Plantations In Subtropical China Research Articles

Introducing N2-Fixing Tree Species into Eucalyptus Plantation in Subtropical China Alleviated Carbon and Nitrogen Constraints within Soil Aggregates

Soil extracellular enzymatic activity (EEA) and extracellular enzymatic stoichiometry (EES) within aggregates indicate variations in soil-nutrient effectiveness and the nutrient requirements of microorganisms. However, the responses of soil EEA and EES after introducing N2-fixing tree species into Eucalyptus plantations are poorly understood. Therefore, we examined soils from a 15-year-old pure Eucalyptus urophylla plantation (PP) and mixed E. urophylla and Acacia mangium plantation (MP) based on the theory of EEA and EES at the aggregate scale. Aggregates were separated into four fractions using a dry-sieving procedure: >2, 1–2, 0.25–1, and <0.25 mm. We measured the EEA of soil carbon (C)-, nitrogen (N)-, and phosphorus (P)-acquiring enzymes, and examined potential factors (soil physicochemical properties, microbial biomass, and litterfall [LF]) that may influence EEA and EES. Significantly higher (p < 0.05) EEA levels in all aggregates were found in MP than in PP. The average natural logarithmic ratio of C-, N-, and P-acquiring enzyme activities in our study was 1.44:1.21:1, which deviated from the global mean ratio of 1:1:1 and implied that soil microbes were limited by C and N. Moreover, the enzyme C:N ratio (EC:N), C:P ratio (EC:P), and vector length (VL) were markedly lower (p < 0.05) in bulk soil and most aggregates in MP compared to PP, suggesting that C limitation was more serious in PP than in MP. Furthermore, while the vector angle (VA) of bulk soil and four aggregate sizes were all <45° in both the PP and the MP, they were markedly higher (p < 0.05) in bulk soil and >2 mm aggregate in MP than in PP. This indicated that mixing N2-fixing species with Eucalyptus alleviated but did not eliminate N limitation. Our study also found that nitrate nitrogen (NO3−-N), total nitrogen (TN), and microbial biomass C:P ratio (MBC:MBP) were the main factors driving changes in EEA, while LF was a key factor controlling EES (p < 0.05). Overall, introducing N2-fixing species into the Eucalyptus plantation alleviated but did not eliminate C and N limitation. The results provide specific recommendations for soil-nutrient management in Eucalyptus plantations in subtropical China.

Soil Fungal Diversity and Functionality Changes Associated with Multispecies Restoration of Pinus massoniana Plantation in Subtropical China

Soil fungi play a critical role in the carbon and nutrient cycling of forest ecosystems. Identifying the composition of soil fungi in response to the broadleaf restoration of Pinus massoniana plantation is essential for exploring the mechanistic linkages between tree species and ecological processes, but remains unexplored. We compared the shifts in soil fungal diversity and guilds by high–throughput sequencing between two P. massoniana plantations at different stand ages, two modes of restoration with broadleaf trees, and a secondary forest in subtropical China. We found that soil fungal taxonomic and functional compositions significantly differed among forests. The highest Chao 1, Shannon, and phylogenetic diversity indices were consistently observed in the two P. massoniana monocultures, followed by the two modes of broadleaf mixing, and the secondary forests. Fungal communities transitioned from Ascomycota-dominated at P. massoniana plantations to Basidiomycota-dominated at other forests in the topsoil. Furthermore, saprotrophs and symbiotrophs were favoured in plantations and secondary forests, respectively. Soil pH exerted the most significant effect on the relative abundance of Ascomycota and Rozellomycota, as well as the saprotrophs. Moreover, the dominant phyla of Ascomycota, Mucoromycota, and Rozellomycota were negatively related to soil microbial biomass nitrogen, ammonium nitrogen, and total nitrogen contents; however, Mortierellomycota benefited from the elevated soil ammonium nitrogen content. On the other hand, soil nitrate nitrogen and available phosphorus contents strongly and negatively influenced the ectomycorrhizal fungi, while the other fungal guilds were mainly affected by soil pH. Our findings guide an evaluation of the consequences of forest restoration and contribute to an improved understanding of the mechanisms behind soil biogeochemical cycling in subtropical forest ecosystems.

Relationships between tree functional traits and leaf nitrogen and phosphorus resorption efficiencies in subtropical young plantations

We examined the relationship between tree functional traits and leaf nitrogen and phosphorus resorption efficiencies across 29 species in 3-year-old pure plantations in subtropical China. The results showed that the average nitrogen (NRE) and phosphorus (PRE) resorption efficiencies in 29 young plantations were 50.5% and 57.3%, respectively. The average NRE of 22 arbuscular mycorrhizal (AM) tree species was 52.7%, significantly higher than that of the seven ectomycorrhizal (EM) tree species (45.1%). NRE was positively correlated with fine root tissue density across the 29 tree species. PRE was positively correlated with root diameter in the seven EM tree species. Functional traits of 22 AM tree species were not associated with NRE and PRE. Among all of the 29 tree species, mycorrhizal type, specific leaf area, fine root tissue density, leaf thickness, and the interaction effects of mycorrhizal type with leaf thickness explained 27% variation in NRE. Specific root length, fine root carbon content, fine root carbon to nitrogen ratio, mycorrhizal type, leaf carbon content, and the interaction effects of mycorrhizal type with leaf carbon content explained 35% variation in PRE. Root functional trait of subtropical species could predict nitrogen and phosphorus resorption efficiencies. The model with multiple functional traits could better reveal the relative importance of different biological factors on nutrient resorption efficiency.

Tree species identity and mixing ratio affected the release of several metallic elements from mixed litter in coniferous-broadleaf plantations in subtropical China

Planting broadleaf trees in coniferous forests has been shown to promote biogeochemical cycling in plantations; however, how species mixing influences litter decomposition and release of metallic elements from mixed coniferous-broadleaf litter remains unclear. An in situ litter decomposition experiment was conducted to examine the effect of 1) a mixture from coniferous litter (Pinus massoniana) with different individual broadleaved litter (Bretschneidera sinensis, Manglietia chingii, Cercidiphyllum japonicum, Michelia maudiae, Camellia oleifera) and 2) their mixing ratio (mass ratios of coniferous and broadleaf litter of 5:5, 6:4 and 7:3) on the release of metallic elements [calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), manganese (Mn), iron (Fe), copper (Cu) and zinc (Zn)] during litter decomposition. We found that the identity of the broadleaf tree species in the mixed litter and the mixing ratio affected the release rates of metallic elements (p < 0.05). After one year of decomposition, K, Mg, Mn and Zn were released, while Na, Ca, Fe and Cu accumulated in the mixed litter. Mixing increased the release of K, Ca, Na, Mg, Fe, Mn, Cu and Zn in more than one-third of the samples, but inhibited the release of K, Fe and Mn in less than 14% of the samples. Increasing the mixing ratio of coniferous to broadleaf litter enhanced the release of Na, Fe, Mn and Zn but decreased the release of Ca and Mg. Overall, these results highlight that mixed litter, particularly tree species identity and mixing ratio, can alter the release and enrichment of metallic elements during litter decomposition, thereby affecting the cycling of metallic elements in plantations with different species compositions.

Deriving tree growth models from stand models based on the self-thinning rule of Chinese fir plantations

Self-thinning due to density-dependent mortality usually occurs during the forest development. To improve predictions of such processes during forest successions under climate change, reliable stand-level models are needed. In this study, we developed an integrated system of tree- and stand-level models by deriving tree diameter and survival models from stand growth and survival models based on climate-sensitive self-thinning rule of Chinese fir plantations in subtropical China. The resulting integrated system, having a unified mathematical structure, should provide consistent estimates at both tree and stand levels. Predictions were reasonable at both stand and tree levels. Because stand-level values aggregated from the tree model outputs are different from those predicted directly from the stand models, the disaggregation approach was applied to provide numerical consistency between models of different resolutions. Compared to the unadjusted approach, predictions from the disaggregation approach were slightly worse for tree survival but slightly better for tree diameter. Because the stand models were developed under the climate-sensitive self-thinning trajectory, the integrated system could offer reasonable predictions that could aid in managing Chinese fir plantations under climate change.

Changes in Bulk and Rhizosphere Soil Microbial Diversity and Composition Along an Age Gradient of Chinese Fir (Cunninghamia lanceolate) Plantations in Subtropical China.

Soil microorganisms play key roles in biogeochemical cycling in forest ecosystems. However, whether the responses of microbial community with stand development differed in rhizosphere and bulk soils remains unknown. We collected rhizosphere and bulk soil in Chinese fir plantations with different stand ages (7a, 15a, 24a, and 34a) in subtropical China, and determined bacterial and fungal community variation via high-throughput sequencing. The results showed that soil bacterial, but not fungal, community diversity significantly differed among stand ages and between rhizosphere and bulk soils (p < 0.05). The differences in Shannon–Wiener and Simpson’s indices between rhizosphere and bulk soil varied with stand age, with significant higher soil bacterial diversity in rhizosphere than bulk soils in 7a and 34a plantations (p < 0.05), but there were no significant difference in soil bacterial diversity between rhizosphere and bulk soils in 15a and 24a plantations (p > 0.05). Soil microbial community composition varied significantly with stand age but not between the rhizosphere and bulk soil. The dominant bacterial phyla at all ages were Acidobacteria and Proteobacteria, while the dominant fungal phyla were Ascomycota and Basidiomycota in both rhizosphere and bulk soil. They showed inconsistent distribution patterns along stand age gradient (7–34a) in the rhizosphere and bulk soil, suggesting distinct ecological strategy (r-strategist vs. k-strategist) of different microbial taxa, as well as changes in the microenvironment (i.e., nutrient stoichiometry and root exudates). Moreover, bacterial and fungal community composition in rhizosphere and bulk soil were governed by distinct driving factors. TP and NH4+–N are the two most important factors regulating bacterial and fungal community structure in rhizosphere soil, while pH and NO3––N, DON, and TN were driving factors for bacterial and fungal community structure in bulk soil, respectively. Collectively, our results demonstrated that the changes in microbial diversity and composition were more obvious along stand age gradients than between sampling locations (rhizosphere vs. bulk soil) in Chinese fir plantations.

Comparing soil organic carbon stock and fractions under natural secondary forest and Pinus massoniana plantation in subtropical China

Comprehensive comparison of soil organic carbon (SOC) stock and its fractions in different forest types with same age is scantily considered, especially between natural secondary forest (NSF) and Pinus massoniana plantation (PMP), which are widely distributed in subtropical China. Thus, we evaluated the differences of C stocks in stand litter and 0–40 cm soil profile of an Andosols and the labile fractions [dissolved organic carbon (DOC), microbial biomass carbon (MBC), coarse particulate organic carbon (cPOC, 250–2000 μm), fine particulate organic carbon (fPOC, 53–250 μm) and easily oxidizable carbon (EOC)] and heavy fraction organic carbon (HFOC) in SOC between NSF and adjacent 70-year-old PMP. We found that NSF enhanced the C stocks in stand litter and 0–40 cm soil epipedon by 15.1% and 37.9% higher those of PMP. Compared with PMP, the stocks of MBC, cPOC, fPOC, EOC and HFOC in 0–40 cm soil epipedon under NSF were increased by 68.2%, 59.8%, 51.6%, 31.7% and 47.18% respectively, but the DOC stock was reduced by 9.9%. The impact of forest type on the SOC pool in surface soil (0–20 cm) was stronger than that in subsoil (20–40 cm). Our results suggested that NSF is more suitable for SOC sequestration and soil C sink in the study region because significantly higher SOC is incorporated as HFOC in the soil rather than stand litter under the naturally generated secondary forest.

Effects of Mixture Mode on the Canopy Bidirectional Reflectance of Coniferous–Broadleaved Mixed Plantations

One of the main initiatives for China to achieve the goal of being carbon neutral before 2060 is transforming monocultures into mixed plantations in subtropical China, because mixed forests possess a higher quality than monocultures in various ways. Very high spatial resolution (VHR) satellite imagery is very promising to precisely monitor the transformation process under the premise of clarifying the canopy reflectance anisotropy of mixed plantations. However, it is almost impossible to understand the canopy reflectance anisotropy of mixed plantations with real satellite data due to the extreme lack of multiangular VHR satellite images. In this study, the effects of the mixture mode on the canopy bidirectional reflectance factor (BRF) were comprehensively analyzed with simulated VHR images. The three-dimensional (3D) Discrete Anisotropic Radiative Transfer model (DART) was used to construct a pure coniferous scene, a pure broadleaved scene, and 27 coniferous–broadleaved mixed plantation scenes containing 3 mixture patterns (i.e., mixed by single trees, mixed by stripes, and mixed by patches) and 9 mixing proportions (i.e., from 10% to 90% with the interval of 10%), and to simulate red (R) and near-infrared (NIR) VHR images for these 3D scenes at both the solar principal plane (SPP) and perpendicular plane (PP) under different solar-viewing geometries. Negative correlations were generally found between the canopy BRF and the ratio of conifers in a mixed stand. The anisotropy of conifer dominated plantations is more prominent than broadleaf dominated plantations, especially for the single tree mixture. Although the level of anisotropy is much lower for PP than SPP, it should not be ignored, especially for the R band. Observations under large viewing zenith angles at PP are more preferred to study the effect of mixing proportions, followed by forward observations at SPP. The R band image has higher potential to distinguish mixture patterns for broadleaf-dominated situations, while the NIR band image has a higher potential for conifer-dominated situations. Furthermore, the canopy BRF generally increases with the solar zenith angle, and one meter can be considered as the optimal spatial resolution for the optical monitoring of the mixture mode. The findings of the current study add some valuable theoretical knowledge for the accurate monitoring of coniferous–broadleaved mixed plantations with VHR imagery.

The Effect of the Conversion from Natural Broadleaved Forests into Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) Plantations on Soil Microbial Communities and Nitrogen Functional Genes

The conversion of forests could change soil characteristics and, in turn, impact the microbial community. However, the long-term effect of forest transformation on bacterial and archaeal composition and diversity, especially on nitrogen functional communities, is poorly understood. This study aimed to explore the response of soil bacterial and archaeal communities, as well as nitrogen functional groups, to the conversion from natural broadleaved forests to Chinese fir (Cunninghamia lanceolate (Lamb.) Hook.) plantations in subtropical China by 16S rRNA amplicon sequencing. Except for soil bulk density (BD) and ammonium nitrogen (NH4+–N) content, other soil properties all decreased with the conversion from natural forests to plantations. Alpha diversity of bacteria and archaea declined with the transformation from natural forests to plantations. The composition of bacteria and archaea was significantly different between natural forests and plantations, which could be mainly attributed to the change in the content of soil organic carbon (SOC), total nitrogen (TN), nitrate nitrogen (NO3−–N), and available phosphorus (AP). The conversion of natural forests to plantations decreased the gene copies of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), and nifH (nitrogen fixation function) but increased denitrification gene copies (i.e., nirS, nirK, and nosZ). In summary, our study emphasizes the long-term negative effect of the conversion from natural broadleaved forests into Chinese fir plantations on the diversity and richness of soil microbial communities, thereby deeply impacting the cycling of soil nitrogen.

A Bimodal Pattern and Age-Related Growth of Intra-Annual Wood Cell Development of Chinese Fir in Subtropical China.

Age plays an important role in regulating the intra-annual changes in wood cell development. Investigating the effect of age on intra-annual wood cell development would help to understand cambial phenology and xylem formation dynamics of trees and predict the growth of trees accurately. Five intermediate trees in each stand (total of 5 stands) in five age groupings of Chinese fir (Cunninghamia lanceolata Hook.) plantations in subtropical China were monitored on micro-cores collected weekly or biweekly from January to December in 2019. We modeled the dynamics of wood cell development with a mixed effects model, analyzed the age effect on intra-annual wood cell development, and explored the contribution of rate and duration of wood cell development on intra-annual wood cell development. We found a bimodal pattern of wood cell development in all age classes, and no matter the date of peak or the maximal number of cells the bimodal patterns were similar in all age classes. In addition, compared with the older trees, the younger trees had the longest duration of wood cell development because of the later end of wood cell development and a larger number of wood cells. The younger trees had the faster growth rate than the older trees, but the date of the maximal growth rate in older trees was earlier than younger trees, which led to the production of more wood cells in the younger trees. Moreover, we found that the number of cells in wood cell formation was mostly affected by the rate (92%) rather than the duration (8%) of wood cell formation.

Soil aggregate stability and its response to overland flow in successive Eucalyptus plantations in subtropical China

Soil aggregates constitute the basic units of the soil structure, and soil aggregate stability is an important indicator of soil erodibility. Successive planting of fast-growing plantations can change the erosion resistance of the soil under rainfall conditions. Pure Eucalyptus plantations in this study (first- to fourth-generations, i.e., I, II, III, and IV, respectively) were investigated. The stability and abrasion characteristics of soil aggregates were analyzed by the wet sieving method, the Le Bissonnais (LB) method and a slope flow scouring experiment. With an increase in successive generations of Eucalyptus, the soil bulk density increased, and the saturated water content, porosity, organic matter and Iron, Aluminum and Manganese (Fe-Al-Mn) oxide contents decreased. Additionally, the wet sieving results showed that the first- and second-generations had higher macroaggregate content than the fourth generation. The mean weight diameter (MWD) values decreased with the number of planting significantly. Based on fast wetting (FW), slow wetting (SW) and mechanical breakdown by shaking after pre-wetting (WS), the aggregate stability was ranked in a decreasing order as MWDSW > MWDWS > MWDFW. The relative dissipation index (RSI) and mechanical crushing index (RMI) increased with increasing number of planting generations. Aggregate stability was significantly negatively correlated with the soil bulk density and was significantly positively correlated with the organic matter and Fe-Mn oxide contents. The extent of aggregate abrasion (Wr/Wi) values and MWD values decreased with increasing scouring distance and slope gradient during the transport process. The α and Wr/Wi values of the scoured aggregates were significantly correlated with aggregate stability. Hence, with successive planting of Eucalyptus, the soil aggregate stability decreased, and the soil was prone to erosion when subjected to slope flow.

Specific responses of canopy conductance to environmental factors in a coniferous plantation in subtropical China

Understanding the responses of canopy conductance (gc) to environmental factors would help ecologists to obtain further insight into the carbon and water exchange processes and forecast future changes in ecosystems under global climate change. Therefore, we investigated the seasonal and interannual variations in the gc based on 12 years of flux data that were observed by an eddy covariance system in a subtropical coniferous forest. Furthermore, by teasing out the dominant factors step by step, the specific gc responses to environmental factors were clarified, and the divergent responses in contrasting climatic years were determined. The multiyear mean gc was 3.46 ± 0.36 mm·s−1, and the gc peaked in April and declined to a minimum in October in most years. The vapor pressure deficit (VPD) was the most important factor that inhibited the gc, and the gc decreased sharply when the VPDs were greater than 1.8 kPa. Without high VPD stress, the gc increased linearly with air temperature (Ta) when the Ta was below 27 °C, after which it decreased. When the high VPD and Ta stresses were excluded, the gc increased with the net radiation (Rn) following a logistic growth model. The gc increased logarithmically with the soil water content at 5 cm depth (SWC5). The effects of the soil water content at 50 cm depth (SWC50) emerged under water stress or temperature stress conditions, which indicated that the forest utilized deep soil water to defend against environmental stress. Additionally, divergent responses of the gc to environmental factors in different climatic years were found. The results of this study provide detailed information on the gc variations and their specific responses to the environmental factors, which would improve the understanding of gc and help accurately estimate gc.

Impact of extensive management system on soil properties and carbon sequestration under an age chronosequence of Moso bamboo plantations in subtropical China

Moso bamboo is widely distributed in southern China and plays an important role in terrestrial C cycling. However, the implications of extensive management on soil properties, C sequestration, and plant restoration in degraded forests remain poorly understood. Here, four representative areas with four comparable stands each were selected for analysis, including those that have been extensively managed (stand age of 19, 37, and 64 years) and intensively managed control stands. Soil was collected and examined from depths of 0–20 and 20–40 cm in all 16 stands. The results show that increases in the duration of extensive management significantly increased the total organic C contents, its fractions, the C mineralization rate, soil pH, and total N in the 0–20 and 20–40 cm soil layers, accompanied by increases in aggregate stability. Amorphous and organically complexed Fe contents also increased. Increased extensive management significantly promoted the establishment, growth, and productivity of understory vegetation, with increases in richness, evenness, biomass, and diversity observed in the shrub layers. This positively affected soil organic C (SOC), aggregate stability, and amorphous Fe, suggesting that increased soil C stocks were driven by increased C input and preservation from enhanced shrub diversity. The physical protection of C fractions and amorphous Fe significantly impacted soil C sequestration considering different management strategies. Protection of shrub diversity may have accelerated the long-term storage and stabilization of SOC and restoration of soil functions.

Decline in nutrient inputs from litterfall following forest plantation in subtropical China

The rapid conversion of natural broadleaved forests to plantations (mainly Cunninghamia lanceolata and Pinus massoniana) in subtropical China has resulted in a substantial decline in soil fertility, but the biogeochemical mechanism behind this decline remains not fully understood. Litterfall is an important source of nutrients that are returned to soils through litter deposition and decomposition. We assessed various types of litterfall (leaves, twigs, flowers, fruits and unidentified litter) monthly and their carbon (C), nitrogen (N), phosphorus (P) and potassium (K) inputs to soils in a natural broadleaved forest (Castanopsis carlesii) and in P. massoniana and C. lanceolata plantations over 10 years. The total litterfall was 21–25% lower in plantations than in the C. carlesii forest, resulting in much lower C (−17% and −14%), N (−62% and −52%), P (−24% and −32%) and K (−53% and −40%) inputs from litterfall in the P. massoniana and C. lanceolata plantations. Leaves accounted for 72–76% of the total litterfall, so we compiled available data on the nutrient release from the three foliar litters during the 1st year of decomposition throughout subtropical China. The decomposition rates of P. massoniana and C. lanceolata needle litter were only half that of C. carlesii leaf litter, and the lower initial quality of P. massoniana and C. lanceolata needle litter relative to C. carlesii leaf litter resulted in significantly lower amounts of N (−95% and −76%) and P (−48% and −25%) being released. These results suggest that low nutrient (N, P and K) inputs due to low litter production and slow nutrient release due to slow litter decomposition are two important biogeochemical reasons for the decline in soil fertility in plantations in subtropical China.

Latitudinal patterns of soil extracellular enzyme activities and their controlling factors in Pinus massoniana plantations in subtropical China

Soil extracellular enzyme activities (EEAs) and enzymatic acquisition ratios are significantly affected by both abiotic and biotic factors across various scales and ecosystems. However, what determines the soil EEAs and their acquisition ratios in planted forests remains poorly understood. In this study, we measured the activities of β-1,4-glucosidase (BG), β-1,4-N-acetylglucosaminidase (NAG) and acid phosphatase (AP) and explored their key controlling factors in Pinus massoniana plantations along a 1100 km latitudinal gradient in subtropical China. The three enzyme activities increased with an increase in latitude and were significantly and negatively affected by mean annual temperature (MAT) and mean annual precipitation (MAP), but were positively correlated with soil available P content. From south to north, the enzymatic C:N (ln(BG):ln(NAG)) acquisition ratio decreased significantly, while the enzymatic C:P (ln(BG):ln(AP)) and N:P (ln(NAG):ln(AP)) acquisition ratios increased significantly. The latitudinal patterns of the enzymatic C:P and N:P acquisition ratios were significantly and negatively affected by MAT and MAP, but were positively affected by soil available P content. In total, climatic factors (e.g., MAT and MAP) accounted for more (14.2% and 25.5%, respectively) of the latitudinal variations in soil EEAs and their acquisition ratios than soil (e.g., available P) and stand (e.g., diameter at breast height). Therefore, these results illustrate that planted forests are susceptible to variations in soil EEAs and their acquisition ratios along a latitudinal gradient and that such changes are mainly driven by climate. Moreover, the similar average enzymatic C:P and N:P acquisition ratios in this pine plantation with the literature values reported for tropical and subtropical natural forests indicate that the pine plantations in subtropical China may be strongly P limited.

Effects of intensive management practices on rhizosphere soil properties, root growth, and nutrient uptake in Moso bamboo plantations in subtropical China

Understanding rhizosphere processes in relation to the aboveground productivity of bamboo stands may shed new light on the optimization of management practices and the ecological functions of belowground components in forest ecosystems. In this paper, Moso bamboo stands under intensive management with high-intensity (deep plowing plus biennial fertilization, T1) and low-intensity (deep plowing plus quadrennial fertilization, T2) practices were compared with those managed conventionally (no-tillage plus no fertilization, CK) to evaluate the impacts of intensive management strategies on the content and forms of soil C, N, and P, their associated microbial biomasses, abundances, and enzyme activities,and their relationships withbamboo root growth, nutrient uptake, and aboveground biomass. Bulk and rhizosphere soil samples from four adjacent extensively–intensively managed Moso bamboo plantation pairs were collected from a subtropical region in Zhejiang Province, China. The results show that compared with CK, intensive management practices, including T1 and T2, significantly decrease the total organic C, labile soil organic C pools including readily oxidizable C, free particulate organic C, and hot-water-extractable organic C, total N, water-soluble organic N, NH4+-N, NO3−-N, total P, organic P, and inorganic P content in the rhizosphere, but increase the content of water-soluble organic N, NH4+-N, and NO3−-N, as well as organic P and inorganic P, in the bulk soil surrounding the plant roots. Furthermore, microbial biomass C, microbial biomass N, bacterial and fungal abundances, and enzymes activities (urease, peroxidase, catalase, polyphenol oxidase, and β-glucosidase) involved in C and N metabolism were significantly decreased in the rhizosphere while acid phosphatase activity was enhanced. These changes were inferred from changes in the active C, N, and inorganic P fractions. Contrastingly, root growth traits, including biomass, length, diameter, and volume, increased continuously in response to the greater soil nutrient heterogeneity in the root zone caused by intensive management, thereby promoting root N, P, and K uptake and increasing the aboveground biomass. In conclusion, intensive management practices significantly promoted root development, growth, and biomass production in bamboo stands, but had notable negative effects on rhizosphere nutrient pools and biological properties, with potential impacts on soil functioning including in soil C sequestration and nutrient cycling.

Disentangling the effects of stand and climatic variables on forest productivity of Chinese fir plantations in subtropical China using a random forest algorithm

A machine learning method called the random forest approach was used to explore the relationship between forest productivity and stand and climate factors. Data was sampled from the long-term spacing trails of Chinese fir plantations in southern China. Results showed that the productivity of Chinese fir plantations increased with increasing value of the Gini coefficient and dominant height (Hd), while it decreased with increasing age (A) and stand basal area (BA). Furthermore, forest productivity was positively correlated with annual precipitation (AP) and summer mean maximum temperature (SMMT); in contrast, it was negatively associated with winter mean minimum temperature (WMMT) and annual heat-moisture index (AHM). Age had the greatest influence on forest productivity compared to a more secondary influence of climate factors. We found that older forests were more vulnerable to climatic stress and the productivity of forests with middle- and high- levels of competition behaved similarly, and was lower than forests with low level of competition intensity. Higher SMMT, AP and lower AHM would enlarge the gap of forest productivity under different levels of stand structure, competition intensity and site quality. Changes in site conditions had little effect on productivity when AP was lower than 1250 mm. Our findings will provide a good framework for Chinese fir plantation management under future climate change.

Long-term structural and functional changes in Acacia mangium plantations in subtropical China

Subtropical China has a large area of Acacia mangium plantations; however, some key aspects of the structural and functional changes and the associated mechanisms after planting are still not well understood. We established a permanent plot in an A. mangium plantation and monitored the plant diversity (i.e., the number of species), biomass, soil physical and chemical properties and light transmittance from 1984 to 2018. Protocols for standard observation and measurement of Chinese Ecological Research Network were adopted. The total number of species in the plantation increased gradually from 3 to 38 during the 34-year period. The biomass in the tree layer initially increased rapidly from 1.60 to 185.01 t/ha within 7 years and then slowly increased to 188.69 t/ha during the following 27 years. The soil fertility of the A. mangium plantation improved continuously over the 34-year period. Vegetation restoration had positive effects on soil properties, such as soil moisture, soil bulk density, soil organic matter and soil total nitrogen and available phosphorus contents. The structure (i.e., plant diversity and light transmittance) and function (i.e., biomass and soil fertility) of the A. mangium plantation were not restored synchronously after 34 years of development. However, considering that the tree species richness in the natural forests in this region was 134, this plantation has slowly transformed from plantation to a natural forest. Artificial intercropping with native tree species can facilitate succession to a natural forest.

Soil carbon and associated bacterial community shifts driven by fine root traits along a chronosequence of Moso bamboo (Phyllostachys edulis) plantations in subtropical China

Moso bamboo (Phyllostachys edulis) is widely considered to be effective in capturing and sequestering atmospheric C, but the long-term effects of extensive management strategies on soil organic carbon (SOC), bacterial communities, fine root (FR, ø ≤ 2 mm) traits, and their inherent connection remain unclear. In this study, we simultaneously measured the SOC content of the bulk and rhizosphere soil fractions, the aggregate stability, the chemical composition of SOC (solid-state 13C nuclear magnetic resonance [NMR]), the bacterial community structure in the rhizosphere, and the FR morphological traits including biomass, specific root length (SRL), and root length density (RLD) along a chronosequence (stand age of 19, 37, and 64 years) of extensively managed Moso bamboo plantations and in an adjacent secondary forest as a control. The organic C content in both the rhizosphere and bulk soil increased rapidly with plantation age in the 0–20- and 20–40-cm soil layers, accompanied by an increase in the aggregate stability. FR traits including biomass, SRL, and RLD also increased continuously in response to soil C:N:P stoichiometry. All of these traits were significantly correlated with SOC, occluded particulate organic C (oPOC), and mineral-associated organic C (MOC), suggesting that FR traits could drive the soil C sequestration with the plantation age. Further analysis indicated that the microbial biomass C (MBC) content, MBC/total organic carbon (TOC) ratio, and bacterial abundance decreased with the plantation age, and the shift from soil oligotrophy to copiotrophy bacteria were mainly driven by changes in FR traits and SOC properties. Such a reassembly of bacterial communities combined with an increase in root biomass is favorable for the accumulation of stable C functional groups (alkyl C or aromatic C). Our findings indicate that extensive management regimes of Moso bamboo plantations could promote long-term soil C sequestration especially in the rhizosphere by promoting the formation of soil aggregates and organic-mineral complexes and by shifting bacterial community composition, and that these changes can be inferred through changes in the FR traits.

Assessments of foliar functional traits of understory shrubs in two 13-year reforested plantations in subtropical China

Understory vegetation is crucial and necessary for the stability and biodiversity in plantation forests. However, the role of understory vegetation is easy to be neglected in traditionally intensity plantations. To date, the foliar functional traits of understory shrub in different plantation types are still in debate. Herein, a field–based restoration experiment with different plantation types was conducted to explore the variation of foliar functional traits of understory shrubs in two 13-year reforested plantations and assess which is the most suitable understory species for the plantation development and succession in subtropical China. The results showed that the foliar functional traits of the understory shrubs were species-specific. Both L. cubeba and I. asprella had higher specific leaf area (SLA) and photosynthetic pigments, suggesting that they can efficiently acquire and utilize light in the understory shade environment rather than M. candidum and R. tomentosa regardless of the plantation types. Also, the leaf N and P concentrations of L. cubeba and I. asprella were significantly larger than those of the other two species. Moreover, the SLA, leaf N and P concentrations were significantly positively related to the leaf photosynthetic pigments (chlorophyll and carotenoid) within the two plantations. These further supported that the L. cubeba and I. asprella can adapt well to the shade environment. Therefore, considering the higher SLA, leaf N and P concentrations, and photosynthetic pigments in L. cubeba and I. asprella, we strongly recommend that L. cubeba and I. asprella can be selective species for the understory management in the subtropical plantations of southern China.