Forest In Central China Research Articles

Assessing the potential of species loss caused by deforestation in a mature subtropical broadleaf forest in central China

Deforestation is a major type of land use change to accommodate growing population, especially in developing countries. The risk of diversity loss due to habitat loss can be estimated using the species-area relationship based on abundance of each species. However, deforestation often occurs before there is any understanding of the impact of deforestation on tree diversity. Here, we assessed the potential effect of forest habitat destruction on the loss of species richness in a mature subtropical broadleaf forest in central China. We surveyed and constructed the species-area relationship for 54 400 m2 plots, and simulated habitat loss scenarios by randomly and aggregately sampling plots. Rank-abundance of the 21 tree species was best fitted by the Zipf-Mandelbrot model, and our sample size was sufficient by the criterion of Hill numbers at orders q = 0, 1, and 2. We found that the number of species lost due to habitat loss was well predicted by the random placement species loss-area loss curve, and was lower than that due to aggregated habitat destruction by less than one species. The probability of losing one species reached 40% when losing 16 plots by aggregated sampling, 10 plots fewer than that by random sampling. Moreover, the probability of losing two species was 10–22 % higher by aggregately sampling than that by randomly sampling when losing 17 – 34 plots (0.68 – 1.36 ha). Considering that aggregated deforestation is common in reality, the results imply that the number of tree species lost due to deforestation could be higher than the theoretical estimation. Our study suggests the importance of assessing the impact of deforestation on tree diversity before selective logging in subtropical forests.

Additive effects of N addition and changing precipitation on soil respiration in a climate transitional forest

Global carbon (C) cycling can be greatly affected by the increasing nitrogen (N) deposition and changing precipitation regimes in terrestrial ecosystems. Nevertheless, whether N enrichment and precipitation variability interactively or additively influence soil C release during a long-term period remain largely elusive. Here, a seven-year (2016–2022) field experiment was conducted to explore the responses of soil respiration to N addition, decreased and increased precipitation in a subtropical-warm temperate forest in Central China. The results showed soil respiration was elevated by 11.0%, 12.2%, and 12.3% under N addition, decreased precipitation, and increased precipitation, respectively, primarily originating from the enhancements in plant root respiration. Increased precipitation led to a temporal increase in soil respiration over the experimental period, mostly due to the elevating understory richness and reducing soil C/N ratio. There were additive effects between N addition and changing precipitation on soil respiration observed in our study, suggesting that the impacts of concurrent global change drivers can be predicted from their respective influences. These findings indicated that global change may intensify soil C release from climate transition forests, thus helping to accurately assess the climate change–C feedback in natural ecosystems.

Environmental and biological controls on the interannual variations of evapotranspiration in a natural oak forest

Investigating the interannual variation (IAV) of evapotranspiration (ET) is important for understanding the water cycle in forest ecosystems. However, there has been little research on interannual variability of ET in warm-temperate forests under a changing environment. In this study, 6-year eddy covariance data (2017–2022) in a warm-temperate natural oak forest in central China were used to examine the IAV of ET and its driving factors. The mean and standard deviation of annual ET from 2017 to 2022 were 610 and 122.8 mm (with a coefficient of variation (CV) of 20 %), respectively. The annual ET did not exhibit an obvious trend over the six years with a larger IAV. Environmental factors had a greater impact on the IAV of annual ET compared to the biological factors, with reference crop evapotranspiration (ET0) exerting the most significant influence on ET. The IAV of monthly ET was primarily controlled by precipitation during the early growing season, but it changed to be controlled by radiation and temperature (i.e., atmospheric evaporation demand) during the mid-growing season. We concluded that atmospheric evaporation demand rather than water supply dominated the IAV of annual ET in the warm-temperate natural oak forest in central China, and occasional seasonal drought only affected the ET in a short time-scale (no more than three years). There is a need to measure three components of ET (transpiration, canopy interception evaporation and soil evaporation) in order to better understand the IAV of ET and the underlying biophysical control mechanisms.

Eleven-Year Canopy Nitrogen Addition Enhances the Uptake of Phosphorus by Plants and Accelerates Its Depletion in Soil

Soil phosphorus (P) is a critical factor that limits plant productivity. Enhanced nitrogen (N) deposition has the potential to modify P transformation and availability, thereby potentially affecting the long-term productivity of forests. Here, we conducted an 11-year-long field experiment to simulate N deposition by adding N to the forest canopy in a N-limited northern subtropical forest in central China and assessed the changes in soil organic P mineralization, P fractions, microbial biomass P content, phosphatase activity, and plant P content under N deposition. Our objective was to establish a theoretical framework for addressing the P supply and sustaining plant productivity in soils with low P availability, particularly in a changing global setting. The results demonstrated a substantial reduction in the levels of total, organic, and available P owing to the canopy addition of N. Furthermore, there was a marked decrease in the proportion of organic P in the total P pool. However, no substantial changes were observed in the soil inorganic P content or the proportion of inorganic P within the total P across different treatments. Canopy N addition significantly enhanced the microbial biomass P content, phosphatase activity, and organic P mineralization rate, suggesting that in soils with limited P availability, the primary source of P was derived from the mineralization of organic P. Canopy N addition substantially increased the P content in leaves and fine roots while concurrently causing a considerable decrease in the N:P ratio. This indicates that N deposition increases P demand in plants. Correlation analysis revealed a significant negative association among the total, organic, and available P levels in the soil and plant P concentrations (p < 0.05). This suggests that the primary cause of the reduced fractions of P was plant uptake following canopy N addition. Various studies have demonstrated that N deposition induces an augmented P demand in plants and expedites the utilization of available P. A substantial reduction in potentially accessible soil P caused by N deposition is likely to exacerbate regional P depletion, thereby exerting adverse impacts on forest ecosystem productivity.

Changes in throughfall induced by low‐severity fire in a coniferous and broadleaved mixed forest

AbstractForest fires, as a prominent main natural disturbance in forest ecosystems, have a critical influence on hydrological processes through the alteration of forest stand structure. Despite throughfall (TF) being the dominant component of rainfall partitioning in forest ecosystems, limited research has thoroughly investigated how forest fire affects the spatial and temporal patterns of TF. This study examined TF patterns in response to low‐severity fire in a coniferous and broadleaved mixed forest in Central China. Our results showed that TF exhibited an increasing trend regarding gross rainfall, and its variability decreased with increasing gross rainfall regardless of burning states. Low‐severity fire induced a considerable enhancement in TF rate, enhancing it from 71.1% to 77.9%, and increased the coefficient of variation from 19.4% to 23.9%. Additionally, TF temporal patterns of burned forest were found to be marginally less stable than unburned forest. We used a sample size of 16 and 25 for unburned and burned forests, respectively, to accurately and reliably acquire TF with an accepted error of 10% and a 95% confidence level. Our findings suggest that low‐severity fire can enhance TF yield and variability, decrease temporal stability. Moreover, we recommend increasing the sample size of burned forest to ensure the collection of representative TF. These findings contribute to a deeper understanding and improve the prediction of forest hydrological processes in response to fire and optimize the sampling scheme of TF.

Differential effects of forest-floor litter and roots on soil organic carbon formation in a temperate oak forest

Both forest-floor litter and roots are known to be involved in the formation of soil organic carbon (SOC), but it remains elusive how the two contrasting types of plant carbon sources would differentiate in their effects on SOC. Here we used field-based soil mesocosms to study the responses of SOC components (i.e. mineral-associated organic C [MAOC], particulate organic C [POC], and dissolved organic C [DOC]) and pathways of production (i.e. plant-derived vs. microbial-derived C compounds) to multi-year treatments of forest-floor litter and roots in a temperate oak forest in central China. After eight years, the concentrations of both POC and MAOC were significantly higher in plots with presence of forest-floor litter than in those exclusive of both forest-floor litter and roots at the soil depth of 0–5 cm; roots predominantly affected POC but not the total SOC. The contributions from the forest-floor litter and roots to total SOC and its components were all non-additive; the plots with co-occurrence of forest-floor litter and roots had 49.7% less total SOC, 36.6% less POC, and 54.7% less MAOC, than the sum of the plots with only forest-floor litter or roots, at the soil depth of 0–5 cm. The concentrations of both lignin phenols and microbial biomass carbon (MBC) were significantly higher in plots with only forest-floor litter than in those exclusive of both forest-floor litter and roots down to a depth of 20 cm, and the plant-derived neutral sugars were significantly higher in plots with only forest-floor litter than in those exclusive of both forest-floor litter and roots at the soil depth of 0–10 cm. In contrast, the concentration of lignin phenols was significantly higher in plots with only roots than in those exclusive of both forest-floor litter and roots below the depth of 5 cm, and the concentrations of MBC and microbial-derived neutral sugars were higher in plots with only roots than in those exclusive of both forest-floor litter and roots at the soil depth of 0–10 cm. The plots with co-occurrence of forest-floor litter and roots had a significantly higher concentration of lignin phenols at all soil depths investigated, as well as higher concentrations of the plant- and microbial-derived neutral sugars and MBC at the soil depth of 0–5 cm, than those exclusive of both forest-floor litter and roots; the effects were all non-additive such that the plots with co-occurrence of forest-floor litter and roots had less plant- and microbial-derived C compounds than the sum of the values between the plots with only forest-floor litter and the plots with only roots. The contributions of amino sugars to SOC did not significantly respond to either the treatments with forest-floor litter and roots or soil depth, albeit variable MBC concentrations. In addition, our results show that the microbial-derived neutral sugars were positively correlated with plant-derived neutral sugars. Findings in this study provide empirical evidence on the primacy of forest-floor litter in SOC pool at the soil depth of 0–5 cm, and demonstrate the differential roles and contributions between forest-floor litter and roots in the pathways of SOC formation and spatial differentiation of carbon compounds.

Post‐fire soil extracellular enzyme activities in subtropical–warm temperate climate transitional forests

AbstractWildfire impacts on soil microbial community structure and functional activity have attracted a growing attention because of the higher sensitivity of soil microbes to wildfire. Soil extracellular enzymes play pivotal roles in biogeochemical processes, such as mediating carbon (C) and nutrient cycling. However, little is known about how post‐fire changes in soil biogeochemical properties and microbial community composition affect soil extracellular enzyme activities. This study explored the responses and regulating factors of C‐, nitrogen‐(N), and phosphorus‐(P) acquiring extracellular enzyme activities across a wildfire chronosequence (i.e., 1, 6, 13, and 50 years after fire) in subtropical–temperate ecotonal forests in Central China. The activities of C‐(β‐glucosidase), N‐(N‐acetylglucosaminidase), and P‐(acid phosphatase) acquiring enzymes declined with time post‐fire, with the highest values one‐year post‐fire. The response of C‐acquiring enzyme activity to fire was positively correlated with bacterial biomass, suggesting that microbial compositions were related to post‐fire changes in extracellular enzyme decomposition. The response of N‐acquiring enzyme activity to fire was positively correlated with soil P availability, while P‐acquiring enzyme activity was positively correlated with soil N availability. Overall, soil extracellular enzyme activity declined with time post‐fire, suggesting wildfire may reduce microbial demand for nutrients over time. Future research is needed to elucidate fire impacts on microbial processes for nutrient‐microbial‐enzyme linkages.

Soil moisture shapes the environmental control mechanism on canopy conductance in a natural oak forest.

Canopy conductance (gc) is an important biophysical parameter closely related to ecosystem energy partitioning and carbon sequestration, which can be used to judge drought effect on forest ecosystems. It is very important to explore how soil moisture change affects the environmental control mechanism of gc, especially in natural oak forests in Central China where frequent extreme precipitation (P) and drought will occur in a context of climate change. In this study, variations of gc and its environmental control mechanisms in a warm-temperate forest over three consecutive years under different hydroclimatic conditions were examined by using eddy-covariance technique. Results showed that the averaged gc in the three growing seasons were 11.2, 11.3 and 7.8 mms-1, respectively, with a CV of 19.7 %. The lowest gc occurred in the year with the lowest P. Using three years of data, we found that vapor pressure deficit (VPD) exhibited the dominate effect on gc, both diffuse photosynthetically active radiation (PARdif) and air temperature (Ta) were positively correlated with gc. When relative extractable water content (REW) was larger than 0.4, however, inhibiting effect of high VPD on gc disappeared and the effect of direct photosynthetically active radiation (PARdir) on gc was larger compared to PARdif. When REW was <0.1, the positive relationship between Ta and gc became negative. Our results indicated that soil moisture ultimately shapes the environmental control mechanism of gc in a natural oak forest.

The species richness pattern and its component dependence of two typical secondary forests in central China

Secondary forests dominated by Masson pine (Pinus massoniana, MPF) and Chinese fir (Cunninghamia lanceolata, CFF) are widespread in the subtropical areas of China, whereas their biodiversity patterns receive far less attention than biomass patterns. This study aimed to gain insights into the species richness (SR) patterns of the typical natural secondary MPF and CFF in central China, with the old-growth zonal vegetation (evergreen-deciduous broadleaved mixed forest, EDBMF) used as reference. We established 45 plots across the three forest types and surveyed all the stems with a diameter at breast height ≥ 1 cm in each plot. Specifically, we compared the local (plot-level) and rarefied SR among the three forest types; at the metacommunity level, we examined the dependence of the rarefied SR difference between MPF and CFF on the number of individuals (N), species abundance distribution (SAD), and conspecific spatial aggregation (CSA); and we also assessed how local SR changed with N and species dominance within MPF and CFF. Our results showed that MPF had local and rarefied SR levels higher than CFF and closer to EDBMF. Both N and SAD had positive contributions to the high rarefied SR level of MPF (relative to CFF), while CSA had negative contributions. The N contributions were significant and dominated at most scale, the SAD contributions were considerable and significant at large scales, and the CSA contributions were significant at small and intermediate scales. Moreover, within MPF and CFF, local SR changed with N more generally than with species dominance. In conclusion, at both local and metacommunity level, MPF was generally more species-rich than CFF, and this difference remarkably depended on N, suggesting that the habitat conditions of CFF may be stressful for species persistence. Our findings also have several implications for biodiversity research and ecosystem management.

Estimating the self-thinning boundary line for oak mixed forests in central China by using stochastic frontier analysis and a proposed variable density model.

A suitable self‐thinning model is fundamental to effective density control and management. Using data from 265 plot measurements in oak mixed forests in central China, we demonstrated how to estimate a suitable self‐thinning line for oak mixed forests from three aspects, i.e., self‐thinning models (Reineke's model and the variable density model), statistical methods (quantile regression and stochastic frontier analysis), and the variables affecting stands (topography and stand structure factors). The proposed variable density model, which is based on the quadratic mean diameter and dominant height, exhibited a better goodness of fit and biological relevance than Reineke's model for modeling the self‐thinning line for mixed oak forests. In addition, the normal‐truncated normal stochastic frontier model was superior to quantile regression for modeling the self‐thinning line. The altitude, Simpson index, and dominant height–diameter ratio (Hd/D) also had significant effects on the density of mixed forests. Overall, a variable density self‐thinning model may be constructed using stochastic frontier analysis for oak mixed forests while considering the effects of site quality and stand structure on density. The findings may contribute to a more accurate density management map for mixed forests.

Different-Sized Vessels of Quercus variabilis Blume Respond Diversely to Six-Year Canopy and Understory N Addition in a Warm-Temperate Transitional Zone

Nitrogen is a necessary macroelement in plant growth and is usually considered a limiting factor in many forest ecosystems. Increasing N deposition has been reported to affect tree growth. However, the effects still remain controversial due to variable N fertilization methods used. In order to study the realistic responses of tree growth to increasing N deposition, we investigated effects of canopy and understory N addition on tree-ring growth and vessel traits of Quercus variabilis Blume. Since 2013, 50 kg N ha−1 year was applied monthly from April to December to either the canopy (CN) or understory (UN) of trees in a warm-temperate forest in Central China. During 2013–2018, tree-ring growth and vessel-related traits (mean vessel area, theoretical xylem hydraulic conductivity (KH), relative ratio of KH, etc.) were analyzed. Tree rings were negatively impacted by both CN and UN treatments, but only the effect of UN was significant. Neither CN nor UN significantly impacted the detected vessel traits. However, some diverging influencing trends were still showed in some vessel traits. Both CN and UN treatments positively affected the percentage of annual total vessel area and vessel density, with the effect of UN on vessel density being more severe. All the detected vessel traits of the large vessels formed at the beginning of the tree-ring responded positively to CN, whereas the opposite response to UN was showed on mean vessel area and the relative ratio of KH. All these diverging responses in different vessel traits likely reflected the compensation and trade-off between maximizing growth and adapting to CN and UN treatments. Six-year long N addition negatively and positively affected tree-ring growth and vessel traits of Q. variabilis in Central China, respectively. UN treatment could not fully simulate the real effect on tree growth, especially on the hydraulic architecture.

Environmental and stomatal control on evapotranspiration in a natural oak forest

AbstractEvapotranspiration (ET) is an important component in water budgets in forest ecosystems, and the associated latent heat flux has important implications for regional climate. However, biophysical control mechanisms on variations in ecosystem ET on intra‐annual scales may have year‐to‐year variations in different years with contrasting precipitation and soil water contents. Related research is lacking in natural oak forests in Central China. Based on eddy‐covariance technique, we investigated variations of ET and controlling factors in a natural oak forest in central China over three years (2017–2019) with contrasting soil water contents (SWCs) and precipitation. The ET of the oak forest was mainly affected by surface conductance (gs) in growing seasons, and the restriction from stoma on ET was aggravated in dry year. High vapour pressure deficit (VPD) promoted gs and ET in wet year but decreased gs and ET in dry year. There was a positive relationship between shallow soil water contents and gs in dry year but no relationship between the two variables in wet year. The ET were 738.1, 750.6 and 513.1 mm, respectively, for 2017–2019, with a coefficient of variation (CV) of 20%, while the corresponding precipitation (P) were 1239.4, 855.8 and 645.6 mm, with a CV of 33%. Annual ET was the lowest in the year with the lowest P, SWC and gs. Short‐term droughts (periods with relative extractable water content less than 0.1 for less than 15 days) only constrained gs and ET in short timescales (e.g., hours). Long‐term droughts (periods with relative extractable water content less than 0.1 for more than 15 days) evidently decreased the gs, thus reducing the ET and affecting energy partitioning in the corresponding period. ET/P were 0.6, 0.88 and 0.79 in 2017–2019, respectively, indicating that ET consumed most of the rain. The ratios of latent heat flux (LE) to net radiation (Rn) were 0.86, 0.88 and 0.6 during growing seasons in 2017–2019, respectively. Our study suggested that the natural oak forest has high ET rate and soil moisture affects the variations of ET.

Changes in carbon inputs affect soil respiration and its temperature sensitivity in a broadleaved forest in central China

Soil respiration, as an important process in global carbon cycle, is drastically stimulated by warming. Changing plant carbon inputs caused by intensified human disturbances and climate change can complexly influence soil respiration and its temperature sensitivity (Q10). Although a number of experiments have been established in the world to explore the effects of changing carbon inputs, it remains a challenge to predict the direction and magnitude of changes in soil respiration and Q10 due to high spatiotemporal variability of climatic zone and ecosystem. Therefore, we conducted a field manipulative experiment to examine the impact of litter removal and root exclusion on soil respiration and Q10 in a deciduous broad-leaved forest in transition zone between subtropical and warm temperate zone. The results showed that litter removal significantly reduced soil respiration, microbial biomass carbon, soil organic carbon, total nitrogen and soil moisture. In addition, litter removal altered soil microbial community composition, expressed in increase of Gram-positive / Gram-negative bacteria ratio and percentage of actinomycetes, and enhanced microbial physiological stress index. Root exclusion significantly increased Q10 by elevating soil C/N ratio, which supporting “C quality-temperature” hypothesis. The study indicates that root exclusion may accelerate soil respiration under warming. Our study provides insights to improve carbon cycling models and accurately predict the consequences of climate change.

Distribution of mercury in foliage, litter and soil profiles in forests of the Qinling Mountains, China

Forest ecosystems have been confirmed to be a sink of the global mercury (Hg) in the biogeochemical cycle. However, few studies have investigated the distribution of Hg in forest ecosystems on a regional scale in China. This work aimed to investigate the concentrations, distribution and influential factors of Hg in the Qinling Mountains forests in central China. Foliage, litter and soil profile samples were collected at 24 sampling sites across the Qinling Mountains forests. The results of the present study showed that the concentrations of Hg in foliage, litter, organic soils and mineral soils were maintained at relatively low levels compared with those in subtropical forests of Southwest China. The average Hg concentrations followed the order litter (74 ± 34 ng g−1) > organic soil (71 ± 37 ng g−1) > mineral soil (34 ± 21 ng g−1) > foliage (31 ± 15 ng g−1). Mercury in foliage showed no obvious spatial pattern, likely due to differences in tree species and ages across the sampling sites. Higher concentrations of Hg in litter were observed on the southern slope (low altitude), while the distribution of Hg in organic soils was the opposite. Both the tree species and environmental parameters (altitude, temperature and precipitation) controlled the Hg concentrations in litter by regulating the decomposition rate of the litter. There were significantly positive correlations between the Hg concentrations and soil organic carbon, nitrogen and sulfur in all soil layers, indicating that organic matter has a high geochemical affinity for Hg in soils. Because of the lower turnover rate and the higher accumulation of organic matter in high altitude and low temperature areas, Hg loss from biogeochemical cycling processes was effectively reduced. The spatial distribution of Hg in forests soil can be shaped by the distribution of organic matter at the regional scale.

Nutrient resorption strategies of three oak tree species in response to interannual climate variability

BackgroundNutrient resorption is critical for plants toward balancing their nutritional requirements and adapting to environmental variabilities, which further impacts litter quality and nutrient cycling. However, the interannual variability of nutrient resorption under climate change remains unclear.MethodsWe investigated the five-year nutrient resorption efficiencies (NuRE, %) of 14 elements in three deciduous oak tree species (Quercus aliena var. acuteserrata, Q. glandulifera, and Q. variabilis) in a warm-temperate forest of Central China and assessed their relationships with interannual climate and soil factors.ResultsNutrient resorption did not differ between species but varied significantly between different years. For each year, N, P, S, K, C, Mg, and Zn were preferentially resorbed in all of the oak species in contrast to Ca, Na, Mn, Ba, Al, Fe, Cu, which were to some extent discriminated. Among the 14 elements, the NuRE of C, N, P, S, Ca, and Mg was more sensitive to interannual climate variations in the three oak species. The carbon resorption efficiency was significantly increased during the driest year of the study (2014); N resorption efficiency was reduced with temperature; whereas N and P resorption efficiency initially decreased and then increased with precipitation. Moreover, the elements with higher NuREs typically had lower coefficient of variation (CV) in all three oak species.ConclusionsDifferent oak species exhibited analogous nutrient conservation strategies in response to annual climate variabilities, and interannual climate variations strongly impacted plant nutrient resorption. Deciduous plants may establish a tradeoff mechanism to rebalance somatic nutrients for regrowth at the end of the growing season.

Effects of biodiversity, stand factors and functional identity on biomass and productivity during the restoration of subtropical forests in Central China

AbstractBiodiversity is found to have a significant promotion effect on ecosystem functions in manipulation experiments on grassland communities. However, its relative role compared with stand factors or functional identity is still controversial in natural forests. Here, we examined their relative effects on biomass and productivity during forest restoration. We investigated stand biomass and productivity for 24 plots (600 m2) across restoration stages in the subtropical forests of Mt. Shennongjia, Central China. We measured five key functional traits and calculated functional diversity (functional richness, evenness and dispersion) and community-weighted mean of traits. We used general linear models, variation partitioning methods to test the relative importance of stand factors (density, stand age, maximum height, etc.), functional identity, species and functional diversity on biomass and productivity. Our results illustrated that stand biomass and productivity increased significantly as forest restoration, and that community species richness increased, while functional dispersion decreased significantly. Variation partitioning analyses showed that diversity had no significant pure effects on biomass and productivity. However, diversity may affect biomass and productivity through the joint effect with stand factors and functional identity. Overall, we found that stand factors had the strongest effect on biomass and productivity, while functional identity significantly affects productivity but not biomass, suggesting that modulating stand structure and species identity are effective ways to enhance forest carbon storage and sequestrations potential in forest management.

Drought Affected Ecosystem Water Use Efficiency of a Natural Oak Forest in Central China

Global climate models project more frequent drought events in Central China. However, the effect of seasonal drought on ecosystem water use efficiency (WUE) and water regulation strategy in Central China’s natural forests is poorly understood. This study investigated variations in WUE associated with drought in a natural oak (Quercus aliena) forest in Central China from 2017 to 2020 at several timescales based on continuous CO2 and water vapor flux measurements. Results showed that the 4-year mean gross ecosystem production (GEP), evapotranspiration (ET) and WUE of the natural oak forest was 1613.2 ± 116 g Cm−2, 637.8 ± 163.3 mm and 2.6 ± 0.68 g Ckg−1 H2O, with a coefficient of variation (CV) of 7.2%, 25.6% and 26.4%, respectively. The inter-annual variation in WUE was large, primarily due to the variation in ET caused by seasonal drought. Drought increased WUE distinctly in summer and decreased it slightly in autumn. During summer drought, surface conductance (gs) usually decreased with an increase in VPD, but the ratios of stomatal sensitivity (m) and reference conductance (gsref) were 0.21 and 0.3 molm−2s−1ln(kPa)−1 in the summer of 2019 and 2020. Strong drought can also affect ecosystem WUE and water regulation strategy in the next year. Decrease in precipitation in spring increased annual WUE. These results suggested that drought in different seasons had different effects on ecosystem WUE. Overall, our findings suggest that the natural oak forest did not reduce GEP by increasing WUE (i.e., reducing ET) under spring and summer drought, which could be due to its typical anisohydric characteristics, although it can also reduce stomatal opening during long-term drought.

The Effects of Biotic and Abiotic Factors on the Community Dynamics in a Mountain Subtropical Forest

From supporting wood production to mitigating climate change, forest ecosystem services are crucial to the well-being of humans. Understanding the mechanisms that drive forest dynamics can help us infer how to maintain forest ecosystem services and how to improve predictions of forest dynamics under climate change. Despite the growing number of studies exploring above ground biomass (AGB) dynamics, questions of dynamics in biodiversity and in number of individuals still remain unclear. Here, we first explored the patterns of community dynamics in different aspects (i.e., AGB, density and biodiversity) based on short-term (five years) data from a 25-ha permanent plot in a subtropical forest in central China. Second, we examined the relationships between community dynamics and biodiversity and functional traits. Third, we identified the key factors affecting different aspects of community dynamics and quantified their relative contributions. We found that in the short term (five years), net above ground biomass change (ΔAGB) and biodiversity increased, while the number of individuals decreased. Resource-conservation traits enhanced the ΔAGB and reduced the loss in individuals, while the resource-acquisition traits had the opposite effect. Furthermore, the community structure contributed the most to ΔAGB; topographic variables and soil nutrients contributed the most to the number of individuals; demographic process contributed the most to biodiversity. Our results indicate that biotic factors mostly affected the community dynamics of ΔAGB and biodiversity, while the number of individuals was mainly shaped by abiotic factors. Our work highlighted that the factors influencing different aspects of community dynamics vary. Therefore, forest management practices should be formulated according to a specific protective purpose.

Nitrogen deposition and increased precipitation interact to affect fine root production and biomass in a temperate forest: Implications for carbon cycling

Fine roots connect belowground and aboveground systems and help regulate the carbon balance of terrestrial ecosystems by providing nutrients and water for plants. To evaluate the effects of atmospheric nitrogen (N) deposition and increased precipitation on fine root production and standing biomass in a temperate deciduous forest in central China, we conducted a 6-year experiment. From 2013 to 2018, we applied N (25 kg N ha−1 yr−1) and water (336 mm, 30% of the ambient annual precipitation) above the forest canopy, and we quantified fine root production and biomass in 2017 and 2018. At 0–10 cm soil depth, the statistical interaction between addition of N and water was not significant in terms of fine root production or biomass. At 0–10 cm soil depth, N addition significantly increased fine root production by 18.1%, but did not affect fine root biomass. Water addition significantly increased fine root production and biomass by 13.6 and 17.0%, respectively. Both N and water addition had significant direct positive effects on fine root production, and water addition had indirect positive effects on fine root biomass through decreasing soil NO3− concentration. At 10–30 cm soil depth, the statistical interaction between N addition and water addition was significant in terms of both fine root production and biomass, i.e., the positive effect of N addition was reduced by water addition, and vice versa. These findings indicate that fine roots and therefore belowground carbon storage may have complex responses to increases in atmospheric N deposition and changes in precipitation predicted for the future. The findings also suggest that results obtained from experiments that consider only one independent variable (e.g., N input or water input) and only one soil depth should be interpreted with caution.

Relative influence of environmental, stand factors and functional traits on allocation of forest productivity during the restoration of subtropical forests in central China

Many studies have examined plant biomass allocation among leaf, stem and root organs in relation to environmental and ontogenetic factors. But the drivers for allocation of stand-level productivity across restoration stages, and the potential role of functional traits, are still poorly understood. Here we investigated 24 plots across four restoration stages in subtropical forests on Mt. Shennongjia, central China. We examined the relative effects of environment (slope, aspect and canopy openness), stand factors (forest height, total basal area, stand density and age), and community-weighted mean (CWM) of key functional traits (wood density and specific leaf area), on the allocation of forest productivity to stems, leaves and roots. The results showed that forest productivity, and the stem fraction of stand productivity, both increased during forest restoration; while leaf and root productivity fractions showed an overall decrease. A higher fraction of productivity was allocated to leaves and roots while lower fraction was allocated to stems towards harsher site condition. Stem fraction was positively related to stand age and forest height, whereas root fraction showed a converse pattern. None of the stem, leaf or root fraction was significantly correlated with the proxies of competition (stand density and total basal area). As for the effect of functional traits, stem productivity fraction was negatively related to the CWM of wood density, while leaf fraction was positively related, which can be explained by the optimal theory and growth strategy together. However, CWM of specific leaf area has no significant effect on productivity allocation. Multivariate analyses showed that topography (slope) and stand age were main drivers for the productivity allocation to woody organs (stem and root), and wood density was also important for stem productivity fraction. Meanwhile, leaf fraction is mainly affected by wood density and aspect. Our results suggest that, in addition to environmental and stand factors, functional traits (e.g. wood density) are also important in affecting the allocation of stand productivity. However, there are still many equivocal results about the influence of functional traits. Allocation of productivity among organs, and key functional traits (e.g. wood density and specific leaf area), are both indispensable parameters in ecosystem processes models. Further studies are needed to explore the mechanisms how functional traits and stand factors affect productivity allocation, to better predict forest carbon balance under future global change, and to better guide forest management to increase ecosystem services and carbon sink.