Drought-related tree mortality profoundly impacts the ecosystem functions and carbon budgets, in which one of the principal mechanisms involved is the catastrophic failure of the hydraulic systems. However, our understanding of tree hydraulic systems and the mechanisms of tree death under extreme drought conditions are limited because the responses of trees to drought stress are multi-dimensional and complex. In this review, we first expounded the indexes of measuring plant drought resistance, and focused on the stomatal safety margin (SSM) that can be used to comprehensively evaluate the drought tolerance of plants. A larger positive value of SSM indicates a stronger coordination between stomata and hydraulic traits, a lower possibility of xylem embolization, and a more conservative hydraulic strategy adopted. Second, we integrated general response processes of woody plants to drought stress. Third, we introduced response mechanisms of different plant organs (leaf, stem and root) to drought stress. The probability of reaching the critical threshold and the duration of tree death are determined by interactions between physiological and morphological traits. Finally, we discussed hydraulic recovery mechanisms of woody plants, and put forward three research priorities in the future: (1) to improve the methodology for measuring leaf hydraulic conductance, especially the xylem and outside-xylem hydraulic conductance, and quantify the relative contributions of the four water transport pathways in mesophyll tissues; (2) to quantify variations in the epidermal permeability for better understanding plant water-use strategies; and (3) to deepen the understanding of the water-carbon coupling mechanisms, and link individual-level structural and physiological traits with patterns and processes at the community and landscape levels, so as to better assessing and monitoring the potential risk of drought-induced tree mortality.

Aims The hydraulic efficiency and safety of tree leaves can respond to changes in water conditions, hence affecting the growth and distribution of trees. This study was conducted to determine the patterns of responses in leaf hydraulic conductivity (K_leaf) and leaf hydraulic vulnerability (P₅₀) in trees to varying water conditions and the influencing factors.

Methods In this study, eight tree species were selected at the study sites of Guandi Mountain and Heicha Mountain in northwestern Shanxi, and their hydraulic traits, leaf vessel and morphological traits were measured. Changes of K_leaf and P₅₀ in those eight tree species were compared between the two locations. The trade-off relationship between leaf hydraulic efficiency and safety was analyzed.

Important findings Within the same tree species, the maximum hydraulic conductivity (K_max) and P₅₀ were higher at the moist Guandi Mountain sites than at the dry Heicha Mountain sites; within the same study areas, K_max and P₅₀ were higher in tree species occurring under high water availability than those in drought-prone environment. There were significant correlations among K_max, P₅₀ and water potential at turgor loss points (TLP). Leaf P₅₀ in trees in the two study areas was significantly and positively correlated with the number of vessels per unit area, the predicted value of vessel collapse ((t/b)³), leaf thickness, and leaf mass per unit area, and negatively with vessel diameter and leaf area. K_leaf and P₅₀ in different tree species were better related with leaf vessel traits than with leaf morphological traits. The changes in P₅₀(δP₅₀) from Guandi Mountain to Heicha Mountain within the same tree species were significantly and positively correlated with changes in leaf mass per unit area and leaf dry mass content, and δP₅₀ was more closely related with the leaf morphological traits than with the leaf vessel traits within the same tree species. The above results indicate that, with deterioration of water conditions, leaf hydraulic efficiency decreases while the hydraulic safety increases. There is a certain trade-off between leaf hydraulic efficiency and safety across different tree species. The differences in leaf hydraulic traits among tree species are more affected by leaf vessel traits than leaf morphological traits. The responses of leaf hydraulic safety to changes in water conditions are mainly driven by leaf morphological traits. An improvement in leaf hydraulic safety occurs at the expenses of structural carbon investment.

Aims Shrubs are an important component of forest ecosystems. This study investigated changes in the stoichiometric characteristics of shrub leaves during forest succession in order to understand and predict the processes of forest succession.

Methods The study was conducted in the Liangshui National Nature Reserve of Heilongjiang Province, with forest stands at different successional stages of mixed broadleaved-Korean pine (Pinus koraiensis) forest representing secondary birch (Betula platyphylla) forest, mixed deciduous broad-leaved forest, mixed coniferous and broad-leaved forest, and mixed broadleaved-Korean pine forest. Measurements were made on carbon (C), nitrogen (N) and phosphorus (P) contents in leaves of the understory shrubs and soil, and the stoichiometric characteristics of shrub leaves and relationships with soil stoichiometry were examined with hierarchical analysis.

Important findings The N content in shrub leaves was significantly higher in the mixed broadleaved-Korean pine forest than in other three forest types; the P content was significantly higher in the mixed broadleaved-Korean pine forest than in two other forest types except the secondary birch forest. Soil N and P contents were significantly and positively correlated with leaf N content at individual scale, and soil P concentration was significantly and positively correlated with leaf P content. At the community level, 82% of leaf N content variation and 62% of leaf P content variation were explained by species diversity and soil chemical properties; the Shannon diversity index was significantly and positively correlated with the N and P contents in shrub leaves, and negatively with the leaf C:N ratio and C:P ratio. In conclusion, shrubs in mixed broadleaved-Korean pine forests at the four successional stages were all N-limited, and species diversity better explains the stoichiometric variations in understory shrubs than soil chemical properties.

Aims The increasing level of soil salinization has been one of the most important factors to limit the development of forestry. The fast-growing Populusspp. are widely used for tree plantations and afforestation around the world and play crucial role in economic and ecological functions. Linking carbon and nitrogen metabolism with the resistance to soil salinity stress, will help to well develop the Populus plantations in salinization area.

Methods The present study used P. deltoidesand P. cathayana for materials, while two salt (NaCl) concentrations and two defoliation treatments were applied. The carbon supply ability and allocation, nitrogen metabolism and allocation of the two poplar species were mainly investigated in different treatments.

Important findings We found that the P. deltoides had higher total biomass and photosynthetic rate than P. cathayana under salinity stress. The chlorophyll concentration and the PSII maximum photochemical efficiency of P. deltoides were significantly higher than those of P. cathayana especially under defoliation with salinity stress, which demonstrated stronger damage on P. cathayana. The defoliation treatment aggravated the damage of NaCl on P. cathayana. The Na⁺ concentration in leaf and stem of P. deltoides was significantly lower than that of P. cathayana under salinity stress, demonstrating that the P. deltoides strongly restricted Na⁺ up-transport from root. Stem and root of P. deltoides had higher concentrations of starch, soluble sugars and sucrose than P. cathayana under salinity stress. The higher adenosine diphosphate glucose pyrophosphorylase activity facilitated the production of starch in P. deltoides than in P. cathayana. The defoliation greatly reduced the resistant ability of P. cathayana to salinity because of lower supply of non-structural carbohydrate to osmoregulation function. The allocation of nitrogen to sodium dodecyl sulfate-soluble protein of P. cathayana was significantly reduced by increasing salt, whereas NH₄⁺ concentration, glutamate dehydrogenase activity and proline concentration were significantly higher than those of P. deltoides. Our results demonstrated the crucial role of non-structural carbohydrate of plant species in resisting soil salinity stress.

Aims The Longdong Loess Plateau in Gansu Province is one of the main apple producing areas in China. Plastic film-mulching is often applied to maintain soil moisture as well as water-saving in apple orchards. It is reported that long-term film mulching may cause the degradation of soil physical properties and inhibition of root growth. The objective of this study were to explore the effect of long-term mulching on the physical properties, stability of the surface (0-20 cm) and subsurface (20-40 cm) layer soil, and to investigate the changes of apple fine root growth characteristics in quantity, morphology, configuration and anatomical traits.

Methods Using soil profile and stratified sampling method, the changes of the physical properties and soil structural stability of the surface and subsurface layer soil was analyzed under film-mulching 2 years (2Y), film-mulching 4 years (4Y) and film-mulching 6 years (6Y), conventional tillage (CK) treatments, and roots of 18-year-old apple trees were collected at rapid growing period (days after fruit harvest and before defoliation) to investigate the spatial distribution by measuring the root length, surface area, specific root length, catheter diameter and catheter density. Principal component analysis was used to extract the main factors of root and soil changes under the condition of plastic film mulching, and to analyze the adaptation strategies for fine root growth of apple trees to the physical degradation of rhizosphere soil.

Important findings Short-term film mulching (2Y) treatment significantly improved the soil water content and total porosity in the subsurface soil layer, increased by 18.04%, 4.53%, respectively, and reduced the soil density by 2.36% than that of conventional tillage (CK) treatments. Growth of fine roots increased in subsurface soil, and the specific surface area was 151% of CK. Film mulching promoted the movement of clay particles to the subsurface soil resulting in obvious deposition and cementation. The physical clay in subsurface soil was higher than that of surface soil. The physical clay in subsurface soil under 2Y, 4Y and 6Y mulching were 115.64%, 115.58% and 114.21% of those in surface soil, which led to soil compaction. Soil texture, aggregate characteristics and organic matter content were selected as the main load factors, which dominated degradation process of subsurface soil, and inhibited the number and configuration characteristics of roots, apple fine roots of long-term film mulching (4Y or 6Y) concentrated in the surface layer of the soil. In the subsurface soil, fine roots were found to be shortened and coarsened with inhibiting elongation growth and increasing catheter diameter, indicating the “intensive” root construction strategy the offset the weakening of absorption function caused by the fine root quantity and weakening of morphological characteristics. In conclusion, the ‘invisible' degradation of subsurface soil physical property occurred in long-term film mulching orchard will have an influence on healthy roots growth and sustainable soil utilization. It is recommended that 2-year was the suitable for continuous film mulching years in Longdong area, and the mulching film should be removed periodically to promote root growth and optimize soil structure.

Aims Based on the ITS sequences, we aimed to analyze the spatial genetic structure, genealogy relationship, and species differentiation of the Amygdalus ledebouriana, A. mongolica, A. pedunculata, A. tangutica in China, and provide data for the future studies on the four species' genetics and evolution.

Methods The median-joining network and principal coordinate analysis (PCoA) were used to reveal haplotype clustering. The maximum likelihood method and Bayesian method were used to analyze the phylogenetic relationships of haplotypes. The “ecospat” package in R 4.0.2 was used to analyze the ecological niche divergence of four almond species and their environmental drivers.

Important findings The total length of the ITS1-ITS4 fragment after corrected alignment was 634 bp, 27 nucleotide variants detected, and a total of 28 haplotypes were identified. The minimum genetic distance among the four almond species is greater than the maximum genetic distance within species, and there are significant genetic differentiations among species. The haplotypes of the four almond species clustered into two branches: A. ledebouriana, A. mongolicaand A. tanguticafor one clade, and A. pedunculatafor the other. The revealed dendrogram relationship of haplotype network and PCoA analysis is consistent with the phylogenetic tree. The significant niche divergence was observed between A. tangutica and A. mongolica, as well as between A. tangutica and A. pedunculata, with annual mean temperature, max temperature of warmest month, min temperature of coldest month and precipitation of warmest quarter as key drivers of niche divergence.

Aims Microbial biomass and their stoichiometric characteristics not only are important parameters of soil nutrient cycling, but also can contribute to prediction of climate changes, improvement of model accuracy, and understanding of terrestrial nutrient cycling. Our objective was to investigate microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and microbial biomass phosphorus (MBP) concentrations and their stoichiometric characteristics in alpine wetlands in the Three Rivers Sources Region.

Methods Using data from 50 sites, we explored MBC, MBN, MBP, their stoichiometry and their relationships with the controlling factors of alpine wetlands in the Three Rivers Source Region.

Important findings Our results showed that 1) MBC, MBN, MBP concentrations were 105.11, 3.79, 0.78 mmol·kg^-1, respectively, and MBC:MBN, MBC:MBP, MBN:MBP, MBC:MBN:MBP were 50.56, 184.89, 5.42, 275:5:1, respectively. 2) Soil physical and chemical properties could significantly affect MBC, MBN and MBP concentration. Soil moisture had significantly negative effects on both MBC:MBN and MBC:MBP, while soil density had positive effects on both MBC:MBN and MBC:MBP. Soil total nitrogen content had negative relationship with MBC:MBP, while having weak effects on MBC:MBN. Soil physical and chemical properties also had weak effects on MBN:MBP. 3) Generally, soil microbial community composition had significant effects on MBC, MBN and MBP concentration. Soil microbial community composition had similar effects on MBC:MBN and MBC:MBP. Total phospholipid fatty acid (PLFA) content, gram-positive bacteria, gram-negative bacteria, bacteria, actinomycete, arbuscular mycorrhizal fungi concentration, and other PLFA content had negative effects on MBC:MBN and MBC:MBP, while fungi:bacteria had positive effects on both MBC:MBN and MBC:MBP, but fungi had weak relationships with both MBC:MBN and MBC:MBP. Except for arbuscular mycorrhizal fungi, MBN:MBP had weak relationships with soil microbial community composition. Soil physical and chemical properties, and soil microbial community composition had significant effects on soil microbial biomass and their stoichiometric characteristics in Three Rivers Sources Regions in the alpine wetlands, which are greatly helpful for deeply understanding of terrestrial high altitude nutrient cycling.

Aims Enhanced duration, intensity, and frequency of freeze-thaw cycles owing to global climate change may significantly affect soil nitrous oxide (N₂O) emission. N₂O is an important greenhouse gas, but our current understanding of soil N₂O emission and its influencing factors during freeze-thaw cycles is still limited.

Methods Here, we adopted the meta-analysis method and collected 30 articles on the effects of freeze-thaw cycles on soil N₂O flux and cumulative emission from peer-reviewed journal articles. Our objectives were to explore the effects of freeze-thaw cycles on N₂O emissions in different ecosystems and to comprehensively explore the influencing factors from the perspectives of experimental settings, soil physical and chemical properties, and the patterns of freeze-thaw cycles.

Important findings Results showed that freeze-thaw cycles significantly increased N₂O instantaneous emission, cumulative emission, and nitrification by 72.34%, 143.25%, and 124.63%, respectively. Freeze-thaw cycles also increased denitrification by 162.56%. Conversely, freeze-thaw cycles significantly decreased microbial biomass nitrogen by 6.39%. The effect of freeze-thaw cycles on N₂O emission was significantly affected by the variations in soil microclimate and soil physical and chemical properties in different ecosystems. When the mean annual temperature (MAT) of a site exceeded 5 °C, freeze-thaw cycles could significantly enhance the N₂O flux by 104.13%, which was significantly higher than that the effect at sites with MAT between 0-5 °C (25.56%) or less than 0 °C (55.29%). When soil moisture was greater than 70%, the increase of soil N₂O flux caused by freeze-thaw cycles was 109.17%, which was significantly higher than that when soil moisture was between 50%-70% (65.67%) or less than 50% (20.37%). The higher soil clay and nutrient contents were, the greater the increase in N₂O emission caused by freeze-thaw cycles became. Freeze-thaw cycles could significantly increase soil N₂O flux by 91.21% in the presence of plants, which was higher than the effect in the absence of plants (54.43%). The impact of freeze-thaw cycles on N₂O emission could be enhanced by soil sieving. In addition, soils sampled during the freeze-thaw cycling period showed more responses to freeze-thaw cycles than soils sampled during other times. The response of cumulative N₂O emissions to freeze-thaw cycles was significantly improved by longer duration of thawing, higher intensity of freezing, and higher frequency of freeze-thaw cycles. When the freezing temperature was lower than -10 °C, freeze-thaw cycles could enhance soil N₂O flux by 100.73%, which was significantly higher than the effect when the freezing temperature was between -10- -5 °C (47.74%) or more than -5 °C (70.25%). The main reason was that higher intensity of freezing could promote the release of more nutrients from soil microorganisms and soil structure, thereby increasing the production and emission of N₂O. Overall, these results can help better understand the response of soil N₂O to freeze-thaw cycles and its influencing factors, and provide scientific data for accurately predicting the impact of global climate change on N₂O emission in the future.

Aims Spatial patterns of vessel in xylem are diverse and closely related with water transportation functions in angiosperms. However, the pattern was generally described qualitatively in anatomy, which were unable to reveal their links to xylem functions and to species distribution. We used point pattern analysis to study vessel spatial pattern in xylem cross-sectional images to quantify their features.

Methods Images of 17 types of vessel configurations were selected in terms of wood porosity, vessel arrangement, and vessel grouping. Optimum Strauss-Hardcore models for coordinates in the images were fitted. Correlations among vessel variables and model coefficients were tested.

Important findings We found that (1) Strauss-Hardcore model fitted all the data well and its three parameters, i.e., hardcore distance, local aggregation distance, and point-pair interaction or point aggregation index, and had apparent biological significance. (2) Classifications of wood xylem by traditional anatomical indices could not precisely present the spatial pattern of vessels compared to spatial point analysis, and local aggregation index from Strauss-Hardcore model was mainly influenced by vessel grouping, especially frequency of radial multiples and vessel clusters. (3) Among the 17 vessel patterns analyzed, diffusive or semi-ring species with xylem consisting of solidary vessels showed negative point-pair interaction and aggregation index was less than 0.4, whereas species with obvious vessel arrangement and multiple or clusters of vessel grouping in xylem owned positive point-pair interaction and bigger aggregation index. (4) The former group of species demonstrated inhibition- inhibition-random pattern at three local scales while the latter species showed inhibition-aggregation-random pattern according to the fitted Strauss-Hardcore models. The findings showed that point process modeling could precisely describe vessel distribution features in 2-D xylem sections and provide insights on vessel development. Therefore, this method may support 3-D vessel system simulation and experimental studies on structure-function of angiosperm xylem.