With the global increase in tree mortality events caused by drought, there have been numerous reports on the mechanism of drought-induced tree mortality both domestically and internationally in recent years. However, the exact mechanism that causes tree mortality remains unclear, which increases the uncertainty of predicting the survival probability of forests under future climate changes. This review systematically analyzed the research progress related to tree death caused by extreme drought events, focusing the prediction of death point and physiological mechanism of drought-induced tree mortality. It highlighted that tree death was the result of multiple physiological processes. Furthermore, previous reports have shown that the death judged by visual symptoms may occur after the tree has already been dead for a period, leading to a lack of early warning signals and making the death inevitable. The review analyzed the main characteristics and possible sequence of physiological variables such as the degree of xylem embolism, radial flow, cell membrane permeability, and cambium activity in the process of drought-induced tree mortality. It suggests that the loss of cambium activity ultimately led to irreversible tree death. Therefore, when discussing the mechanism of drought-induced tree mortality, quantifying the loss rate of cambium activity is crucial for accurately determining the time of tree death, which is worth further studying. This paper also proposed relevant issues and research directions in the field of drought-induced tree mortality, providing reference ideas for accurately predicting tree death and formulating efficient and appropriate solutions to future climate change.

Aims Forests serve as the largest carbon pool in terrestrial ecosystems. Promoting forests carbon sequestration and carbon sink is a key approach to achieving the “double carbon” target. Biodiversity is a crucial foundation for maintaining ecosystem functions. Clarifying the relationship between forest biodiversity and carbon sink function is an important prerequisite for enhancing forest carbon sequestration and carbon sink. However, the relative contribution of biodiversity to forest carbon sink function along temperate forest succession, as well as the corresponding ecological processes are not clear.

Methods This study focuses on the primary Korean pine (Pinus koraiensis) - broadleaf mixed forest and the secondary forests in Changbai Mountains. Based on the data from two-phase forest community surveys, we calculated forest functional diversity and functional composition, which reflect the niche complementarity effect and the mass ratio effect, respectively. Additionally, we used forest initial aboveground carbon storage to represent the green vegetation effect. Finally, utilizing structural equation modeling, we examined the impact of different ecological effects on forest carbon storage and carbon sequestration rate, and tested the impact changes with forest succession.

Important findings We found the ecological mechanisms underlying the relationship between forest biodiversity and carbon sink function changed with forest succession. In the secondary poplar-birch forest (i.e., early successional stage), the mass ratio effect, niche complementarity effect, and vegetation quantity effect jointly affected the carbon sink function. In the secondary conifer-broadleaf mixed forest stage (i.e., middle successional stage), the mass ratio effect was the main mechanism affecting forest carbon sink function. In the primary Korean pine-broadleaf mixed forest (i.e., climax stage), the mass ratio effect and vegetation quantity effect exhibited more significant impacts. Additionally, the local environment also significantly influenced forest carbon storage and carbon sequestration rate. This study revealed the relationship between forest biodiversity and carbon sink function, as well as its underlying mechanisms, and their changes with forest succession in Changbai Mountains. These results deepen our understanding in the complex mechanisms of carbon sink function in temperate forests, and provide scientific support for the ecological restoration and management of secondary forests in Northeast China.

Aims Litter is an important source of organic matter in forest soils, and litter decomposition is crucial in the global carbon cycle. There is a large topographic relief that has formed various vegetation types and ecosystems along the altitudinal gradient in the Wuyi Mountain, China. Studying the differences in the decomposition regularities and driving factors of leaf litter and fine roots of Cunninghamia lanceolata at different altitudes can provide scientific theoretical basis for protecting and managing ecosystems in the study area.

Methods In this study, C. lanceolata plantation forests at three altitudes in the Wuyi Mountain were selected as the research platform for climate change research. A 3.5-year-experiment on litter and fine root decomposition was conducted using the decomposition bag approach along the altitude gradient.

Important findings The decomposition rate of leaf litter was higher than that of fine roots across the altitude gradient. The decomposition rates of leaf litter and fine roots decreased with increasing altitude, while we observed increased differences in decomposition rates between leaf litter and fine roots. Furthermore, the release rates of carbon, nitrogen, and phosphorus from leaf litter and fine roots reduced with increasing altitude, which had an inhibitory effect on lignin decomposition in litter and roots. The temperature was found to drive fine root decomposition and nutrient changes, while soil nutrient status and microbial communities across the altitudinal gradient acted primarily on leaf litter, suggesting divergent drivers controlling litter and root decomposition across the altitude gradient. Taken together, this study explores the impact of temperature, soil, and microorganisms on leaf litter and fine root decomposition, thereby deepening the understanding of differences in aboveground and belowground litter decomposition of the same tree species and their response to climate change.

Aims To examine stable isotope (D, ¹⁸O) variations in different water pools along the soil-plant-atmosphere continuum (SPAC) in irrigated maize (Zea mays) farmland of arid and semi-arid oasis regions and the influencing factors.

Methods The isotopic compositions of water sampled from the above-referred SPAC system were analyzed to partition those related to evapotranspiration using local meteoric water line, correlation analysis, the Craig-Gordon model, and isotopic mass balance.

Important findings (1) The isotopic composition of soil water was directly influenced by atmospheric precipitation, irrigation, and evapotranspiration. Surface wind speed had a significant effect on the isotopic values of soil water. (2) Water transported from maize roots to stems underwent stable isotope depletion. Maize roots and stems shared a common water source, with the former exhibiting isotopic enrichment relative to the mixed soil water within the 0-100 cm depth profile. (3) Atmospheric water oxygen stable isotope ratio (δ¹⁸O) showed vertical stratification, with its value at 2 m height being consistently higher than that at the 10 m height throughout the study, and the environmental sensitivity of hydrogen stable isotope ratio (δD) at 10 m being greater than that of δ¹⁸O.

Aims The spatial distribution patterns of plant populations result from the combined effects of multiple ecological processes, such as dispersal limitation and environmental filtering. The plants distributed in alpine treeline ecotones are highly sensitive to climate change due to their unique habitats. Therefore, studying the spatial distribution patterns of these plants and their correlations is critical for understanding and predicting the dynamics and trends of forest communities in alpine treelines.

Methods This study is based on the inventory data collected from a 20 hm²dynamics plot of a subalpine cold-temperate coniferous forest in Shangri-La, Yunnan, China. The dominant tree species identified were Abies georgei, Lonicera tangutica, Dipelta yunnanensis, Rhododendron rubiginosum, and Sorbus rehderiana. The spatial point pattern method was used to analyze the spatial distribution pattern of each dominant species, the intraspecific association of A. georgei at different developmental stages, the interspecific association between A. georgei and the other dominant species, and the interspecific association among the other dominant species. Additionally, the Torus-translation method was applied to test the associations between these plants and topographic factors.

Important findings (1) Sapling and juvenile trees of A. georgei demonstrated aggregated distributions, primarily driven by dispersal limitation and habitat heterogeneity. In contrast, adult trees exhibited a predominantly random distribution, suggesting that density-dependent competition may be the primary factor influencing the distribution of individuals in large-diameter classes. The dominant species in both the subtree layer and shrub layer also demonstrated aggregated distribution. However, the posterior partial advantage of the environmental heterogeneity transformed into a random distribution, indicating that environmental filtering might be responsible for driving the spatial distribution pattern of these tree species. (2) Positive associations were observed between sapling and juvenile trees of A. georgei indicating that small-diameter individuals tend to congregate due to an enhanced capacity to cope with external environmental stresses. Conversely, saplings and juvenile trees were negatively correlated with adult trees. This was mainly due to the infestation of specific pathogens and phytophagous insects caused by density constraints and asymmetric competition of large individuals against smaller ones. (3) There were positive and negative correlations between the saplings and the dominant species in the subtree layer and the shrub layer, respectively. The juvenile trees and other dominant species revealed predominantly negative correlation, while the adult trees showed predominantly positive correlation. The majority of the dominant species in the tree layer and shrub layer exhibited positive correlation, indicating a complex dynamic balance within the dominant species in the subalpine cold-temperate coniferous forest. The long-term coexistence of each dominant species in the plot is achieved through their unique survival strategies and resource utilization, and ultimately leading to the formation of a relatively stable successional climax community dominated by A. georgei. (4) Slope was found to be significantly negatively correlated with sapling and juvenile trees of A. georgei, and significantly positively related to R. rubiginosum and D. yunnanensis. This suggests that the slope ecological niche differentiation occurred between A. georgei and other dominant species. Additionally, convexity was found to exert a significant effect on the distribution of dominant species due to adverse conditions such as prolonged snowpack in winter. In conclusion, the habitat filtering driven by topography is the main driver that maintains community assembly in subalpine cold-temperate coniferous forests.

Aims Changes in precipitation characteristics, such as drought, prolonged dry season, and increased dry-wet alternation, lead to variations in plant functional traits. These changes trigger adjustments in the cooperative relationship of plant functional traits within a single organ or between multiple organs. Consequently, plant behavior and adaptation strategies change accordingly. However, the quantitative relationships and mechanisms behind this process are still unclear. This study aims to measure the specific responses of common species to climate across regions along a precipitation gradient, quantify the trait-environment relationship, elucidate the regulatory mechanism, and reveal the regional differentiation of functional traits and adaptation strategies of common species. This study will provide data support and solid scientific basis for climate management.

Methods The study focused on Ulmus pumila as the experimental subject. Ten sites were selected along a precipitation gradient from southeast to northwest China, where 28 functional traits of branches and leaves were measured. We analyzed the regional differentiation of branch and leaf traits, as well as their trade-offs. Furthermore, we quantified the regional differentiation of collaborative relationships among functional traits of branches and leaves along the precipitation gradient, revealing the adaptation strategies of U. pumila to varying moisture environments.

Important findings The results showed that: (1) In humid regions, U. pumila branches exhibited the highest hydraulic conductivity (K_s) and the lowest cavitation resistance (P₅₀); as precipitation decreased, leaf thickness and leaf tissue structure tightness increased, enhancing U. pumila’s drought resistance. (2) Across the entire precipitation gradient, there was an efficiency-safety trade-off within branches and between branches and leaves of U. pumila; however, at the regional scale, this trade-off relationship decoupled with decreasing precipitation. (3) Correlation analyses of branch and leaf functional traits revealed that, across the entire precipitation gradient, maximum net photosynthetic rate (P_n) and leaf mass per unit area were negatively correlated with K_s and positively correlated with P₅₀. Ulmus pumila regulated photosynthesis through coordinated adjustments of branch water transport capacity and leaf functional traits. The coordination and adjustment of branch and leaf functional traits are crucial mechanisms for U. pumila to adapt to varying moisture environments.

Aims Understanding the physiological mechanism of wood productivity and its capacity to adapt to climate change is crucial for developing sustainable plantation management strategies, which requires an investigation into the xylem formation process. The objective of this study was to clarify the phenology and dynamics of xylem formation and their link with environmental factors in two tree species under subtropical climate.

Methods In 2022, we monitored the seasonal dynamics of xylem formation of Cunninghamia lanceolata and Schima superba using the microcoring method at Qianyanzhou Subtropical Forest Ecological Research Station. We also collected environmental data to analyze the correlations with xylem growth rates.

Important findings The results showed that in late March, C. lanceolataand Schima superbastarted to produce enlargement cells. In April, the cell walls thickened, and in May, the cells began to mature. Schima superba completed cell enlargement in July and finished cell lignification by the end of August, 48 days and 21 days earlier than C. lanceolata, respectively. Despite having a longer growing season, C. lanceolataexhibited a much lower growth rate than S. superba, resulting in a smaller final growth volume. The xylem growth rates of C. lanceolata and S. superba correlated favorably with air temperature and soil water content on an annual basis. Furthermore, C. lanceolatadisplayed a significant positive response to vapor pressure deficit and photosynthetically active radiation, while it was negatively impacted by relative humidity. Additionally, there was a substantially positive correlation between soil temperature and S. superbaxylem growth rate. The primary factors influencing the xylem growth of C. lanceolataand S. superba in the study area were temperature and the soil water conditions.

Aims The mountainous region of Southwestern China has been recognized as one of the biodiversity hotspots in the world. The extremely complicated and fragmented geography promotes the process of population divergence, and increases species diversity in this area. Some species in the Amorphophallus genus are important cash crops that native to Southern China and Indo-China Peninsula. Wild resources of this genus are distributed as isolated populations. The demographic history and differentiation mechanisms of natural Amorphophallus populations need to be investigated.

Methods Five chloroplast DNA fragments were used to characterize the phylogeographic pattern among 16 populations of A. yunnanensis, a species that mainly resided in mountainous region of Southwestern China. The factors that contribute to the genetic differentiation pattern of A. yunnanensis were also investigated.

Important findings Genetic diversity analyses found a low level of genetic variation (nucleotide diversity ranged from 0.000 07 to 0.001 82) within populations of A. yunnanensis, and a high level of genetic differentiation with an average fixation index value of 0.363. Phylogenetic analysis revealed a clear east-west genetic differentiation, with two distinct genetic lineages inhabiting Guizhou Plateau and Yunnan Plateau, respectively. We discovered demographic expansion of the Guizhou Plateau lineage and recent hybridization in populations at the contact region. Based on the population demographic history as well as the significant level of isolation by distance, mountain systems, historical river capture events and Pleistocene climatic changes might contribute to differentiation of A. yunnanensis.

Aims The reed (Phragmites australis) is a long-rhizomed clonal plant, which is distributed worldwide and has the plasticity and adaptability to change its morphology, even growth form with environmental change. The creeping ramets of reeds are a special growth form generated from their rhizomes extending out of alkaline soil patches. This study aims to explore the growth of creeping ramets and their underlying mechanisms.

Methods Using methods such as regular tracking of ramet growth by hanging tags, measurement of photosynthetic physiology of leaves of different ages, and determination of ¹⁵N isotope transfer, we measured and analyzed the growth rhythms and patterns of creeping reed ramets, their photosynthetic characteristics, and indicators of physiological integration between ramets.

Important findings We found that in highly alkaline areas, reeds’ creeping ramets displayed different growth patterns compared to control, upright ramets. After 120 days of growth, the creeping ramets had an average length of (685.25 ± 118.75) cm, and their average growth rate during the observation period was (6.64 ± 3.51) cm·d^-1. This was 15.4 times faster than the control, upright ramets, indicating a logarithmic allometry growth process that started quickly but then slowed down. In contrast, the control ramets showed a relatively stable linear isogonic growth process. Young leaves at the top of the creeping ramets had the same maximum photosynthetic capacity as mature functional leaves. The net photosynthetic rate of leaves on the creeping ramets varied in a logistic curve with increasing leaf order, while the control ramets varied in a quadratic curve that first increased and then decreased. Moreover, the theoretical maximum net photosynthetic rate of creeping ramets was 19.4% higher than that of the control ramets. Creeping ramets treated with ¹⁵N isotopes had significantly higher levels of ¹⁵N abundance in various organs compared to untreated creeping ramets. Creeping ramets are a new adaptive feature of this widespread plant in extremely harsh alkaline habitat patches. The superior growth of creeping ramets is attributed to the high photosynthetic rate of young apical leaves and the physiological integration between tufted basal ramets and creeping ramets. This study provides new insights into the adaptation of reeds to extreme habitats and offers a method for analyzing the superior growth of creeping ramets based on matter production and physiological integration, with important theoretical implications.

Aims Climate warming will lead to a transient warming in subtropical regions during winter. Different overwintering strategies were observed in evergreen species. However, it is not clear how the photosynthetic physiology of evergreen broadleaf plants responds to transient warming at winter in subtropical region. This research aims to explore the responses of photosystem II (PSII) function of evergreen broadleaf species with different cold tolerance to transient warming at winter.

Methods Three evergreen broadleaf species Photinia × fraseri (high cold resistance), Magnolia grandiflora(moderate cold resistance), and Ficus concinna (cold sensitive) planted in subtropics were selected. The chlorophyll fluorescence parameters of shade leaves and sun leaves were examined during the transient warming period (the daily maximum temperature exceeding 15 °C and lasting for 3 d) at winter.

Important findings The PSII function of three evergreen broadleaf species was inhibited by low temperature at winter. Transient warming would promote the recovery of PSII function, however, different responses of PSII function to transient warming were observed in species with different cold tolerance. The winter PSII photoinhibition (WPI) of P. × fraseri was reversible for both shade and sun leaves, the maximum photochemical efficiency (F_v/F_m) of PSII was recovered to normal levels (>0.80) under the transient warming at winter. Moreover, the transient warming stimulated the overcompensation recovery of PSII function in both shade and sun leaves of P. × fraseri, manifested by the photochemical quenching (q_P) and non-photochemical quenching (NPQ) recovered to a higher level under the transient warming condition than before winter cooling (October). To compare the response difference between shade leaves and sun leaves of P. × fraseri, the improvement of photochemical reaction was better in shade leaves, whereas the recovery of thermal dissipation was better in sun leaves. The WPI of shade leaves of M. grandiflora was mainly reversible, while WPI of sun leaves could only partially recover. The transient warming promoted a significant recovery of PSII function in M. grandiflora. Overall, transient warming had a greater promoting effect on the improvement of energy dissipation in M. grandiflora than photochemical reactions. However, the recovery of energy dissipation in shade leaves of M. grandiflora was relatively better than that in sun leaves, while the recovery of photochemical reactions in sun leaves was better during the transient warming periods. For F. concinna, the WPI of shade leaves was mainly reversible, however, the sun leaves were damaged by severe photoinhibition by cold temperature with high light. During the transient warming period, partial recovery of PSII function was observed in the shade leaves of F. concinna, but not in the sun leaves. As same as the M. grandiflora, transient warming was more conducive to the recovery of energy dissipation than photochemical reaction in leaves of F. concinna. WPI was positively correlated with the cold tolerance of the three evergreen broadleaf species. Transient warming stimulated the overcompensation recovery of PSII function in P. × fraseri, and had a greater promoting effect on energy dissipation in M. grandiflora than photochemical reactions, but promoted the recovery of energy dissipation only in F. concinna.

Aims As one of the important species in shelterbelt forests of the northern China, the large-scale decline of the Populus simonii has a serious impact on the healthy development of the ecosystem and the sustainable management of shelterbelt forests, and the investigation on the causes of P. simonii degradation in the context of climate change can provide a reference for the management of plantation forests.

Methods The study investigated three P. simonii plantation forests with different degradation levels in Zhangbei, and compared their basal area increment (BAI), intrinsic water use efficiency (iWUE), tree ring carbon stable isotopes ratio, and stomatal regulation strategies, in order to analyze the impacts of climate change on the growth of P. simonii and its intrinsic water use efficiency.

Important findings The results showed that: 1) CO₂ concentration and air temperature were the main drivers of iWUE changes, with a significant increasing trend in iWUE under the combined effects of increasing atmospheric CO₂ concentration, climate change and physiological conditions. 2) The tree growth in the three P. simonii stands with different levels of degradation was mainly determined by air temperature, and in most cases the increase in iWUE did not promote tree growth. 3) Declining trees were more sensitive to drought in the context of climate change, and more stringent stomatal strategies were adopted by highly degraded stands under drought stress. 4) The negative effects of increased drought stress on tree physiology could not be counteracted by increased CO₂ concentrations and increased air temperatures, and prolonged drought stress might lead to further decline in the growth of degraded trees.

Aims To explore the biomass allocation of trunk, branch, leaf, coarse root, fine root, total tree, aboveground and belowground of young Catalpa bungei trees, thus to develop corresponding allometric growth models.

Methods Different components of 41 sample trees, with a diameter at breast height (D) ranging from 3.2 to 24.8 cm, were collected from 3 to 8-year-old C. bungei plantation forests at four sampling sites in three neighboring provinces. We utilized the whole mass method to determine the biomass of different components and analyzed their allocation patterns. With D, tree height (H) and their composite form D²H as predictive variables, biomass models for trunk, branch, leaf, coarse root, fine root, total tree, above- and below-ground parts of C. bungei were developed using simple power function. The accuracy of the model was then validated.

Important findings There were obvious allometric growth relationship between the biomass of various components of C. bungei. In average, 80.54% of the total biomass was allocated to above-ground, with an average of 49.29% to the trunks, far exceeding the portion of below-ground biomass, with only 0.29% of the total biomass was allocated to the fine roots. For trees with D≤ 10 cm, the proportion of branch biomass increased, while the coarse root biomass proportion decreased, resulting in a gradual increase in the difference between above- and below-ground biomass with increasing D. Whilst for trees with 10 cm < D< 25 cm, the changes in the proportional biomass of each component diminished. As for allometric models, among the three predictive variables, the accuracy ranking was approximately D > D²H > H. The models using D as a single predictive variable showed highest accuracies for the trunk, branch, leaf, coarse root, total tree, above- and below-ground part biomass, whilst D²H was the best single predictive variable for fine root biomass. Sampling of various diameter classes of C. bungei was used to validate model accuracy, and the results indicated high estimation accuracy of the optimal model for each component. Young C. bungei trees allocated their biomass according to the following order: trunk > branch > coarse root > leaf > fine root. The proportion of above-ground biomass allocation increased as Dincreased. D is the most reliable single variable for predicting the biomass of all components of the C. bungei, except for biomass of the fine roots, which is best predicted by D²H. The optimal allometric growth models constructed can predict the growth rule of young C. bungei accurately, providing significant reference for the selection and breeding of fine C. bungei clones.