In arid and semi-arid regions, improving water efficiency is imperative for the sustainable advancement of both forestry and agriculture. The water demand period for Xanthoceras sorbifolium Bunge, an economically and ecologically important tree grown in the Horqin Sandy Land of China, is unknown. This knowledge gap has hindered the development of optimized deficit irrigation (DI) schemes aimed at conserving water while maintaining yield and quality. To identify this key period and better understand the water-use dynamics of this species, we conducted a two-year field experiment (2021-2022). Eight irrigation treatments were applied across three key phenological stages: flowering (F), fruit setting to expansion (S), and fruit-expansion to maturity (M). The irrigation treatments included full-stage (FSM), two-stage (FS, FM, and SM), single-stage (F, S, and M), and no irrigation (NI). The application of DI decreased fruit yields by 8.36-58.01 % (p < 0.05), while two-stage irrigation significantly reduced water consumption and evapotranspiration compared with full-stage irrigation (p < 0.01). FS significantly improved water productivity (WP), irrigation water productivity (WPI), and fruit quality. All two-stage irrigation treatments demonstrated yield response factors (ky) < 1. The FS treatment reduced irrigation volume by 43.7 %, while the yield decreased by only 8.36 %, suggesting that the irrigation savings did not significantly compromise yield. In summary, the FS treatment is recommended as the most optimal irrigation schedule, followed by SM and FM, for the production of X. sorbifolium in drylands. This approach conserves water while minimally impacting productivity, thus representing a sustainable water management strategy.

Cities worldwide are promoting large-scale greening initiatives to expand public green spaces (PGS) and enhance ecosystem services. However, such efforts often fall short in advancing equitable accessibility from an environmental justice perspective. This study investigates Beijing’s Dual-Phase “One Million-Mu (666 km2) Afforestation Project” as a critical case. High-resolution mapping of PGS larger than one hectare was conducted through visual interpretation of land cover and functional classification of urban green spaces (UGS) in three scenarios: S0 (simulated pre-afforestation baseline, 2011), S1 (the actual condition of post-Phase I, 2015), and S2 (derived post-Phase II built upon S1, 2022). Public green space accessibility (PGSA) and social equity were quantified using an enhanced Gaussian-based two-step floating catchment area (G2SFCA) method and the Gini coefficient analysis. Key findings include: (1) Plain-area green coverage increased from 24% (S0) to 42% (S2), with PGS proportion rising from 12% (S0) to 46% (S2). Per capita PGS surged from 23.11 m2 to 68.21 m2. Medium (5-10 ha) and large (10-20 ha) patches in suburban plains constitute most of the green expansion. (2) Afforestation significantly improved PGSA across the plains, yielding maximum gains of 224.25 m2/person at a 30-minute walking threshold. Walking-mode PGSA improvements dramatically outperformed driving-mode gains. PGSA exhibited pronounced core-suburban spatial heterogeneity after the afforestation, with higher values in peripheral districts (especially Daxing, Tongzhou, and Shunyi). (3) PGSA equity improved across village, township, and district scales, demonstrating afforestation’s positive role in equitable PGS allocation. Optimal equity (Gini=0.2-0.3) occurred under 30-minute driving thresholds at township and district levels after the afforestation, though shorter-distance equity still requires further intervention. This study delivers evidence-based pathways for optimizing large-scale urban greening policies by achieving spatially equitable PGS distribution and redress environmental disparities, facilitating sustainable cities and society.

Thinning is widely used to improve stand structure in poplar plantations, but its short-term seasonal effects on leaf physiology remain unclear. We examined how different thinning intensities influence leaf water status, nutrient dynamics, and photosynthetic function of Populus tomentosa during dry and rainy seasons. Seasonal physiological adjustments were assessed using leaf functional traits, chlorophyll fluorescence, gas-exchange measurements, and nutrient stoichiometry. Thinning increased leaf relative water content and moderated seasonal variation in leaf water potential, indicating short-term buffering of leaf water status. Moderate thinning altered photochemical parameters in a season-dependent manner. In contrast, gas-exchange traits were driven mainly by seasonal conditions rather than thinning. Leaf nutrient stoichiometry exhibited clear seasonal contrasts, with N-P co-limitation during the dry season and N limitation during the rainy season. Structural equation modelling showed that tree growth was more strongly associated with photochemical traits in the dry season and with gas-exchange traits in the rainy season. Overall, these findings demonstrate that thinning primarily influences short-term seasonal coordination among leaf physiological processes rather than inducing rapid shifts in structural traits, highlighting the importance of seasonal context when evaluating thinning effects in poplar plantations.

Transcriptome analysis identified the hub gene SmNDUFS4, which promotes graft union formation in Sapindus mukorossi by accelerating callusproliferation and vascular reconnection, thereby enhancing grafting efficiency. Grafting is widely employed to shorten the seedling stage and enhance fruit quality, while theoretically serving as a model for studying the transport and rearrangement of genetic material. Despite grafted plants exhibiting superior characteristics in multiple aspects, the underlying healing mechanisms remain poorly understood. This study identified four critical phases: isolation layer formation, callus proliferation (7 DAG), cambial bridge establishment (14–30 DAG), and vascular reconnection (after 30 DAG). Subsequently, transcriptome sequencing was performed on ten sample groups from two tissues (scion and rootstock) across five developmental stages. Analysis based on GO, KEGG, WGCNA, and correlation studies identified the key candidate gene SmNDUFS4. Genetic engineering established Arabidopsis transgenic lines overexpressing 35S::SmNDUFS4. Functional validation in Arabidopsis showed that SmNDUFS4 overexpression significantly accelerated cell division at the graft interface and enhanced vascular continuity. Transcriptome sequencing findings corroborated morphological observations, indicating that SmNDUFS4 overexpression promotes graft union formation by modulating plant stress responses. This study provides the first dynamic characterization of the graft union formation process in S. mukorossi and identifies SmNDUFS4 as a key gene in graft union formation. It establishes a crucial foundation for mechanistic research into graft union formation in S. mukorossi and offers a potential biotechnological strategy for enhancing grafting efficiency in woody plants.

Abstract Poplar seed fibers cause environmental and health concerns, yet their developmental mechanisms remain poorly understood. Here, we constructed a high-resolution spatiotemporal transcriptomic atlas of female poplar capsules by integrating single-nucleus and spatial transcriptomics. We delineated the developmental trajectory of seed fibers, confirming their origin from placenta cells, and identified three functionally distinct fiber cell subtypes involved in initiation, metabolic support, and elongation. Weighted gene co-expression network analysis (WGCNA) identified several hub transcription factors, including PtoMYB , PtoHDT1 , PtoEIF6 and PtoPDF2 , that may serve as key regulators of fiber development. Our study provides a cellular-resolution framework for understanding trichome development in woody perennials and offers candidate targets for functional characterization toward breeding low-fluff poplar cultivars. Highlights A spatiotemporal transcriptomic atlas of poplar capsule development is constructed at single-cell resolution Fiber cells originate from placenta cells and comprise three functionally distinct subtypes Provides molecular targets for breeding low-fluff poplar cultivars to mitigate environmental pollution

The MADS-box gene family plays a pivotal role in regulating the transition from vegetative to reproductive growth, as well as flower, gametophyte, and fruit development. This study aimed to investigate the MADS-box gene family in Sapindus mukorossi. A total of 106 MADS-box genes were identified and classified into Type I (Mα, Mβ, Mγ) and Type II (17 subgroups). Comprehensive analyses included gene structure, conserved motifs, cis-regulatory elements, phylogeny, and protein interactions. Type I genes were primarily expressed in vegetative organs and during pericarp and seed development, whereas Type II genes were predominantly expressed in flowers, with expression varying across seven developmental stages in male and female flowers. Quantitative real-time PCR analysis revealed distinct temporal and spatial expression patterns of APETALA1 (SmAP1) and APETALA3 (SmAP3) during S. mukorossi flower development. SmAP1 expression peaked at the floral differentiation stage (FF4/MF4) and was highly enriched in both aborted and functional pistils, whereas SmAP3 reached maximum expression earlier, at the microsporocyte meiosis stage (FF2/MF2), with predominant accumulation in functional pistils. Functional validation through overexpression in Arabidopsis thaliana demonstrated that both SmAP1 and SmAP3 individually accelerated flowering and enhanced gynoecium development compared with wild-type plants, with SmAP1 showing a more pronounced effect. These results highlight the early and pistil-specific roles of SmAP1 and SmAP3 and provide a foundation for dissecting the molecular mechanisms controlling flowering and reproductive organ development in Sapindus species.

Seed dormancy helps seeds to survive in nature but often reduces uniform germination under cultivation. In Sapindus mukorossi , seed dormancy is influenced by both physical and chemical factors. Hard seeds of S. mukorossi , which remain firm even after prolonged soaking, germinate earlier, while soft seeds that soften after soaking exhibit delayed germination. To unravel the mechanisms underlying dormancy variation, we combined allelopathic bioassays, UPLC–MS/MS–based metabolite profiling, and SEM-guided anatomical characterization of the seed coat. Bioassay experiments revealed that soft-seed kernel extracts of S. mukorossi caused strong allelopathic inhibition in Chinese cabbage seeds germination. The 100% soft-seed kernel extract (100g/L) reduced germination percentage to only 6%. In contrast, the 100% hard-seed kernel extract showed much weaker allelopathic effects, with germination percentage reaching up to 100%. At 100% concentration, soft-seed kernel extracts completely inhibited root growth while soft-seed coat extracts allowed limited root elongation, with roots reaching to 2.9 cm. Metabolomic profiling identified 1,120 metabolites in soft and hard seed kernels, of which 129 differed significantly between the two seeds tissue. Notably, soft seed kernels were enriched in 33 unique flavonoids and 11 unique alkaloids, including Hesperetin-7-O-glucoside, (-)-epicatechin, phlorizin derivatives, strictamine, methyl dioxindole-3-acetate, hexadecanamide, and caffeine. These compounds act as allelochemicals and inhibited germination and seedling growth. SEM showed that soft seeds had a rough outer surface with palisade layers containing spaces, crystal-like formations, and a porous endotesta. In contrast, hard seeds exhibited a porous outer surface but a highly compact palisade layer and interconnected endotesta. Overall, our results indicate that delayed germination in S. mukorossi seeds is primarily caused by allelopathic inhibitors accumulated in the seed kernel rather than by physical restrictions of the seed coat.

From 2016 to 2021, a field experiment was conducted in the North China Plain to study the long-term effects of drip irrigation and nitrogen coupling on the growth, biomass allocation, and irrigation water and fertilizer use efficiency of short-rotation triploid Populus tomentosa plantations. The experiment adopted a completely randomized block design, with one control (CK) and six water-nitrogen coupling treatments (IF, two irrigation levels × three nitrogen application levels). Data analysis was conducted using ANOVA, regression models, Spearman's correlation analysis, and path analysis. The results showed that the effects of water and nitrogen treatments on the annual increment of diameter at breast height (ΔDBH), annual increment of tree height (ΔH), basal area of the stand (BAS), stand volume (VS), and annual forest productivity (AFP) in short-rotation forestry exhibited a significant stand age effect. The coupling of water and nitrogen significantly promoted the DBH growth of 2-year-old trees (p < 0.05), but after 3 years of age, the promoting effect of water and nitrogen coupling gradually diminished. In the 6th year, the above-ground biomass of Populus tomentosa was 5.16 to 6.62 times the under-ground biomass under different treatments. Compared to the I45 treatment (irrigation at soil water potential of -45 kPa), the irrigation water use efficiency of the I20 treatment (-20 kPa) decreased by 88.79%. PFP showed a downward trend with the increase in fertilization amount, dropping by 130.95% and 132.86% under the I20 and I45 irrigation levels. Path analysis indicated that irrigation had a significant effect on the BAS, VS, AFP, and TGB of 6-year-old Populus tomentosa (p < 0.05), with the universality of irrigation being higher than that of fertilization. It is recommended to implement phased water and fertilizer management for Populus tomentosa plantations in the North China Plain. During 1-3 years of tree age, adequate irrigation should be ensured and nitrogen fertilizer application increased. Between the ages of 4 and 6, irrigation and fertilization should be ceased to reduce resource wastage. This work provides scientific guidance for water and fertilizer management in short-rotation plantations.

Sapindus is an important forest tree genus with utility in biodiesel, biomedicine, biochemistry and forestry. Similar to many perennial crop plants, its breeding is hampered by long generation times and lack of genetic resources. To understand the genome evolution underlying the important bioeconomic traits, we carried out a common garden experiment with 100 Sapindus core germplasm individuals representing three endemic species and 60 populations sampled throughout China. Whole genome sequencing identified a split into six populations according to species and geography. The previously uncharacterized S. delavayi and S. rarak are diploid species, and here we propose hypotheses for their speciation. Selective sweeps suggested stress responses as well as alleles of the genes CYP716A, CAMTA and HD-ZIP involved in triterpenoid saponin biosynthesis to have been under selection in natural populations, while genome-wide association analysis revealed several homologues of fatty acid biosynthesis genes to be associated with kernel fatty acid quality. Our findings elucidate the genetic structure of Sapindus in China, provide target loci for selection and suggest cultivar materials for genetic improvement.

Grafting has been extensively utilized across various plant species to enhance productivity and stress resistance. Successful grafting necessitates an effective transport system between the rootstock and scion; otherwise, the scion may perish rapidly. Despite its widespread application, the mechanisms underlying graft formation remain poorly understood. To elucidate this process, we monitored hormone levels and genome-wide gene expression changes in grafted Sapindus mukorossi. Our observations revealed that the tissues above and below the graft exhibited asymmetry during the early stages of grafting (7, 14, 20, 30, 45 days after grafting), with the expression of numerous hormones and genes being significantly higher on one side compared to the other. This asymmetry resulted in distinct cellular activities occurring simultaneously in the tissues above and below the graft. Our findings indicate that the rootstock serves as the primary source of callus formation during grafting, while the scion is chiefly involved in the reconnection of vascular tissues. These two processes are separated in time. Subsequently, TuxNet was employed to predict the regulatory network of the tissues above and below the graft, identifying three genetic hubs that facilitate the development of the scion graft: SmHCA2, SmMYB117, and SmCKC1. Additionally, we identified two genetic hubs that promote the healing of the rootstock graft: SmHAM3 and SmSPL3. These findings provide critical insights into the hormones and genes implicated in the graft healing process of Sapindus and serve as a valuable reference for grafting research in other species.

Fertilization can improve soil nutrition and increase the yield of Sapindus mukorossi, but the response of soil microbial communities to fertilization treatments and their correlation with soil nutrition and Sapindus mukorossi yield are unclear. In order to investigate the characteristics of soil physicochemical qualities and the bacterial community, we carried out a field experiment comparing various quantities of nitrogen (N), phosphorus (P), and potassium (K) fertilizers to the unfertilized control treatments and the yield of Sapindus mukorossi in raw material forests in response to different applications of fertilizers and to try to clarify the interrelation among the three. Results showed that (1) there are significant differences in the effects of different fertilization treatments on the soil properties of Sapindus mukorossi raw material forests. The increase in the application rates of nitrogen or phosphorus fertilizers significantly reduced the soil pH value. (2) Compared with control, the α-diversity of bacterial communities was significantly lower in N3P2K2 and N1P1K2 treatments. Among the dominant groups of soil bacteria at the phylum level, the relative abundance of Chloroflexi showed an increase and then a decrease trend with the increase in N application. The relative abundance of Firmicutes, Bacteroidota, and Fusobacteriota was positively correlated with the application of P and K fertilizers, while the relative abundance of Acidobacteriota and Verrucomicrobiota decreased with the increase in P and K fertilizers. (3) The N2P2K2 treatment produced the highest sapindus yield (1464.58 kg/ha), which increased by 258.67% above the control. (4) Redundancy analysis (RDA) showed that the primary determinants of bacterial community structure were soil pH, total K, and effective P concentration. (5) Structural equation modeling (SEM) showed that soil nutrient content was the main direct factor driving the yield of Sapindus mukorossi, whereas the bacterial community attributes (e.g., diversity and structure) had minor effects on the yield. In summary, the rational use of formulated fertilization can change the bacterial community structure, improve the bacterial diversity, and increase the soil nutrient content, with the latter exerting a significant effect on the improvement of the yield of Sapindus mukorossi.

Grafting compatibility refers to the successful healing between stock and scion, as well as the normal functioning of their tissues. This article investigates the survival rates and growth conditions of various Sapindus grafting combinations and establishes a new evaluation system for Sapindus grafting compatibility. This system aims to provide theoretical support for predicting Sapindus grafting compatibility and enhancing the development of improved varieties. The study utilizes 15 Sapindus grafting combinations as research materials, assesses their survival rates and growth conditions, employs the CRITIC method to calculate weights, and develops a Sapindus grafting compatibility evaluation model. By analyzing the physiological characteristics of the scion leaves from these 15 grafting combinations, variance analysis, cluster analysis, and linear regression models were utilized to assess the accuracy of the grafting compatibility evaluation model. The results indicate that the growth of grafted plants serves as a critical evaluation index of grafting compatibility, providing a more scientific approach than relying solely on survival rate metrics. This research offers a theoretical foundation and data support for predicting Sapindus grafting compatibility; an effective prediction model will substantially enhance the efficiency of breeding and promoting improved varieties, thereby fostering the growth of the Sapindus industry.

The Basic Leucine Zipper (bZIP) transcription factors play a vital role in plant responses to abiotic stress. Despite being studied in various plant species, the function of the bZIP gene family in Soapberry (Sapindus mukorossi Gaertn.), a significant tree species for forestry biomass energy, remains unclear. In this study, we conducted a genome-wide analysis of the bZIP gene family in Soapberry, based on the observation that bZIP transcription factors were enriched in the transcriptome data of Soapberry-grafted stem segments, as revealed by both GO and KEGG analyses. For the first time, we identified 31 SmbZIPs and provided detailed information regarding their physicochemical characteristics, gene structures, protein motifs, phylogenetic relationships, cis-regulatory elements (CREs), and predicted transcriptional regulatory networks. According to our prediction of the SmbZIP-mediated regulatory network and CREs in the promoter region, SmbZIPs may be associated with plant growth and development as well as responses to mechanical wounding stress. By integrating RT-qPCR and RNA-seq analyses, we determined that the expression patterns of SmbZIPs were specific to the graft-healing stages and locations. In conclusion, our study elucidates the potential role of the bZIP gene family in responding to plant wounding stress and facilitating graft healing, thereby providing valuable insights for future functional genomics studies of Soapberry.

Soapberry (Sapindus mukorossi) is an important industrial raw material tree species with multiple applications. However, little progress has been made in the comparative genomics, phylogeny, and breeding of soapberry due to a lack of genomic resources. In this study, the complete chloroplast (cp) genomes of cultivated soapberry species (S. mukorossi 'Yuanhua') was sequenced using Illumina and PacBio sequencing for the first time. Its length is 160,463 bp, encoding 86 protein genes, 8 transfer RNA genes, and 37 ribosomal RNA genes. 49 long repeat sequences and 88 simple sequence repeats (SSRs) were detected. In addition, to elucidate the characteristics of the chloroplast genome of S. mukorossi 'Yuanhua', it was compared with 11 other chloroplast genome sequences of Sapindaceae, demonstrating that sequence variations are considerably greater in the small single copy (SSC) and the large single copy (LSC) than inverted repeat (IR) regions. Meanwhile, protein-coding regions are more stable than non-coding sequences. Contraction and expansion of the IR region of the chloroplast genome were analyzed through comparison of the boundaries between IR, LSC, and SSC regions. Maximum likelihood phylogenetic analysis was conducted based on 28 complete chloroplast genome sequences. The results support the classification concept of a broad Sapindaceae (Dodonaeoideae, Sapindoideae, Hippocastanoideae, and Xanthoceroideae). This study enriches the chloroplast genome database of Sapindaceae, laying the foundation for species identification of Sapindus. The results obtained here define the phylogeny of Sapindaceae and further support the concept Sapindaceae sensu lato.

Sapindus mukorossi Gaertn., commonly referred to as soapberry, soapnut, or Chinese soapberry, is a deciduous tree species within the genus Sapindus of the family Sapindaceae. It is predominantly distributed south of the Yangtze River in China and extends to Southeast Asia, India, and Japan (Sun et al. 2018). Known for its appealing morphology and robust root system, soapberry is characterized by summer blossoms, golden autumn foliage, and fruit, making it a preferred species for landscaping and soil and water conservation in tropical and subtropical regions (Zhao et al. 2019).

Soapberry (Sapindus mukorossi Gaertn.) is a monoecious species with unisexual flowers, valued for its fruit oil and saponins, used in biofuels, detergents, and pharmaceuticals. However, its industrial potential is limited by low fruit yield caused by an imbalanced ratio of female to male flowers. The developmental processes underlying flower formation and the critical stages of sex differentiation in this species remain poorly understood. This study investigated the cymose panicle structure and the spatial distribution of male and female flowers, and conducted detailed morphological and cytological analyses across 15 developmental stages. The observations revealed that both male and female flowers initially passed through a bisexual phase before one set of sexual organs was arrested and underwent abortion during a broad developmental window. Ovule abortion in male flowers primarily occurred during the pre-meiosis to the pre-mitosis stage (S10-S11), while in female flowers, pollen abnormalities appeared at the post-meiosis stage, specifically during the first mitotic division (mitosis I, S11), preceding the arrest of the styles and filaments. Distinct features, including abnormal cell plate degradation and formation, pollen degradation, calcium oxalate crystal-associated anther dehiscence, extensive programmed cell death (PCD) and other abnormalities were observed in the sterile anther of female flowers. These results identified megaspore mother cell (MMC) pre-meiosis (S10c) as the point of sex determination, and MMC meiosis and pollen mitosis I (S11) as the onset of visible sex differentiation. This developmental framework provides new insights into sex regulation in soapberry flowers and lays a foundation for targeted breeding programs aimed at enhancing fruit yield, oil production, and saponin output.

Purpose Fine roots and soil properties show distinct vertical patterns, reflecting their coupled responses to thinning and water-fertilizer management. This study aimed to elucidate soil-root interactions and provide insights for the sustainable management of plantations. Methods A split-plot design was established with three thinning intensities (no thinning, moderate, heavy) and three water-nitrogen treatments (control, irrigation, irrigation + nitrogen). Soil profiles (0–6 m) and fine roots were sampled to assess changes in soil moisture, nutrient dynamics, and fine root traits. Multivariate analyses were used to identify key regulatory drivers. Results Soil water content (SWC) peaked at 300–400 cm and was sensitive to management in the 20–500 cm layer. Thinning and irrigation increased SWC, whereas water-nitrogen input reduced it in mid-depth layers. Thinning enhanced nitrogen accumulation, while water-nitrogen input offset nitrogen loss but increased nitrate leaching risk. Fine root biomass density was highest in the 0–20 cm layer, with deeper layers remaining stable. Water-nitrogen addition increased specific root area, with SWC as the main determinant after thinning, and both phosphorus and SWC driving responses under fertilization. Conclusion Thinning improved water availability but constrained nutrients, while water-nitrogen input shifted fine roots toward an acquisitive strategy, highlighting management-specific soil-root interactions.

The purpose of this study was to evaluate the effects of different nitrogen (N), phosphorus (P), and potassium (K) fertilization ratios on the carbon (C), N, and P contents and their ecological stoichiometric characteristics in the leaf–soil–microbial system of Sapindus saponaria and elucidate their relationship with yield. A “3414” experimental design was employed in a 6-year-old Sapindus saponaria woodland located in Fujian Province of China. Fourteen N–P–K fertilization treatments with three replicates were established. Leaf, soil, and microbial samples were collected and analyzed for C, N, and P contents. Redundancy Analysis (RDA), Partial Least Squares Path Modeling (PLS–PM), and the entropy-weighted technique of ranking preferences by similarity to optimal solutions (TOPSIS) were utilized to assess the relationships among variables and determine optimal fertilization strategies. It was found through research that different fertilization treatment methods have a significant impact on both the soil nutrient content and the C, N, and P contents of soil microorganisms. Compared with the control group, soil organic C, total N, and total P, and microbial C, N, and P contents increased by 14.25% to 52.61%, 3.90% to 39.84%, 9.52% to 150%, 6.65% to 47.45%, 11.84% to 46.50%, and 14.91% to 201.98%, respectively. Results from Redundancy Analysis (RDA) indicated that soil organic C, total N, and total P exerted a significant influence on the leaf nutrients. PLS-PM demonstrated that fertilization indirectly affected leaf nutrient accumulation and yield by altering soil properties, with soil total phosphorus and leaf phosphorus being key determinants of yield. Additionally, soil microbial entropy impacted yield by regulating microbial biomass stoichiometric ratios. The entropy-weighted TOPSIS model identified the N2P2K2 treatment (600 kg/ha N, 500 kg/ha P, and 400 kg/ha K) as the most effective fertilization strategy. Optimizing N–P–K fertilization ratios significantly enhances leaf nutrient content and soil microbial biomass C, N, and P, thereby increasing Sapindus saponaria yield. This research clarifies the underlying mechanisms through which fertilization exerts an impact on the C–N–P stoichiometry within the leaf–soil–microbial system. Moreover, it furnishes a scientific foundation for the optimization of fertilization management strategies in Sapindus saponaria plantations.

In order to explore the management strategies for cultivating and improving the stem quality of Populus tomentosa plantations under the background of climate change, this study focuses on P. tomentosa plantations over 10 years old in the North China Plain. Using linear mixed models and ordered logistic models, the impacts of cultivar, tree size, stand age, competition, and climate on the stem quality of P. tomentosa (including crown base height, tapering, branching grade, and straightness grade) were analyzed. The study found that: cultivar significantly affected all stem quality indicators (P < 0.05). Compared to other cultivars, the P. tomentosa f. yixianensis had a 23 % increase in branch height, an 8 % reduction in taper, and the risk of having poorer branches and stem form decreased by 96 % and 80 %, respectively. In addition, taller and bigger-diameter trees had better external stem quality. The impacts of competition-related indicators on stem quality were inconsistent: reduced canopy openness could improve stem quality by enhancing light competition, however, increased tree density increased the risk of deteriorating branching and straightness grade by 1.2 % and 0.9 %, respectively. Among all factors, cultivar and individual tree size had the greatest relative importance for various stem quality indicators, followed by competition-related factors, while stand age and climate factors have no significant impact on P. tomentosa stem quality (P > 0.05). Currently, climate change has little impact on the external stem quality characteristics during the cultivation of P. tomentosa plantations. Management strategies for stem quality can focus on cultivar selection and competition regulation. It is worth noting that increasing tree density in the North China Plain may not necessarily improve stem quality of P. tomentosa plantations, so caution is needed in the process of regulating competition.