Mixed forests often exhibit higher community productivity; however, opposite results have also been reported, and the underlying mechanisms remain unclear. We hypothesized that, under the condition of stable water and nutrient niches and consistent source-sink relationships across species, enhanced productivity in diversified plant communities primarily depends on community leaf area index (LAI), community-weighted mean photosynthetic capacity (CMW-Pn), and the degree of temporal niche differentiation. To test this, we studied Pinus tabuliformis Carr. (P) and Quercus variabilis Blume (Q) mixed forests with varying mixing ratios (P9Q1: 90 % P, 10 % Q; P6Q4: 60 % P, 40 % Q; P2Q8: 20 % P, 80 % Q). Basal area increment (BAI) served as a proxy for the mean productivity in the last five years, while microcore technique was applied to capture monthly and daily dynamics of growth. Monthly changes in LAI and CMW-Pn were monitored, and stable isotope tracers, hydraulic traits, and soil nutrients were used to evaluate water and nutrient niches. Results showed neither changes in moisture or nutrient niches, nor changes in non-structural carbohydrate patterns. The P6Q4 community had the highest 5-years mean annual productivity. Mean annual productivity showed a significant positive correlation with community LAI but not with CMW-Pn. Monthly scale analysis revealed that CMW-Pn contributed little to monthly productivity (3.95 %), whereas monthly Delta LAI was the main driver (59.37 %). Further, under simplified assumptions, we found that growth temporal niche differentiation increased community annual productivity by about 15 % per hectare, compared to no temporal niche differentiation. Our results suggest that regulation of community LAI, as well as the degree of interspecific differentiation in growth temporal niches, is a key ecological mechanism determining productivity in mixed forests.

Pinus tabuliformis (Chinese pine) is the dominant conifer species in northern China, comprising over 87% of nursery-raised plantings and covering a significant portion of the country’s 79.54 million hectares of timberland. It plays a central role in China’s “Carbon Double” policy, which aims to peak carbon emissions by 2030 and achieve carbon neutrality by 2060. However, the long-term response of this species’ growth to changing climatic conditions in the semi-arid temperate zone remains insufficiently quantified. In this study, we applied the process-based 3-PG model to simulate the growth and biomass dynamics of P. tabuliformis under two Representative Concentration Pathways (RCP 4.5 and RCP 8.5). Model calibration integrated field biophysical measurements and climate data from 24 urban/suburban forest parks in Beijing. Independent validation of plot-level growth was performed on 35 re-measured plots from the 8th National Forest Inventory (2009), enabling direct comparison of 14-year growth increments. Morris sensitivity analysis (μ*, σ) identified FR as the most influential parameter. Segmented regression established a significant breakpoint at FR ≈ 0.60 (p < 0.001), above which marginal gains diminished. A secondary inflection at 0.71 was not statistically significant (p = 0.23). The calibrated model showed strong agreement with observed values for diameter at breast height (DBH; R2 > 0.91, RMSE = 1.02 cm), stem biomass (R2 > 0.95, RMSE = 2.30 t·ha−1) and foliage biomass (R2 > 0.98, RMSE = 0.39 t·ha−1). Future projections revealed divergent trajectories. Under RCP 8.5, biomass increased ~40% by 2070 but declined ~30% by 2,100 under intensifying warming and drought stress. Under RCP 4.5, more stable biomass gains of ~22% sustained through 2060. These results demonstrate that current afforestation practices risk maladaptation unless they incorporate (1) FR-informed thinning protocols, (2) selection of drought-tolerant provenances, and (3) integrated water-conservation measures. To our knowledge, this study provides the first empirically parameterized and validated 3-PG framework for P. tabuliformis in northern China, offering process-based evidence to align Chinese pine management with national carbon-neutrality goals under climate stress.

The increasing frequency and severity of heatwaves pose a significant threat to forest viability, potentially inducing growth anomalies or mortality. However, the physiological mechanisms governing mature tree acclimation to recurrent heatwaves remain poorly understood. Here, automatic, continuous dendrometer and xylem sap flow sensors were used to assess the dynamics of stem growth (RGR), tree water deficit (TWD), and stem sap flow density (Js) in three mature Populus plantations (Tongzhou, TZ; Gaotang, GT; Wenxian, WX) across North China during summer heatwaves in 2023. During the initial heatwave, trees exhibited growth suppression while sustaining higher TWD. Relative to pre-heatwave levels, tree growth during the second heatwave still declined, but the magnitude of the decline varied regionally. Trees in GT with a sufficient 10-day recovery window showed greater growth recovery than the first heatwave period, whereas those in TZ and WX with insufficient recovery intervals (2-4 days) experienced persistent growth inhibition. Furthermore, environmental factors strongly affected RGR during repeated heatwave periods, whereas TWD and Js were associated with RGR during the second heatwave period. Specifically, high Js promoted growth, whereas increased TWD reduced growth. Collectively, this study emphasizes the importance of hydraulic regulation and adequate recovery windows in shaping thermal adaptive strategies, providing valuable insights to guide sustainable forest management and silvicultural practices under increasingly frequent compound extremes.

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.

In the course of fighting climate change, bamboo forests are increasingly recognized as a modern nature-based solution. Developing bamboo-based carbon projects can bring triple-bottom-line benefits to livelihood, climate, and industry, but they can also face various barriers. Based on a qualitative research framework, this paper discusses the key challenges and lessons learned from China. It then describes some of the innovative approaches that have been adopted to overcome these challenges. We identified four overarching challenges: economic, market, technical, and social. First, the rising labor costs and declining market demand for bamboo products are critical economic challenges, leading to high upfront project development costs and increasingly lower financial viability of bamboo-based investments. Second, the low transaction demand and the shifts in the national offset market have contributed to the market challenges. Moreover, many bamboo-rich regions face technical difficulties, such as lacking forestry infrastructure and skilled bamboo-specific carbon experts. Fourth, social challenges exist regarding the information asymmetry between farmers and project developers and the difficulties encountered when managing forest land-use rights in China. Inspired by several recent innovations, this paper recommends a green financing model integrating large-scale, professional forest management and the essential downstream bamboo industry development via strategies such as concessional loans and carbon-linked subsidies. There is a need for internationally standardized methodologies for bamboo forest management that incorporate advanced carbon accounting for selective harvesting and product carbon pools, enhancing credibility and scalability in compliance and voluntary markets. Such developments are needed if global policymakers, especially from some of the bamboo-based economies of the Global South, are to transform bamboo resources effectively for climate change mitigation, environmental protection, and local livelihood enhancement.

Three-dimensional models of trees can help simulate forest resource management, field surveys, and urban landscape design. With the advancement of Computer Vision (CV) and laser remote sensing technology, forestry researchers can use images and point cloud data to perform digital modeling. However, modeling leafless tree models that conform to tree growth rules and have effective branching remains a major challenge. This article proposes a method based on 3D Gaussian Splatting (3D GS) to address this issue. Firstly, we compared the reconstruction of the same tree and confirmed the advantages of the 3D GS method in tree 3D reconstruction. Secondly, seven landscape trees were reconstructed using the 3D GS-based method, to verify the effectiveness of the method. Finally, the 3D reconstructed point cloud was used to generate the QSM and extract tree feature parameters to verify the accuracy of the reconstructed model. Our results indicate that this method can effectively reconstruct the structure of real trees, and especially completely reconstruct 3rd-order branches. Meanwhile, the error of the Diameter at Breast Height (DBH) of the model is below 1.59 cm, with a relative error of 3.8–14.6%. This proves that 3D GS effectively solved the problems of inconsistency between tree models and real growth rules, as well as poor branch structure in tree reconstruction models, providing new insights and research directions for the 3D reconstruction and visualization of landscape trees in the leafless stage.

Gaps have a significant influence on forest structure and development. This study analysed characteristics of gaps between secondary and old–growth forests in Kon Ka Kinh National Park, Gia Lai Province, Vietnam. One hectare inventory plots were established in each forest stage. Data on gap size, coordinates, average height of surrounding trees, slope and directions of main axes were recorded. Results showed that the secondary forest had slightly higher number of gaps, compared to old–growth forest. However, the average size of gaps was smaller. Frequency distributions were significantly different between the two forest stages. The size of the gaps correlated with the height of the surrounding forest canopy but there was no correlation with slope. The length direction of gaps was most concentrated at East–North–East in both forest types. The distribution of gaps were regular at a scale of 0 to about 15 m, but were random at larger distances. The gap size spatial distribution was random in the secondary forest, but 60

In forestry data management and analysis, data integrity and analytical accuracy are of critical importance. However, existing techniques face a dual challenge: first, sensor failures, data transmission interruptions, and human errors lead to the prevalence of missing data in forestry datasets; second, the multidimensional heterogeneity and environmental complexity of forestry systems not only increase the difficulty of missing value estimation, but also significantly affect the accuracy of resolving the potential correlations among data. In order to solve the above problems, we proposed the L2 model using the aspen woodland as the experimental object. The L2 model consists of a complementary model and a predictive model. The L2 complementary model integrates low tensor tensor kernel norm minimisation (LRTC-TNN) to capture global consistency and local trends, and combines long and short-term memory and convolutional neural network (LSTM-CNN) to extract temporal and spatial features, which is effective in accurately reconstructing the missing values in forestry time-series data. We also optimised the LRTC-TNN model to handle multi-class data and incorporated a self-attention mechanism into the LSTM-CNN framework to improve performance in the case of complex missing data. The L2 prediction model adopts a dual attention mechanism (temporal attention mechanism and feature attention mechanism) based on LSTM to construct a stem diameter prediction model, which achieves high-precision prediction of stem diameter variation. Then we further analyzed the effects of various factors on stem diameter using SHAP (Shapley Additive Explanations).Experimental results demonstrate that our L2 significantly improves data completion accuracy while preserving the original structure and key characteristics of the data. Moreover, it enables a more precise analysis of the factors affecting stem diameter, providing a robust foundation for advanced forestry data analysis and informed decision making.

Forest structure is a key to effective forest management. This study analyzed diameter and height data collected from two plots in Tuyen Quang, Vietnam to elucidate the structural characteristics and carbon stocks of the forests. The forest volume varied between 77.59 and 103.93 m3, while the carbon stock ranged from 32.53 to 43.83 (Ton/ha). Additionally, the number of carbon credits spanned from 119.38 to 160.86. The best function for simulating the diameter frequency distribution was Wakeby and Gen. Gamma (4P). Meanwhile, the best function for the height frequency distribution was Dagum and Log-Logistic (3P). The regression between diameter and height was best described by the Power and Naslund equation. A strong association between diameter, height and forest tree quality was revealed in both plots. Good trees were typically clustered within groups with larger diameters and heights, whereas bad trees were predominantly found in the smallest groups. These findings will support the assessment of a forest's ecological functions, contributing to effective carbon sequestration initiatives and climate change mitigation efforts in the future.

Plant hydraulic traits primarily define the water regulation strategy, thus enabling a better understanding of vegetation structure, function and dynamics under varying hydro‐environments. Despite being intensively documented in woody species, the variation and correlation of hydraulic traits across herbaceous species remain largely understudied. Here, we report on the leaf hydraulics of nine herbs with contrasting growth forms (graminoid and forb). Traits quantifying drought resistance, including leaf water potential thresholds triggering xylem embolism (P x ), stomatal closure (P gs ) or leaf turgor loss point (P tlp ), and minimum conductance (g min ), together with leaf gas exchange, morphological traits and biomass allocation, were measured on pot‐grown plants. In addition, an in situ dry‐down was imposed on four representative species, with leaf gas exchange, water potential and level of xylem embolism being continuously monitored during dehydration to determine the dynamics of stomatal closure and leaf xylem embolism. We found that the studied graminoids tended to be more drought tolerant than forbs, although the difference in hydraulic safety margin for stomatal closure (HSM st ) did not differ significantly between these growth forms. Across species, P x was coordinated with P gs and P tlp , but was decoupled from gas exchange traits, including maximum photosynthetic rate and stomatal conductance. Furthermore, no correlations were found between hydraulic traits and specific leaf area or the ratio of aboveground to belowground biomass. For plants that experienced in situ dehydration, stomatal closure always preceded the onset of xylem embolism in leaves. Moreover, species exhibited a distinct stomatal regulation strategy during the dehydration despite belonging to the same growth form. Our findings contribute to the understanding of herb hydraulics, which will inform prediction on the dynamics of grassy ecosystems by providing traits data and guiding the classification of plant functional types in ‘grassy’ ecosystems. Read the free Plain Language Summary for this article on the Journal blog.

Plantations are an important component of global forest coverage, but their performance is increasingly affected by water limitation due to climate change. Employing a rainfall exclusion facility, we report on the impacts of reduced rainfall on leaf water relations and organ morphological traits, in six Populus varieties commonly used for afforestation across North China. We exposed trees to 2 years of 50% rainfall exclusion and found that leaf hydraulic traits conferring drought resistance, including water potential thresholds triggering xylem embolism, leaf pressure-volume characteristics and metrics quantifying the risk of hydraulic dysfunction (i.e., hydraulic safety margin), were not improved, despite slightly but significantly decreased predawn leaf water potential and growth rate. Interspecific variation in response to rainfall exclusion was observed for some morphological traits, yet the adjustments were unlikely to benefit drought resistance. Overall, our results demonstrate an overall lack of physiological adaptive adjustments for leaves in response to rainfall reduction at early growth stage for these trees. If this response persists as trees age, the function of these trees will be potentially reduced due to increased risk of hydraulic failure, if the drying trend continues in their planting region.

Studying key leaf functional traits is crucial for understanding plant resource utilization strategies and growth. To explore the patterns and driving factors of key leaf functional traits in forests along elevational gradients under global change, we collected survey data from 697 forests across China from 2008 to 2020. This study examined the elevational patterns of Specific Leaf Area (SLA, m²/kg), Leaf Dry Matter Content (LDMC, g/g), Leaf Nitrogen (LN, mg/g), and Leaf Phosphorus (LP, mg/g), and their responses to climate, soil nutrients, and stand factors. The results showed distinct differences in these key leaf traits at different elevational gradients. Generally, as elevation increased, SLA decreased, while LDMC significantly increased (P < 0.001), and LN first increase and then decreased (P < 0.001). The direct influence of elevation on the spatial variation of key leaf traits was greater than its indirect effects (through environmental and stand factors). The elevational patterns of leaf traits related to resource utilization strategies (SLA and LDMC) were mainly influenced by climate (temperature and precipitation) and soil nutrient factors, showing opposite trends in response to environmental changes. The patterns of leaf nutrient traits (LN and LP) along elevational gradients were primarily influenced by climatic factors, with LN exhibiting greater environmental plasticity. Compared to other stand factors, forest age predominantly influenced the spatial variation of key leaf traits, especially SLA. These findings have significant theoretical implications for revealing how plants adapt to global change.

1. Hydraulic traits are major determinants of plant fitness, thus exerting control over vegetation structure, function and distribution. Yet it remains unclear whether and how hydraulic traits respond to environmental stimuli (i.e. phenotypic variation of hydraulic traits; PVHT) and if the coordination between different hydraulic traits and the trait-climate relationship are affected by PVHT. 2. Here, we synthesized data of PVHT (maximum hydraulic conductivity and water potential inducing 50% loss of hydraulic conductivity) and potentially related morphological and anatomical traits (e.g. sapwood density, branch Huber value, mean and hydraulic weighted conduit diameter). We analysed the magnitude, direction and source of variation of the plastic response, as well as the influence of environmental factors on trait coordination. Additionally, we compared the intra- and inter-specific variation between key hydraulic traits and climate metrics (mean annual precipitation and mean annual temperature) at the site of growth, as well as across the population range. 3. PVHT was highly variable in both magnitude and direction, which was contingent on the environmental factor. The variation in PVHT mainly occurred at high taxonomic levels (i.e. family and genus), whereas phenology explained little variation for PVHT. Despite the high variability, trait correlation remained robust in the presence of environmental stimuli. Moreover, trait-climate relationships differed at inter-specific and intra-specific levels. The intra-specific variation of hydraulic traits in most species showed no correlation with climate metrics compared with the high correlation of hydraulic traits with climate metrics across species. 4. Our findings suggest that the high variability of PVHT does not affect the trait correlation which may be valuable in predicting vegetation dynamics under varying environments. The distinct trait-climate relationships highlight the need to unravel the driving force of PVHT, as well as the adaptive strategy across populations. Read the free Plain Language Summary for this article on the Journal blog.

Although the relationship between biodiversity and ecosystem functionality (BEF) has been studied comprehensively, how the mixing ratio of tree species in mixed forests affects the response of trees to climate and drought remains an unexplored and rather unknown question. Hence, we established tree-ring chronologies for Pinus tabuliformis Carr. (P) and Quercus variabilis Blume. (Q) mixed forests with different mixing ratios. In the temperate region of China, we investigated three mixing ratios: 90% P and 10% Q (P9Q1), 60% P and 40% Q (P6Q4), and 20% P and 80% Q (P2Q8). We collected tree ring samples using three tree size categories: dominant, intermediate, and suppressed trees. We explored the climate sensitivity of these trees and their drought tolerance indices-resilience (Rs), resistance (Rt), and recovery (Rc) under two drought conditions: short-term drought (1993 drought) and long-term drought (1999-2015 drought). P6Q4 made P. tabuliformis more sensitive to the Palmer drought severity index (PDSI) from the previous year than the other two ratios. The effect of the mixing ratio on drought response was insignificant under short-term drought in both tree species. Rt, Rc, and Rs of P. tabuliformis decreased with an increasing Q. variabilis:P. tabuliformis ratio in long-term drought. Rt, Rc, and Rs of Q. variabilis were the highest in P6Q4. The sensitivity of trees to PDSI varied among classes and was influenced by the mixing ratio. Dominant trees were most sensitive to PDSI in P6Q4 and P2Q8, whereas intermediate and suppressed trees were more sensitive to PDSI in P9Q1. The impact of tree size on drought tolerance indices varied according to drought type and mixing ratio. These findings showed that the mixing ratio has a confounding effect on the drought sensitivity of temperate tree species. Differences in hydrological niches allow Q. variabilis to benefit from mixing with P. tabuliformis. Mixing with optimal proportion of P. tabuliformis maximizes the drought resilience of Q. variabilis. Additionally, weakly competitive species (P. tabuliformis) do not benefit from mixed forests during prolonged water deficits. This result complements previous arguments that species mixing reduces the biological vulnerability of individuals. This study emphasizes the importance of species selection based on the biological and physiological characteristics of tree species in the afforestation of mixed forests. It highlights the critical role of species mixing ratios in the resistance of mixed forest ecosystems to climate change, which may provide a reference for sustainable forest management.

Aims Nighttime sap flow can be lost through the leave stomata as nocturnal transpiration or stored in the stem as nocturnal refilling.Distinguishing transpiration and refilling from nighttime sap flow has been a challenging and pressing problem that needs to be addressed.Although the water-refilling forecasting method is widely used due to its convenience,its accuracy is highly questionable. Methods To systematically analyze the accuracy and applicability of four water-refilling forecasting methods for the division of nighttime sap flow components,we conducted a study using Populus tomentosa as the test material.The thermal dissipation probe(TDP)were utilized to measure the nighttime sap flow.By combining the accuracy advantage of the height difference method in stem water-refilling with that of the water-refilling forecasting method in nighttime transpiration,we compared and analyzed the estimation effects of the four commonly used water-refilling forecasting methods in this study. Important findings In terms of estimating the amount of the nocturnal transpiration using the four methods based on sap flow atdifferent heights,only the linear decay model method(Line Method)showed no significant difference,while the other methods exhibited large deviations.When comparing the estimated stem water refilling using the four water-refilling forecasting methods with the results calculated by the height difference method,only the prediction method based on transpiration inversion(Et method)showed a significant difference,while the other methods did not.Additionally,among the four water-refilling forecasting methods,the Line method had the smallest deviation.Thereby,we propose using the sap flow height difference method for water-refilling forecasting to divide nighttime sap flow into three components,namely stem water-refilling,canopy water-refilling,and nocturnal transpiration.This method improves the estimation accuracy of stem water-refilling below the canopy by applying the height difference method.Furthermore,it enhances the accuracy in differentiating canopy water-refilling and nocturnal transpiration through the Line method with the smallest error.Using the new method,the calculated amount of nocturnal refilling was approximately 76.5％,which was 19.8％-26.5％higher than the findings of existing studies.The responses of the divided nighttime sap flow components to environmental factors indicated that vapor pressure deficit(VPD)and shallow soil water content were the main factors influencing nocturnal refilling.The proportion of nocturnal transpiration in nighttime sap flow exhibited a negative and nonlinear correlation with VPD.

Developing non-grain forestry woody fuel industries such as Jatropha curcas L. (JCL) aligns with the national conditions of countries with food shortages and large populations, such as some countries in Africa and Asia, and is an ideal alternative to replace fossil fuels. Determining appropriate planting densities suitable for different regions is one of the most critical practices for agriculture and forestry to achieve high yields and sustainability. This study conducted a life cycle assessment (LCA) of the JCL biodiesel's environmental impact, energy balance, and economic analysis (3E) of two densities and three of the most suitable planting regions in China. Findings reveal that the density of 2500 plants/ha had a better 3E performance than 1600 plants/ha, and Yunnan province outperformed Guizhou province and Sichuan province. JCL biodiesel effectively mitigates the greenhouse effect (1141.14-1936.59 kg CO2 eq/t) compared to conventional fossil diesel in different cases. Key factors influencing environmental performance include seed yield, urea applied during cultivation, methanol for biodiesel conversion, and electricity for irrigation. The Net Energy Balance (NEB) and Net Energy Ratio (NER) demonstrate the superior energy efficiency of JCL biodiesel over fossil diesel. The cost of producing JCL biodiesel decreased with the maturation of the trees, with water use and labor salary accounting for over 80% due to the necessity of the cultivation stage. Although the Financial Net Present Value (FNPV) was negative when considering only JCL biodiesel, it became positive (12111.64-27342.44 CNY) with the inclusion of by-products (glycerine, press-cake, and shells) in high-density cases. Our findings suggest that JCL biodiesel with appropriate density can serve as a sustainable biofuel alternative, replacing a portion of the fossil diesel currently on the market and significantly contributing to climate change mitigation and nonrenewable energy conservation.

Hydraulic traits dictate plant response to drought, thus enabling better understanding of community dynamics under global climate change. Despite being intensively documented in woody species, herbaceous species (graminoids and forbs) are largely understudied, hence the distribution and correlation of hydraulic traits in herbaceous species remains unclear. Here, we collected key hydraulic traits for 436 herbaceous species from published literature, including leaf hydraulic conductivity (Kleaf), water potential inducing 50 % loss of hydraulic conductivity (P50), stomatal closure (Pclose) and turgor loss (Ptlp). Trait variation of herbs was analyzed and contrasted with angiosperm woody species within the existing global hydraulic traits database, as well as between different growth forms within herbs. Furthermore, hydraulic traits coordination was also assessed for herbaceous species. We found that herbs showed overall more negative Pclose but less negative Ptlp compared with angiosperm woody species, while P50 did not differ between functional types, regardless of the organ (leaf and stem). In addition, correlations were found between Kleaf and P50 of leaf (P50leaf), as well as between Pclose, P50leaf and Kleaf. Within herbs, graminoids generally exhibited more negative P50 and Ptlp, but lower Kleaf, relative to forbs. Within herbs, no clear pattern regarding hydraulic traits-climate relationship was found. Our analysis provided insights into herb hydraulic, and highlighted the knowledge gaps need to be filled regarding the response of herbs to drought.

Water is a vital resource for tree growth, and changes in plantation and canopy structure can affect stand transpiration (Ec), consequently influencing water use efficiency (WUE). Populus tomentosa is a fast-growing and productive timber species in China. In recent years, thinning combined with pruning has become a widely used silvicultural practice for timber management. However, its effect on water utilization has been less well studied. To address this gap, we designed experiments with two thinning intensities and three pruning treatments. Thermal dissipation probes were employed to monitor tree sap flow density (Js), and estimated Ec and canopy conductance (gc). We established a relationship between the canopy transpiration per unit leaf area (EL) and gc and climatic factors. Finally, we compared basal area increment (BAI) and WUE among treatments under different rainfall conditions. The results indicated that: (1) The pattern of transpiration changes was consistent at both the individual tree and stand level. (2) The combined effect of T1 (thinning intensity of 833 trees per hectare) and pruning reduced Ec, decreasing the sensitivity of tree transpiration to the climate, with no discernible impact on EL and gc. Conversely, T2 (thinning intensity of 416 trees per hectare) and pruning increased EL and gc but had no effect on Ec, enhancing the sensitivity of tree transpiration to the climate. The sensitivity of gc to VPD suggested a flexible stomatal regulation of transpiration under different combined thinning and pruning treatments. (3) Under T1, only P2 (4 m pruning from ground) promoted WUE, while pruning effects significantly reduced WUE under T2. Overall, the WUE of T2P0 (thinning intensity of 416 trees per hectare combined with no pruning) was significantly higher than that of the other treatments, and that of T1P0 (thinning intensity of 833 trees per hectare combined with no pruning) was significantly lower than that of the other treatments. Additionally, significant differences in Ec and BAI were observed among treatments under different rainfall conditions, with the promotion effect of Ec on BAI being more pronounced in the dry season.

Understanding the response characteristics of fine roots to soil drought of different degrees is essential for revealing the ecological adaptability of trees to different water environments and diverse plant resource absorption strategies. This study focused on a Chinese white poplar (Populus tomentosa) plantation stand, which gradually experienced the process of deep soil drying. In 2019 and 2021, by measuring the fine-root length density (FRLD), mean root diameter (MRD), specific root length (SRL), and root tissue density (RTD) of 1920 root samples and continuously monitoring the soil water content (SWC) in 0–600 cm soil layers, we explored the response characteristics of fine-root distributions and morphological traits relative to soil drought of different degrees. The results showed that P. tomentosa primarily changed the fine-root vertical distribution rather than the total amount of fine roots for coping with soil drought of different degrees. Shallow soil drought induced more fine-root distributions in the deep soil layer, while drought in both shallow and deep soil further aggravated this trend. Shallow soil drought restrained shallow fine-root growth, yet deep soil drought promoted deep fine-root growth. The very deep fine roots (400–600 cm) were more sensitive to soil drought than shallow fine roots. The shallow soil drought significantly increased the SRL of very deep fine roots; in contrast, when deep soil drought also occurred, the MRD and SRL significantly increased and decreased, respectively. In addition, fine-root morphological traits exhibited significant vertical spatial and temporal variation. MRD increased and then decreased, and the RTD gradually decreased with depth, while SRL had an increased trend in the very deep soil layer (400–600 cm). When the rainy season came, MRD and SRL increased and decreased, respectively. In conclusion, when facing gradual deep soil drying, P. tomentosa will use a large range of rooting patterns to meet the water demand of the canopy. These patterns range from “drought tolerant strategies” by distributing more fine roots in the deeper soil layer where water is abundant to “drought tolerant strategies” by changing very deep fine-root morphological traits to improve water-absorbing and transporting efficiencies. Our findings provide insight into the ecological adaption strategy of tree root systems relative to soil drought of different degrees in arid and semi-arid regions and provide crucial theoretical support for developing water management technologies to cope with deep soil drying under climate change.