothesis that microbial activity in tropical forest soils is not primarily limited by P availability.

Stand structure and environment jointly determine negative air ion concentrations in forests: Evidence from concurrent on-site monitoring in four typical subtropical forests during the growing season

Negative air ions (NAIs) have been extensively studied by scientists and administrators due to their crucial role in assessing the benefits of forest health to humans and improving air quality. This paper presents the results of a study conducted in four typical subtropical forests, where 24-hour on-site monitoring of NAIs, air temperature, relative humidity, wind speed, direct radiation, and particulate matter was conducted during the growing season. The vegetation characteristics of these forests were also investigated. Correlation and random forest analyses were employed to jointly evidence the effects of stand structure and environmental factors on NAI concentrations using mean data for the monitoring period. Additionally, regression analyses were employed to discuss the response of NAIs to continuous changes in meteorological factors using screened nighttime data. Our analysis revealed that NAI diurnal dynamics followed a unimodal pattern, with peaks observed in the afternoon (13:30–16:30) and troughs in the early morning (4:00–7:00). Significant variations in NAI concentrations were observed among the forest types, with mixed broadleaved forest showing the highest concentrations, followed by phoebe forest, Moso bamboo plantation, and Chinese fir plantation. The differences in NAI concentrations among forest types were primarily influenced by plant density at monitor heights, plant diversity, leaf area index, and tree height diversity. Overall, NAI concentrations were positively influenced by temperature and relative humidity and negatively influenced by particulate matter, while the influence of wind speed varied with forest type. Furthermore, our findings indicate that changes in particulate matter had a greater contribution to NAI concentrations compared to meteorological factors. By quantifying the spatial and temporal characteristics of NAIs in typical subtropical forests and their relationships with influencing factors from the perspectives of both stand structure and environment, our study provides new insights into scheduling human forest therapy activities and managing therapeutic forests.

Multi-ecosystem services differently affected by over-canopy and understory nitrogen additions in a typical subtropical forest.

Obtaining a holistic understanding of the impacts of atmospheric nitrogen deposition on multiple ecosystem services of forest is essential for developing comprehensive and sustainable strategies, particularly in heavy N deposition regions such as subtropical China. However, such impacts remain incompletely understood, with most previous studies focus on individual ecosystem function or service via understory N addition experiments. To address this knowledge gap, we quantified the effects of over-canopy and understory N additions on multiple ecosystem services based on a 7-year large-scale field experiment in a typical subtropical forest. Our results showed continued over-canopy N addition with 50 kg ha-1 year-1 over a period of 4-7 years significantly increased plant nutrient retention, but did not affect the services of soil nutrient accumulation, water yield, C sequestration (in plants and soil), or oxygen release. There were trade-offs between the soil and plant on providing the services of nutrient accumulation/retention and C sequestration under over-canopy N addition. However, without uptake and retention of tree canopy, the trade-off between soil and plant were more weaken under the understory N addition with 50 kg ha-1 year-1 , and their relationships were even synergetic under the understory N addition with 25 kg ha-1 year-1 . The results suggest that understory N addition cannot accurately simulate the effects of atmospheric N deposition on multiple services, along with mutual relationships. Interestingly, the services of plant N, P retention, and C sequestration exhibited a synergetic increase under the over-canopy N addition but a decrease under the understory N addition. Our results also found tree layer plays a primary role in providing plant nutrient retention service and is sensitive to atmospheric N deposition. Further studies are needed to investigate the generalized effects of forest canopy processes on alleviating the threaten of global change factors in different forest ecosystems.

Long-term nitrogen and phosphorus addition have stronger negative effects on microbial residual carbon in subsoils than topsoils in subtropical forests.

Highly weathered lowland (sub)tropical forests are widely recognized as nitrogen (N)-rich and phosphorus (P)-poor, and the input of N and P affects soil carbon (C) cycling and storage in these ecosystems. Microbial residual C (MRC) plays a crucial role in regulating soil organic C (SOC) stability in forest soils. However, the effects of long-term N and P addition on soil MRC across different soil layers remain unclear. This study conducted a 12-year N and P addition experiment in two typical subtropical plantation forests dominated by Acacia auriculiformis and Eucalyptus urophylla trees, respectively. We measured plant C input (fine root biomass, fine root C, and litter C), microbial community structure, enzyme activity (C/N/P-cycling enzymes), mineral properties, and MRC. Our results showed that continuous P addition reduced MRC in the subsoil (20-40 cm) of both plantations (A. auriculiformis: 28.44% and E. urophylla: 28.29%), whereas no significant changes occurred in the topsoil (0-20 cm). N addition decreased MRC in the subsoil of E. urophylla (25.44%), but had no significant effects on A. auriculiformis. Combined N and P addition reduced MRC (34.63%) in the subsoil of A. auriculiformis but not in that of E. urophylla. The factors regulating MRC varied across soil layers. In the topsoil (0-10 cm), plant C input (the relative contributions to the total variance was 20%, hereafter) and mineral protection (47.2%) were dominant factors. In the soil layer of 10-20 cm, both microbial characteristics (41.3%) and mineral protection (32.3%) had substantial effects, whereas the deeper layer (20-40 cm) was predominantly regulated by microbial characteristics (37.9%) and mineral protection (18.8%). Understanding differential drivers of MRC across soil depth, particularly in deeper soil layers, is crucial for accurately predicting the stability and storage of SOC and its responses to chronic N enrichment and/or increased P limitation in (sub)tropical forests.

Comparison of distinctive diversities, co-occurrence patterns and potential ecological functions of microbial communities in heterogeneous soil areas in typical subtropical forests, southeast China

Although microbes are crucial in ecological cycling in forest soil, the response of the network structure and ecological functions of these microorganisms to soil heterogeneity remains elusive. Here, we analyzed the composition and co-occurrence network patterns of microorganisms in heterogeneous soil areas and evaluated their potential ecological functions, taking the scenic area (SA) and nature reserve (NR) of Wuyishan National Park as examples. Our results showed that the soil properties of SA and NR could almost be clustered separately, and soil heterogeneity between them has been found. Prokaryotic communities in both areas were also divided and showed higher similarities individually than those of eukaryotes. The prokaryotic community composition in both areas was mainly affected by pH and ammonia, while the eukaryotic community composition was mainly affected by total carbon and the carbon to nitrogen ratio. Both the microbial network and ecological function network in SA were more complex than those of NR, while those of NR were more stable than those of SA. The keystone taxa of pro- and eukaryotes with important ecological functions were enriched in SA and NR, respectively. Moreover, significant discrepancies between SA and NR were found in the prokaryotic functions related to nitrogen cycling, sulfur cycling, and compound degradation. Our study illuminated that the soil heterogeneity in subtropical areas changed microbial community diversity, interaction patterns, and keystone taxa composition, and might further affect their ecological functions.

Climatic drivers of litterfall production and its components in two subtropical forests in South China: A 14-year observation

Understanding the climatic triggers of litterfall production is essential for simulating nutrient cycling in forest ecosystems. However, the mechanisms underlying litterfall dynamics in subtropical forests remain unclear because of the complex canopy phenological strategies. Herein, we explored this by investigating a unique litterfall dataset (including components such as leaves, branches, flowers and fruits, and miscellaneous, etc.) spanning a 14-year observation period (2005–2018) in two typical subtropical evergreen forest ecosystems in South China: a mixed conifer-broadleaf forest (MF) and a monsoon evergreen broadleaved forest (MEBF). We found that the annual total litterfall production, often dominated by leaf litterfall, was significantly higher in MF (10.21 ± 2.09 t∙hm−2) than in MEBF (7.74 ± 2.21 t∙hm−2) (P < 0.01). Generally, forest litterfall production occurred mostly during the wet season and was profoundly correlated with the monthly maximum wind speed (rMF = 0.70, rMEBF = 0.71, P < 0.01), it contributed exceeded 80% to the total litterfall and the leaf and branch components. Whereas during the dry season, the forest litterfall process was dramatically weakened, showing a significant correlation with the mean temperature (rMF = 0.48, rMEBF = 0.68, P < 0.01). Further analysis showed that during the dry season, the cumulative contribution rates of temperature factors to the total litterfall and the leaf and branch components exceeded 60%, indicating a leading role of temperature in triggering forest litterfall processes. Notably, when excluding the typhoon disturbances, it is observed that temperature, alongside wind speed, emerges as a significant determinant of forest litterfall production during the wet season. Those two factors collectively account for almost 60% of the total litterfall and the leaf and branch components. Our findings have potential significance in improving our understanding of carbon and nutrient cycling in subtropical forest ecosystems under climate change conditions.

Distribution pattern and factors affecting litter stocks at the slope-scale in a karst forest in subtropical China

Litter distribution significantly affects nutrient cycling, energy transfer, and various ecosystem services. However, little is known about how microtopography affects litter accumulation and distribution on the forest floor. Distribution characteristics of litter stocks at the slope-scale and their relationship with topography and rock exposure rate (RER) in a typical subtropical karst forest in China were determined by measuring the litter stock and RER of 300 grids of1 × 1 m in a 10 × 120 m square on a limestone hill. The results showed that (1) the average litter stock of karst forest was 691.54 ± 332.60 g∙m-2, among which 65.81% was undecomposed litter; (2) the spatial distribution of litter stocks was uneven with moderate variation, presenting five aggregated areas and four sparse areas on the entire slope; (3) there was a significant negative correlation between RER and litter stock (R = −0.37, P < 0.01), which increased with increasing slope gradient and slope position; and (4) the RER coupled with topography in karst areas directly affected litter accumulation via the redistribution process and indirectly affected the decomposition rate by altering the microenvironment, which reshaped the slope-scale distribution pattern of litter stocks. Our research highlights the importance of the complex slope-scale morphology of karst ecosystems in altering nutrient cycling and energy transfer by mediating litter redistribution, providing new insights into the formation of the high environmental heterogeneity in karst habitats.

Increasing plant species diversity enhances microbial necromass carbon content but does not alter its contribution to soil organic carbon pool in a subtropical forest

Increasing plant species diversity benefits soil organic carbon (SOC) accumulation in forest ecosystems, but how microbial necromass contributes to SOC accumulation in response to plant species diversity and the underlying mechanisms have not been well explored. In the present study, 45 plots covering a natural gradient of plant species diversity as indexed by Shannon's diversity index from 0.15 to 3.57 were selected in a typical subtropical forest with calcareous soil. Soil microbial necromass C including fungal necromass C and bacterial necromass C was estimated using amino sugars, with multiple soil biotic and abiotic variables being determined to explore the mechanisms underlying the effects of plant species diversity on microbial necromass C. Total microbial necromass C contents ranged from 4.28 to 85.7 g kg−1 soil with an average of 36.3 ± 17.6 g kg−1 soil, and accounted for 54.2% ± 22.2% of the SOC pool across the 45 plots. Fungal necromass C had a greater contribution to SOC than bacterial necromass C. Increasing plant species diversity was significantly and positively correlated with microbial necromass C content, but was not significantly related to the contribution of microbial necromass C to the SOC pool. Though increasing plant species diversity enhanced microbial biomass via stimulating C use efficiency and substrate availability, microbial biomass was not significantly linked to microbial necromass C accumulation. Instead, increasing plant species diversity benefited microbial necromass C accumulation via enhancing the mineral protection due to its positive effect on soil exchangeable calcium. Due to the proportionate increase of microbial necromass C and SOC, and the unaltered soil C:N ratio and C:N imbalance between soil organic matter and microbial biomass, the contribution of microbial necromass C to the SOC pool was not affected by plant species diversity. Our study highlights the key role of mineral protection in soil microbial necromass C accumulation, and provides a microbe-dependent linkage between plant species diversity and SOC accumulation.

Multiple Factors Jointly Lead to the Lower Soil Microbial Carbon Use Efficiency of Abies fanjingshanensis in a Typical Subtropical Forest in Southwest China

Abies fanjingshanensis trees are the only remaining Abies species in a type of subtropical forest of southwest China and are in imminent danger. Previous studies suggested that the massive death of Abies was caused by the unbalanced chemometrics and nutrients in the soil. To the best of our knowledge, for the first time, we evaluated the microbial carbon use efficiency (CUE) in the rhizospheric topsoil and subsoil of A. fanjingshanensis, at high elevation, middle elevation, and low elevation as well as investigated their physicochemical indices, soil enzyme activities, bacteria, fungi, and microbial biomass. The results showed that the physicochemical parameters (TP, SOC, AK, AP, MC, TN, NO3-N, NH4-N and cation exchange capacity) of the topsoil were higher than those of the subsoil. Acidobacteria, Proteobacteria, Planctomycetes, and Actinobacteria were the dominant phyla in the two soil layers. Candidatus_Koribacter was the main indicator species in the rhizospheric topsoil and subsoil. The positive correlation in the bacterial co-occurrence networks implied that cooperation was dominant between the bacteria in four soil types, and the same phenomenon was found in the co-occurrence networks of fungi. A structural equation model confirmed that pH was the most important factor affecting microbial CUE in the topsoil and subsoil. We inferred that the microorganisms in the acidic soil environment were forced to consume more energy to maintain cellular pH, while less energy was used for growth. The increased solubility of some toxic metals in the acidic soil affected the microbes, resulting in a lower microbial CUE in the A. fanjingshanensis rhizospheric soil. Our results highlight that pH values in soil mainly affected microbial CUE, and a lower microbial CUE may be another important factor in the death of large numbers of A. fanjingshanensis. Several measures must be carried out to improve the microbial CUE in the rhizospheric soil of A. fanjingshanensis by the department of forest management, such as adding the appropriate biochar and nitrogenous fertilizer.

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.

Environmental variables drive medicinal plant composition and distribution in various forest types of subtropical region in Pakistan

Analyzing the ecological distribution patterns of medicinal plants under the influence of environmental variables can provide important information about how these plants are affected by environmental factors. This analysis can also serve as a conceptual foundation for medicinal plant management strategies. We examined the composition and distribution of medicinal plants as well as how and which environmental factors influence the plant communities in subtropical forest types from Pakistan. 11 forest sites over the course of two years (2020−2021) were sampled, measuring 10 environmental variables and documenting medicinal plants that were present in the study area. Plants data viz.; cover, density, and frequency were recorded from randomly selected sites through quadrat method. Soil parameters i.e., pH, nitrogen, phosphorus, electrical conductance, potassium and soil organic matter were recorded from each site. After calculating importance value index for each species, the data was analyzed utilizing a multivariate ordination approach. The most prevalent family observed during present investigations were Asteraceae followed by Fabaceae and Solanaceae. The life form spectra outcomes exhibited the predominance of the therophyte species, followed by chamaephyte species. The results of the regression analysis exhibit that soil organic matter, nitrogen, potassium, and phosphorus have a positive influence on medicinal plant richness, while electrical conductance (EC) and soil pH have a negative influence. Canonical correspondence analysis revealed that the soil parameters and anthropogenic factors (grazing pressure, land degradation, over-collection) had a significant (p ≤ 0.002) effect on the distribution of medicinal plants. The seasonal pattern of the life cycle revealed its peak during the spring season (41%), followed by monsoon (32%). The leaf size spectra results showed dominance of microphylls, over nanophylls and leptophylls. The results would be significant in establishing plant conservation strategies in Pakistan, especially due to over-exploitation of medicinal plants and the negative effects of climate change make it imperative to protect them on prior basis.

Water use efficiency at multi-time scales and its response to episodic drought and wet periods in a typical subtropical evergreen forest of Southeast China

Water use efficiency (WUE) quantifies the trade-off between carbon gain and water loss. A deep understanding of its relationships with the controlling factors is essential for predicting ecosystem responses to climate change. Our study utilized the eddy covariance measurements to analyze the variability of WUE and its influencing climate factors at the seasonal, monthly, 8-day, daily, and half-hourly scales and during the episodic drought and wet periods at a typical subtropical evergreen forest of Southeast China. The results show that ET is the dominant factor influencing the WUE variability with a relative contribution of 57.56 ± 2.26% for different seasons in this study area. Secondly, WUE responds differently to climatic drivers from monthly to half-hourly scales. Our results show that WUE has a remarkably high sensitivity to soil moisture at multi-time scales (from monthly to half-hourly), and the connection strength increases with increasing time scale. Thirdly, there is no significant relationship between VPD and WUE as time scale expands. In addition, WUEs during the episodic drought period have higher values than those during the episodic wet period on the monthly scales. The half-hourly WUE during the episodic drought period is close to the WUE during the episodic wet period from 8:30 to 19:00 but is higher from 6:00 to 8:00. These results indicate that soil moisture plays an important role on influencing WUE variation at multi-time scales and under extreme climatic conditions.

Stand, plot characteristics, and tree species diversity jointly dominate the recruitment biomass of subtropical forests

Forest dynamics is largely affected by the recruitment of trees. Despite several investigations on forest recruitment biomass (FRB), it is still not well explored how tree species diversity, stand characteristics, climate, and topography regulate FRB. Moreover, there have been limited studies and discussions on FRB in typical subtropical forests in karst areas. We examined the relationship between influencing factors and forest biomass from the perspective of tree recruitment using data from 2334 Chinese National Forest Inventory (CNFI) plots in Guizhou Province. The best predictors within each group were chosen using a stepwise regression model, and the effects of five categories, including tree species diversity, stand factors (e.g., stand density), climatic factors (e.g., mean annual temperature and annual precipitation), topographic factors (e.g., elevation and slope), and plot factors (e.g., canopy density and forest composition) on FRB were determined using structural equation models (SEM). Besides this, the relative importance of each variable among five categories in determining FRB was also quantified by hierarchical partitioning analysis (HPA). Our results indicated that diversity of tree species had the highest relative importance in determining FRB (28.85 %), followed by average DBH (21.06 %), canopy density (10.29 %) based on HPA. While for five categories, stand factors contributed the most to FRB (37.04 %), followed by plot factors (29.01 %), tree species diversity (28.85 %), climatic factors (3.81 %), and topographic factors (1.29 %). Moreover, SEM showed FRB was strongly directly and significantly controlled by tree species diversity (β = 0.63, P < 0.05), while stand factors had a strong direct effect (β = 0.45, P < 0.05) and indirect effect (β = -0.78, P < 0.05). We conclude that niche complementarity is the primary mechanism driving this forest's FRB. Our findings highlight the significance of tree recruitment in maintaining subtropical forests' carbon storage capacity. Furthermore, increasing tree species diversity, particularly by combining conifers and broad-leaved trees, will encourage the recruitment of young trees.

Dynamics of heavy metals in the fine sediments from a subtropical forest headwater stream during a rainy season

ABSTRACT Headwater streams, as the capillaries of rivers, largely control the functional integrity of the entire river network. Headwater stream sediments, especially fine sediments, are essential ecosystem components and dominant reservoirs for heavy metals, with these reservoirs significantly affecting their cycling in streams. However, the dynamics of heavy metals in headwater stream sediments are not well assessed in the literature. Here, we assessed the temporal and spatial storage dynamics of several common heavy metals (i.e., Mn, Zn, Cu, Cr, Cd, and Pb) in the fine sediments from a typical subtropical forest headwater stream during the rainy season of 2021 (Mar–Aug). We found that (1) the mean storage of fine sediment of heavy metals was ordered Mn (21.57 g m−2) > Pb (2.53 g m−2) > Zn (2.27 g m−2) > Cr (0.66 g m−2) > Cu (0.12 g m−2) > Cd (0.006 g m−2); (2) their storage had a transient increase during the rainy season, followed by a decrease, but storage of both Zn and Cr was unevenly distributed from stream source to mouth; and (3) the key factors controlling heavy metal storage were water alkalinity, temperature, and pH, as well as monthly precipitation. These findings suggest that forest headwater streams can store heavy metals, but the amounts vary across temporal and spatial scales. Our results help us better understand the role of headwater streams in storing and transferring materials in forest ecosystems.

Topographic Factors Drive Short-Term Understory Revegetation in Burned Areas

Fire is a disturbance factor in forest ecosystems which has an important influence on vegetation succession. In order to reveal the natural recovery characteristics of the understory vegetation and its response to topographic factors (elevation, slope position, aspect and slope) after fire in subtropical forests, a typical subtropical forest (Xide County, China) was selected as the study area. Based on quadrat surveys and correlation analyses, the main results of this study were as follows. (1) Within four months after a fire, a total of 71 species, 52 genera and 20 families of understory vegetation species were increased, representing a rapid recovery after a fire. The total number of genera and species of understory vegetation increased by 157% and 209%, respectively. (2) Different burning intensities had different effects on the recovery of vegetation species diversity. The change in Margalef richness index was the most significant, increasing by 5.44 and 5.16 in lightly and severely burned areas, respectively. (3) The vegetation community could be divided into six types after restoration. (4) Elevation and slope were significantly correlated with the diversity and distribution characteristics of understory vegetation in the burned area. This study is useful to understand the characteristics of the short-term natural recovery of understory vegetation in subtropical forests after fire, to identify the relationship between vegetation and topographic factors in burned areas, and to provide a scientific basis for vegetation recovery.

Simulated net ecosystem productivity of subtropical forests and its response to climate change in Zhejiang Province, China.

Net ecosystem productivity (NEP) is an important index that indicates the carbon sequestration capacity of forest ecosystems. However, the effect of climate change on the spatiotemporal variability in NEP is still unclear. Using the Integrated Terrestrial Ecosystem Carbon-budget (InTEC) model, this study takes the typical subtropical forests in the Zhejiang Province, China as an example, simulated the spatiotemporal patterns of forest NEP from 1979 to 2079 based on historically observed climate data (1979-2015) and data from three representative concentration pathway (RCP) scenarios (RCP2.6, RCP4.5, and RCP8.5) provided by the Coupled Model Intercomparison Project 5 (CMIP5). We analyzed the responses of NEP at different forest age classes to the variation in meteorological factors. The NEP of Zhejiang's forests decreased from 1979 to 1985 and then increased from 1985 to 2015, with an annual increase rate of 9.66 g C·m-2·yr-1 and a cumulative NEP of 364.99 Tg·C. Forest NEP decreased from 2016 to 2079; however, the cumulative NEP continued to increase. The simulated cumulative NEP under the RCP2.6, RCP4.5, and RCP8.5 scenarios was 750 Tg·C, 866 Tg·C, and 958 Tg·C, respectively, at the end of 2079. Partial correlation analysis between forest NEP at different age stages and meteorological factors showed that temperature is the key climatic factor that affects the carbon sequestration capacity of juvenile forests (1979-1999), while precipitation is the key climatic factor that affects middle-aged forests (2000-2015) and mature forests (2016-2079). Adopting appropriate management strategies for forests, such as selective cutting of different ages, is critical for the subtropical forests to adapt to climate change and maintain their high carbon sink capacity.