With gradual global warming and drying up, forest fires not only drive changes in the structure and function of forest ecosystems, but also affect the physiology and growth of trees. The thermal damage caused by forest fires can trigger a series of complicated physiological responses in trees. Revealing the response mechanisms of postfire tree physiology can guide the further understanding of the carbon-water relationship and how it influences the postfire growth recovery limitations of trees. Furthermore, the accuracy of tree mortality prediction after fires must be improved. Starting with a description of the pathways in which forest fires affect trees, this review elaborates on the damage caused by different forms of forest fires (canopy, surface, and ground fires) on various parts (crowns, trunks, and roots) of trees. In particular, this review discusses the direct and indirect effects of forest fires on tree physiology and the tree physiology-abiotic/biotic interactions after fires. Cambium and phloem necrosis and xylem hydraulic dysfunction are the main response mechanisms of postfire tree physiology. The two physiological functional limitations—carbon starvation and hydraulic failure—caused by the two aforementioned mechanisms seriously affect the carbon-water relationship of trees, further influencing the growth recovery of trees or their delayed death after fires. The physiological mechanisms of trees after fires are also closely related to drought, insect attack, microbial invasion, and other factors. The quantitative analyses of forest fire intensity and the accurate judgment of the plant tissue death threshold are urgent tasks, and the interaction of tree physiology with the functional traits of trees and other factors after fires must be explored. Accurately evaluating the relationship between tree physiological mechanisms is crucial in fully understanding how forest fires affect the tree functional integrity of trees and contributes to the improvement of forest fire risk assessments and mortality model predictions. In the context of high-frequency and high-intensity forest fires driven by future climate warming and drying, a profound understanding of tree physiological responses can also enhance the study of the dynamics of postfire ecosystems and their interrelationships with climate factors.

The light response curve of electron transport rate is an important tool to investigate plant physiology and ecology. It can provide a theoretical basis for quantifying the absorption and transmission of light energy in primary reaction. In this paper, the mathematical characteristics, the advantages, and the potential weaknesses in practical application and research trends of the light response models of electron transport rate are reviewed and discussed. The primary reaction, which includes absorption of light energy, excitation and de-excitation of photosynthetic pigment molecules (including photochemical reaction, fluorescence emission and heat dissipation), and electron transport rate stemming from charge separation in the photosystem II (PSII) reaction center caused by exciton resonance, is consisted by a series of complex physical and biochemical reactions. The classical and semi-mechanistic models of light response curve of electron transport rate are difficult to explain the dynamic down-regulation of PSII, light adaptation and light protection of algae and higher plant, because they did not involve or only partly involved the primary reaction process. However, the mechanism model takes into account the important role of the physical parameters of the photosynthetic pigment molecule (e.g., the eign-absorption cross-section of light energy (σ_ik), the average life-time of the molecule in the lowest excited state (τ_min), the energy level degeneracy of the molecule and the number of photosynthetic pigment molecules in the excited state (N_k)) in the whole primary reaction process. This model can not only obtain the maximum electron transfer rate and its corresponding saturation light intensity of algae and higher plant, but also get some important physical parameters such as σ_ik and τ_min. Meanwhile, it also can obtain the laws about light-response of the effective light energy absorption cross-section ($\sigma_{\mathrm{ik}}^{\prime}$) and of N_k. It may be the developmental direction of the mechanism model of electron transport rate response to light in the future when the mechanistic model was coupled with some environmental factors (e.g., temperature and CO₂ concentration) and the relationship between light intensity and $\sigma_{\mathrm{ik}}^{\prime}$ and N_k were determined.

Aims Nei Mongol is an important ecological security barrier in northern China, and the study of changes in its vegetation productivity is of great significance to the ecological security of the northern region.

Methods Based on multi-source remote sensing data such as Eddy Covariance-Light Use Efficiency Gross Primary Productivity (EC-LUE GPP) in Nei Mongol from 1982 to 2017, this paper uses trend analysis and correlation analysis to analyze the temporal and spatial variation characteristics of vegetation gross primary production (GPP) in Nei Mongol and its correlation with air temperature, precipitation and soil moisture. On this basis, multiple linear regression and residual analysis methods were used to decompose and quantify GPP under the influence of climate changes and human activities, divide different time periods to carry out its impact on vegetation GPP, and explore the impact of different vegetation types on the driving factors response.

Important findings (1) Three meteorological elements showed good correlation with vegetation GPP, among which precipitation and soil moisture had higher correlations with GPP. (2) During the period 1982-1990, vegetation GPP showed an insignificant increasing trend with large fluctuations and the remaining three time periods (1991-2000, 2001-2010, 2011-2017) showed an insignificant downward trend. The areas with an overall downward trend accounted for 55% of the total area, and the other 45% showed a significant upward trend. (3) Except for the period from 2001 to 2010, climate changes played a decisive role in vegetation restoration in the other three time periods (1982-1990, 1991-2000, 2011-2017), explaining 20%, 16% and 13% of vegetation restoration, respectively. Human activities dominated vegetation degradation areas, explaining 13%, 19% and 20% of vegetation degradation, respectively. The research results can provide scientific reference for the implementation of ecological environmental protection and management policies and green and sustainable development in Nei Mongol.

Aims Populations are the basis for the formation and development of the structure and function of grassland ecosystems. However, long-term grazing and global climate change like nitrogen addition profoundly impact the growth and reproduction of populations, such as Stipa bungeana, a dominant species in typical steppe of the Loess Plateau that has a high ecological and economic value. This study investigated how grazing and nitrogen addition affect the growth of S. bungeana.

Methods The study was based on a long-term rotational sheep grazing experiment in the typical steppe of the Loess Plateau. A completely randomized split-plot experimental design was employed, with stocking rate (0, 2.7, 5.3, 8.7 sheep·hm^-2) as the main factor and nitrogen addition levels (0, 5, 10, 20 g·m^-2) as the secondary factor. Morphological traits, aboveground biomass, the proportion of population biomass to total community aboveground biomass (PPB) and the relationship between them in S. bungeana were examined to investigate the effects of stocking rate, nitrogen addition and their interaction.

Important findings As the stocking rate increased, the plant height, canopy diameter, tiller density, seedlings, aboveground biomass, and PPB all followed a “single peak” curve trend, while the population density decreased. Nitrogen addition increased the plant height, canopy diameter, reproductive branch density, tiller density, aboveground biomass and PPB, while density of seedlings initially increased and then decreased as the nitrogen addition levels rose. The total effect of grazing on population aboveground biomass and the PPB was small compared with that of nitrogen addition. Specifically, grazing had a direct negative effect on aboveground biomass and affected PPB by regulating tiller density, population density and aboveground biomass. Nitrogen addition had a positive effect on aboveground biomass, both directly and indirectly through increasing plant height, reproductive branch density. It also impacted the PPB through regulating population density, canopy diameter, tiller density, and reproductive branch density. Overall, nitrogen addition increased canopy diameter and reproductive branch density and grazing increased density of seedings. The interaction of grazing and nitrogen addition significantly affected reproductive branch density. Stipa bungeana had maximum aboveground biomass or community status at a stocking rate of 4.10 or 5.29 sheep·hm^-2. These results indicated that grazing and nitrogen addition regulated the aboveground biomass and community status of S. bungeana through affecting its morphological characteristics, providing a basis for the scientific management and sustainable development of grassland populations.

Aims Physiological responses at the leaf level are determined by anatomical structure. Quantitative analysis of the responses of leaf anatomy to grazing in desert steppe is of great theoretical implications to reveal the ecological adaptation mechanism of plants to harsh environment.

Methods We measured 13 anatomical indices for leaves of Cleistogenes songorica, a dominant species in desert steppe under different grazing intensities, including control (CK), lightly grazing (LG), moderately grazing (MG), and heavy grazing (HG). Many indices measured were related to photosynthesis, including protective tissue, vascular tissue, and Kranz structure area. Comparative analysis for all the indices was made among different grazing intensities.

Important findings The results showed that: (1) cuticle thickness, as well as the ratio of cuticle thickness to leaf thickness first decreased and then increased with increasing grazing intensity. The thickness of motor cells decreases first and then increases with the increase of grazing intensity, compared with control and moderately heavy grazing areas, the thickness of motor cells in lightly grazing areas significantly decreased. (2) In terms of vascular tissue, the area of vascular bundle, vessel area, and phloem area were firstly increased and then decreased with the increases of grazing intensity. xylem area first decreased and then increased with the increases of grazing intensity. In terms of vascular tissue proportion, the ratio of xylem to vascular bundle area increased with the increases of grazing intensity, while the ratio between dominant vessel to vascular bundle area decreased with the increases of grazing intensity. The phloem area increased first and then decreased with the increases of grazing intensity. Compared with the control, phloem area significantly decreased in the three grazing treatments. (3) The area of Kranz structure increased with the increases of grazing intensity. Compared with that in the control, the area of Kranz structure was significantly increased in the three grazing treatments.

Aims Old trees not only record the climate and environment change information, but also witness the history of social changes. Understanding the history of growth change and disturbance of old trees is useful to accurately assess the health of old trees in the context of global warming and to develop conservation plans.

Methods We used tree-ring methods to analyze the disturbance history of the old trees (Pinus tabuliformis) in the Xishan Mountain of Beijing.

Important findings There were six major growth suppression (1831-1837, 1855-1868, 1882-1891, 1920-1930, 1960-1970 and 1980-1986) and five release events (1820-1830, 1869-1881, 1909-1918, 1947-1959 and 2004-2010). The response analysis of tree growth to climate factors and the historical data showed that drought events and subsequent climatic improvement were the main reasons for growth suppression and release events. Further studies showed that the average growth rate of diameter at breast height of old trees was 2.70 mm·a^-1 before 1840, and then the growth rate of diameter at breast height gradually decreased. After 1980, there was an obvious growth suppression, and the annual average growth rate decreased to 0.38 mm·a^-1. A stronger negative correlation between tree growth and temperature emerged after 1965, and a significant positive correlation emerged for Palmer Drought Severity Index (PDSI). These results suggest that the warming and drying trend in Beijing in recent years may further negatively affect old trees in the urban area. These results are helpful for further understanding the regularity of forest disturbance in the Beijing area during historical period, and are of great significance for accurately assessing the healthy status of elderly trees in the context of climate change.

Aims Species diversity is a crucial component of biodiversity, directly affect ecosystem function and stability during forest succession. Therefore, it is of significant importance to investigate the changes in species diversity and their influencing factors during forest succession for accurately predicting forest ecological processes and biodiversity patterns.

Methods The study focused on three successional stages in the northern Da Hinggan Mountains: Betula platyphylla forest in the early stage, Betula platyphylla - Larix gmelinii mixed forest in the middle stage and Larix gmeliniiforest in the late stage. The study employed a space-for-time substitution to analyze the changes in species diversity during forest succession and explore the relationship between species diversity and environmental factors in cold temperate zone of China.

Important findings The results demonstrated that: (1) Species richness and diversity significantly increased during forest succession. The Margalef richness index and Shannon-Wiener index were 2.42 and 2.69 in the early stage of succession, and 5.90 and 3.43 in the late stage of succession. However, Pielou evenness index showed no significant difference. (2) As succession proceeded, the dissimilarity between plant communities gradually increased, indicated by the increased Jaccard, Sorenson and Bray-Curtis indices. (3) Soil pH, soil organic matter content, total nitrogen content, and total phosphorus content significantly influence species diversity during succession. Specifically, total nitrogen content and total phosphorus content were the primary factors affecting species diversity in the early and middle stages of succession, while soil pH and soil organic matter content were the dominate factors influencing species diversity in the late stage. (4) The influence of stand spatial structure on species diversity also increased over succession, where angular scale and size ratio emerge as the main spatial structure factors. These results underscore the dominant role of soil factors in shaping species diversity during forest succession in the northern Da Hinggan Mountains, while the influence of stand spatial structure on species diversity in the process of forest succession should not be overlooked.

Aims Patterns of leaf trait variation and correlation have long been a key aspect in unraveling plant responses to climate change. However, the specifics of how these patterns of leaf structural traits and photosynthetic physiological characteristics align and differ across growth types of broadleaf plants remain unclear.

Methods This study focused on 18 dominant or common broadleaf species in a mixed broadleaf-Korean pine (Pinus koraiensis) forest. We measured four structural traits (leaf area (LA), leaf thickness (LT), leaf dry matter content (LDMC) and leaf mass per area (LMA)) and four photosynthetic physiological traits (leaf chlorophyll value (SPAD), intercellular CO₂ concentration (C_i), stomatal conductance (G_s) and net photosynthetic rate (P_n)). We analyzed the variation and correlations between structural and physiological traits across different growth types of broadleaf plants.

Important findings Leaf functional traits displayed variation ranges spanning from 7.73% to 74.54%. Interspecific differences accounted for the majority of variability for LA and LT, while growth type primarily drove variation in C_i, SPAD, LDMC, and LMA. G_s and P_n variation originate mainly from intraspecific differences. There were significant differences among growth types for all leaf traits. Specifically, herbs showed significantly higher LA, LT, and C_i compared to shrubs and trees, trees exhibited significantly elevated LMA, LDMC, SPAD, P_n, and G_s compared to shrubs and herbs. There were significant isometric relationships between P_n and LMA, LDMC among growth types, with slopes above 1. For SPAD versus LA, LT, LDMC, LMA, along with C_i versus LT, LDMC, LMA, slopes remained under 1, indicating allometric growth relationships. Herbs displayed a resource-acquisitive strategy, while trees adopted a relatively conservative strategy. Falling intermediate, shrubs struck a balance, possibly linked to the light levels across their habitats. Investigating trade-off and connections between leaf structure and photosynthetic physiology proves critical for understanding resource acquisition and allocation mechanism in plants.

Aims Revealing the response patterns of soil bacterial community to changes in vegetation type during secondary succession can improve our understanding of the mechanisms that structure the above- and below-ground interactions in ecosystems.

Methods To investigate how soil bacterial diversity, taxa network structure and biomarkers shift with vegetation succession, this study measured soil carbon, nitrogen and phosphorus contents, as well as soil bacterial community properties across shrubland, deciduous broadleaf forest and evergreen broadleaf forests, representing the early-, middle- and late-successional stages, respectively, on Dajinshan Island, Shanghai.

Important findings Soil nutrient contents were significantly higher in evergreen broadleaf forest than in deciduous broadleaf forest. However, soil bacterial diversity was significantly higher in deciduous broadleaf forest than in evergreen broadleaf forest, while soil nutrient content and bacterial diversity were medium in deciduous shrubland. The correlation network nodes, density and complexity of soil bacteria were highest in deciduous broadleaf forest, medium in deciduous shrubland, and lowest in evergreen broadleaf forest. The dominant soil bacteria in deciduous shrubland and broadleaf forest was Rhizobiales and Burkholderiales, respectively, which belong to functional group of nitrogen-fixing. The dominant soil bacterial in evergreen broadleaf forest were characterized by functional groups of pathogenicity and resistance such as Xanthomonadales and Thermogemmatisporales, and functional group associated with cellulose degradation such as Acidobacteriales. These results suggest that changes in plant species composition and soil nutrient availability during island vegetation succession can greatly reshape species diversity, community composition, interactive network structure and biomarkers of soil bacteria. In evergreen broadleaf forest, lowered soil bacterial diversity, simplified bacterial network structure, and emerged biomarkers of functional groups of pathogenicity and resistance suggest a response of belowground to the degraded trend of aboveground in the studied climax forest.

The valley forest in the Ertix River Basin is the germplasm bank of Salicaceae species and has important genetic diversity value of Altay, Xinjiang, China. It is of great significance for the protection and utilization of precious resources to study the distribution of plant species and vegetation types under the current human activities such as climate change, hydropower project and grazing. In this study, 80 squares of 20 plots were investigated at 4-8 km intervals in the Burqin River, Haba River and Bilizik River. The constructive species, quantitative characteristics and diameter class structure of the community were analyzed, and the community types were classified. The results showed that: (1) 121 species, 92 genera and 34 families of plants were investigated in the main tributaries of the Ertix River Basin, with a large number of perennial herbaceous plants. (2) Betula pendula and Populus laurifolia were constructive species in the Burjin River and Haba River, while P. alba was constructive species in the Bilizik River. P. nigra, Populus × berolinensis var. jrtyschensis, and P. canescens are associated species, while Salix alba is mainly distributed alone along the riparian edge and rarely mixed with poplar species. (3) The number of individuals of the dominant tree species Betula pendula reached the maximum at the middle altitude of the three tributaries, while the number of individuals of Populus laurifolia was smaller at the middle altitude. (4) The diameter class structure of tree layer species was greatly affected by grazing in summer, and the individuals of large diameter class were the most, and the number of saplings and seedlings were less. (5) The valley forest communities were divided into 6 formations and 34 associations.