The fine root phenology of forests is an important indicator to observe the impact of global warming. It can reflect not only the growth status of forests under the background of global change but also the dynamics of the carbon cycle and below-ground carbon distribution of terrestrial ecosystems. Meanwhile, the response of forest fine root phenology to climate change is considered a hotspot and challenge in the field of global change effects, and thus has been studied extensively. Recent studies claim that soil warming will prolong the growing season of fine roots in forests as the spring phenology and growth peak will begin earlier in some areas of the northern hemisphere, however, atmospheric warming may inhibit the growth of fine roots and delay the phenological events. In addition, some studies found that the root phenology in the surface soil may be more affected by warming than that in the deep layer. Concurrently, some researchers associated fine root phenology with rhizosphere soil environment, microorganisms and above-ground phenology to reveal the response mechanism. However, the response of fine root phenology to climate warming and its underlying mechanisms have not been fully explained. This paper systematically reviewed changes in fine root phenology in forests under global warming, and aimed to provide references for the research on below-ground phenology, and the response and adaptation mechanism of forests to global changes. Future studies should enhance the research on the following aspects: 1) improving and exploring more accurate simulation warming devices and carrying out long-term quantitative research; 2) exploring the relationship between different functional modules of roots (such as absorbing root/transporting root, fibrous root/pioneering root) and their phenology under changing environments, i.e. “environment-traits- phenology”; 3) considering variations of the control factors of root phenology under different below-ground phenological phases (the beginning, peak and end of root growth), species and soil layers; 4) focusing on the relationship between below- and above-ground phenology, and its impacts on plant productivity; 5) focus on the changes of forest below-ground phenology and ecosystem functions (such as carbon sinks, nutrient cycling, etc.) under the combined effect of warming and other environmental factors (CO₂ concentration, nitrogen deposition, etc.).

Aims How nitrogen (N) addition impacts the emission of greenhouse gases (GHGs) is now becoming a hot issue in the study of global change. We aim to delineate the effects of N addition on the emission of major greenhouse gases (CO₂, CH₄and N₂O).

Methods In order to achieve this goal, the flux of the three major GHGs was measured using static chamber gas chromatography during the growing seasons (May through September) of 2020 and 2021 in a meadow steppe of Hulun Buir in Nei Mongol. The experiment was conducted by applying NH₄NO₃ to simulate the atmospheric N deposition, which involved six N addition levels (i.e., 0, 2, 5, 10, 20, 50 g·m^-2·a^-1) and two grassland utilization levels (i.e., mown and unmown).

Important findings The results showed that the response of the three GHGs to N addition showed clear nonlinear patterns, but there was a remarkable difference in the patterns among the three GHGs. The emission of CO₂ was increased with increasing N addition but saturated at around 10 g·m^-2·a^-1. The uptake of CH₄ was promoted with increasing N addition when N addition was low (0-5 g·m^-2·a^-1), but this promotion effect was diminished with further increase in N addition (5-10 g·m^-2·a^-1), and the uptake of CH₄ was inhibited when N addition reached 50 g·m^-2·a^-1. The emission of N₂O increased significantly with the increase of N addition rates, but the response patterns and amplitude showed remarkable difference between the two years. With the data in the two years pooled, the CO₂ flux had a significant positive correlation with precipitation and nitrate nitrogen (NO- 3-N) content, and a significant negative correlation with pH; CH₄ absorption flux was significantly positively correlated with precipitation and ammonium nitrogen (NH+ 4-N) content, while negatively correlated with pH; N₂O flux was significantly positively correlated with soil temperature and NH+ 4-N content, while significantly negatively correlated with NO- 3-N content. Our findings demonstrated that the response of the three GHGs to increasing atmospheric N deposition was largely nonlinear, and the response patterns were remarkably different among the three GHGs. These findings may be of great importance for controlling N fertilizer use, selecting appropriate grassland use, and evaluating grassland ecosystem warming potential under increasing atmospheric N deposition.

Aims Sphagnum, as the dominant species in Sphagnum wetland, is the most important carbon (C) sequestration plant. The physiological and morphological characteristics of Sphagnum determine the carbon sink potential of Sphagnum wetland. Nitrogen (N) deposition has a significant effect on the physiological and morphological characteristics of Sphagnum,but the effects of N deposition on such characteristics of Sphagnum in the wetland environment remain controversial. Moreover, there are few reports on the physiological and morphological characteristics of Sphagnum in subtropical wetlands.

Methods We selected a Sphagnum wetland in southwestern Hubei Province as the study area. Different concentrations of NH₄Cl solution were sprayed in situto investigate the effects of simulated N deposition on physiological and morphological characteristics of Sphagnum. Four N concentrations were applied, namely 0 (N0), 3 (N3), 6 (N6) and 12 g·m^-2·a^-1 (N12), with N0 representing the control (CK).

Important findings (1) Nitrogen deposition had significant effects on the contents of total C and total N in Sphagnum. Among the treatments, the contents of total C and total N in Sphagnumunder the N3 treatment were the highest, and compared with those of the CK, increased by 3.78% and 88.52%, respectively. (2) Nitrogen deposition had no obvious effect on chlorophyll content and fluorescence activities of Sphagnum. However, N deposition significantly promoted the antioxidant enzyme activities and the contents of osmotic substances in Sphagnum,especially the contents of soluble sugars and peroxidase activity. (3) With the increase in N deposition, the height, branch number, mass per plant, and leaf cell area of Sphagnum tended to initially increase and thereafter decrease. The maximum values were observed in response to N deposition of 3 g·m^-2·a^-1. (4) Sphagnum was sensitive to N deposition, and there was a certain load value of the effect of N deposition on physiological and morphological characteristics of Sphagnumafter 2 years treatment, which was approximately 3 g·m^-2·a^-1. When the amount of N deposition is greater than 3 g·m^-2·a^-1, the effects on morphological indicators of Sphagnum are detrimental and the stress to Sphagnumwill increase significantly. The results of this study indicated that the current natural atmospheric N deposition is beneficial to the growth of Sphagnum in the wetland in southwestern Hubei Province. However, continual or doubled N deposition might be harmful to the growth of Sphagnum.

Aims Biodiversity is important for maintaining multiple ecosystem functions and enhancing community resilience to disturbance. Selection effect and niche complementarity effect are two widely discussed mechanisms for maintaining ecosystem function, but the understanding of how these two mechanisms maintain forest ecosystem multifunctionality (EMF) under climate change is still limited. It is essential to deepen our understanding of these mechanisms, particularly in assessing whether there are differences in their effectiveness across different climatic zones.

Methods Based on plots distributed in natural forests of middle temperate and cold temperate zones in northeastern China, we used functional trait diversity (FD_{q= 0}), single and multidimensional trait functional dispersion indices (FDis) to represent the niche complementarity effect, and community weighted mean trait values (CWM) to represent the selection effect. We also explored the driving force of EMF to climate change by using multivariate linear models and partial least squares path modeling (PLS-PM; structural equation model).

Important findings (1) In middle temperate forests, two attributes of biodiversity (tree species diversity (SR) and FD_{q= 0}) had significant positive effects on EMF, and FD_{q= 0} had stronger effects than SR. In cold temperate forests, no significant relationship between biodiversity and EMF (BEMF) was found. (2) In middle temperate forest communities, the effects of SR on EMF were mediated by trait differences and community weighted mean maximum tree height (CWM_Hmax) value. Both selection effect and niche complementarity effect simultaneously maintained EMF in middle temperate forests, with selection effect slightly higher than complementarity effect. CWM_Hmax was the main biotic factor influencing cold temperate forest EMF, and selection effect was the main driving force on EMF in these forests. SR and trait differences did not have a significant promoting effect on EMF. (3) Due to the “insurance effect” of biodiversity, middle temperate forests had a stronger resistance to climate change. Climate factors had no significant impact on SR, trait differences, CWM_Hmax and EMF. However, cold temperate forests were sensitive to climate change, and climatic factors were important abiotic factors affecting EMF. Higher annual mean air temperature and precipitation significantly altered community trait composition (e.g., CWM_Hmax), diluting the contribution of species with high competitiveness and fitness traits (e.g., maximum tree height (H_max) trait) to ecosystem functions, and reducing the strength of the selection effect. This study highlights the importance of biodiversity for maintaining forest EMF, and demonstrates that both selection effect and complementarity effect are driving forces for temperate forest EMF in northeastern China.

Aims Tree crown plays vital roles in carbohydrate accumulation and productivity formation, clarifying the variation pattern of tree growth and crown development, as well as their responses to the growing environment such as slope aspects, can help us to understand the adaption strategies of trees.

Methods In this study, Larix olgensis var. changbaiensis from 60 sites in east Liaoning were selected, and the responses of tree growth and crown development to different slope aspects (shady or sunny slope) and age classes (15-16 a, 23-26 a, 49-56 a) were analyzed. The three types of crown indicators, including objective indicators, subjective indicators, and composite indicators, were used to analyze the crown development in this study.

Important findings The results showed that: (1) No significant difference was found in diameter at breast height (DBH), tree height (H), wood volume, and relative dominant tree height among age classes. In comparison, the differentiation of DBH on the sunny slopes was significantly higher than that on the shady slopes in the mature age class. (2) By the comprehensive analysis of the three types of crown indicators, the crown exhibited distinct growth adaptation strategies but depending on slope aspects. Tree growing on the sunny slopes generally showed small crown length and width with uniform crown layer and high crown production efficiency. By contrast, tree growing on the shady slopes showed large crown length and width with large crown volume and low crown production efficiency. (3) Except for the ratio of H to DBH, all objective crown indicators were positively correlated to growth indicators. On the sunny slopes, the contribution of the crown to wood volume decreased with stand age, which could be proved by the significant negative correlation between crown length, crown ratio, and growth indicators. In addition, the crown position and crown fullness ratio were also the main factors that promoted stand growth at the shady slopes. (4) All the objective indicators were positively correlated to the biomass of each organ. On both slope aspects, whole tree biomass was positively correlated to crown light exposure, crown surface area, and crown volume, but negatively correlated to crown dieback. On the sunny slopes, whole tree biomass decreased with the increase of foliage transparency, ratio of crown density to DBH, and crown ratio, while on the shady slopes, biomass increased with higher crown position and crown fullness ratio. (5) Among all indicators, crown surface area showed the best indicator for explaining the biomass of each organ in this study. Overall, better crown development could significantly promote tree growth on Larix olgensis var. changbaiensis plantation, but the trade-off between crown development and tree growth that varies with slope aspects should be considered in such stand management.

Aims Explore the allometry between lamina and rachis and clarify the leaf cost-benefit relationship of pinnate compound-leaf temperate tree species.

Methods In the Maoershan Forest Ecosystem Research Station, Heilongjiang Province, the lamina and rachis traits were measured for five pinnate compound-leaf species (Fraxinus mandshurica, Juglans mandshurica, Phellodendron amurense, Maackia amurensisand Aralia elata), and the allometry between lamina and rachis was developed by using the standardized major axis regression.

Important findings There was a significant allometric scaling between lamina area and rachis mass, and the slope was closer to the theoretical value of 3/4 of the West, Brown and Enquist (WBE) model than to the theoretical value of 2/3 of the geometric similarity model. However, there was an isometric relationship between lamina mass and rachis mass in most cases. There were significant allometric relationships between compound-leaf area and rachis length and between lamina mass and rachis length, and the common slopes for all the species were 1.853 and 2.322, respectively. There was a significant allometry between rachis mass and rachis length, with all the allometric exponents being greater than 2, indicating that the increasing rate of spatial expansion benefit achieved by the rachis lengthening was lower than the increasing rate of lamina mass cost. Significant allometric scaling between lamina area and lamina mass and between lamina area and compound-leaf mass indicated a trend of diminishing returns in compound-leaves. The bipinnate compound-leaves of A. elata had a higher carbon return to investment ratio than the pinnate ones of the other species. In summary, the significant allometric scaling between lamina and rachis of the pinnate compound-leaf species suggests rachis elongation helps light acquisition, but the increase in investment cost caused by the rachis elongation limits the expansion of the compound leaf area.

Aims Leaf construction cost (LCC) is a measure of the energy cost of building leaves per unit mass or area. Its differences and changes can reflect the energy utilization strategies and environmental adaptation characteristics of plants. This study assumes that desert plants of different functional groups have different leaf energy utilization strategies, which are conducive to their adaptation to arid environments.

Methods Taking the main desert plants in the desert area of the Hexi Corridor as the research object, the differences of LCC in different ecosystems and different plant functional groups were compared, and the relationships between LCC and other leaf traits were analyzed, as well as the change of LCC with environmental factors.

Important findings LCC of plant unit mass in this desert ecosystem was significantly lower than that in forest and grassland ecosystems. In desert ecosystems, LCC per unit mass of succulent plants is significantly lower than that of non succulent plants, while LCC per unit area has no significant difference between the two plant groups. The correlations between leaf carbon, nitrogen contents, calorific value and ash content and LCC indicate that ash content is the internal determinant of LCC difference between succulent plants and non succulent plants. The contribution of environmental factors to the spatial variation of unit mass LCC is very limited (11.22% for succulent plants, 25.30% for non succulent plants, and 24.99% for all plants), and only soil conductivity, annual mean air temperature, and annual precipitation among the six environmental factors show significant independent contributions. LCC per unit mass of succulent plants decreases with the decrease of annual average temperature, while LCC per unit mass of non succulent plants decreases with the decrease of annual precipitation and annual mean air temperature. The results of this study show that reducing the cost of leaf construction per unit mass rather than per unit area is more beneficial for plants to adapt to drought and high salt desert habitats.

Aims The xylem water transport resistance and leaf transpiration rate of tall trees increased with tree height, resulting in a water supply-demand paradox along the tree height gradient. Quantitative analysis of the variation and coordination of related functional traits along tree height will be conducive to deeply understanding the water supply and demand mechanism of plants.

Methods Here, Taxodium distichum and its variety T. distichumvar. imbricatum grown in a mesic common garden were studied, with hydraulic (sapwood-specific hydraulic conductivity (K_s), leaf specific conductivity (K_l), the water potential causing 50% loss of conductivity (P₅₀), maximum transpiration rate (T_r), midday leaf water potential (ψ_MD), Huber value (H_v), etc.), photosynthetic (maximum photosynthetic rate (P_n)) and carbon economic traits (leaf mass per unit area (LMA), wood density (WD)) measured. Traits variation and coordination along the tree height and traits differences between the same canopy of T. distichum and T. distichumvar. imbricatum were analyzed by a series of methods, including regression analysis, one-way ANOVA, and path analysis.

Important findings We found that: (1) K_l, H_v, P_n and LMA in T. distichum and T. distichumvar. imbricatum increased along the height, and the increase of P_n may be related to the decrease of T_r and maximum operational stomatal conductance (G_s) in the middle canopy. (2) Coordination relationships between intraspecific traits: K_s in T. distichum and T. distichumvar. imbricatum was significantly negatively related to H_v, WD in T. distichum was significantly positively related to K_s, and WD in T. distichumvar. imbricatum was significantly negatively related to H_v. (3) There was water limitation in the higher canopy of T. distichum and T. distichumvar. imbricatum. The theoretical water supply and demand ratio (r) calculated by Darcy’s Law and T_r confirmed this limitation. The theoretical maximum height when r = 0: T. distichum 32 m (upper bound of 95% confidence interval: 57 m); T. distichumvar. imbricatum 21 m (upper bound of 95% confidence interval: 27 m), was consistent with the maximum height recorded historically. (4) K_l, H_v and LMA in canopies of T. distichumvar. imbricatum were significantly higher than those of T. distichum,while P_n, T_r and G_s were significantly lower; hydraulic safety margin (HSM) in the middle and higher canopies of T. distichumvar. imbricatum was significantly higher, and P₅₀was significantly lower: The conservative hydraulic strategy of T. distichumvar. imbricatum corresponded to lower resource acquisition capacity, which in turn resulted in lower maximum growth height. The radical hydraulic strategy of T. distichum corresponded to higher resource acquisition capacity, which in turn resulted in higher maximum growth height.

Aims Vulnerability segmentation is one of physiological mechanisms underlying drought resistance for tree species. The aim of this study is to clarify the patterns of vulnerability segmentation and its physiological significance for woody species from drought-prone tropical and subtropical karst forests.

Methods A total of 57 typical woody species were selected from tropical and subtropical karst forests. We measured their leaf and stem vulnerability curves and minimum water potential to calculate vulnerability segmentation (P50_leaf-stemis the difference in cavitation resistance between leaf and stem, with larger values indicating stronger vulnerability segmentation) and hydraulic safety margin. We compared the differences in P50_leaf-stem between different plant taxa and explored the relationships between P50_leaf-stem and hydraulic safety margin.

Important findings (1) The P50_leaf-stem across the woody species ranged from -1.28 MPa to 4.63 MPa, with an average value of 1.32 MPa. Out of the 57 species, 49 species showed a positive P50_leaf-stem. (2) Shrub species showed higher P50_leaf-stem than tree species, and species from karst ridge showed higher P50_leaf-stem than those from karst valley. However, there was no significant difference in P50_leaf-stem between evergreen and deciduous species. (3) During the drought period, species with higher P50_leaf-stem tended to have smaller leaf hydraulic safety margin but larger stem hydraulic safety margin, indicating that occurrence of cavitation in leaves can reduce hydraulic risks of stems. This study confirms that vulnerability segmentation is an important hydraulic strategy for most tropical and subtropical karst woody species to deal with drought stress.

Aims Seasonal drought often occurs in the northern rocky mountainous area due to annual variation in precipitation. Exploring the effects of changes in precipitation on the sap flow characteristics and water sources of Platycladus orientalis is of great significance for structuring a stable ecosystem.

Methods In this study, the plantation of P. orientalis in the rocky mountainous area of Beijing was taken as the research target. The thermal dissipation probe (TDP) and hydrogen/oxygen isotope tracer technology were used to observe the P. orientalisunder different watering treatments. Meteorological, soil moisture, and other environmental factors were simultaneously monitored.

Important findings The results showed that natural precipitation and double precipitation > half precipitation > no precipitation were the main characteristics of P. orientalis sap flow. Precipitation increased the relative effective water content (REW) of soil, thereby stimulated the response of P. orientalis sap flow to environmental factors. Platycladus orientalis sap flow is mainly affected by the atmospheric vapor pressure deficit (VPD), and the impact of solar radiation (R_s) and wind speed (WS) is relatively low. The water source of P. orientalis changed with precipitation amounts. When the soil water content increased, its water source gradually changed to shallow soil. Compared with precipitation before, P. orientalis have increased the utilization ratio of 0-40 cm soil water after precipitation, except that P. orientalis in no precipitation treatment that exhibited no noticeable change. This change is more pronounced in natural precipitation and double precipitation treatments where a period of high relative water content after precipitation was more evident. To sum up, P. orientalis can adjust sap flow and the depth of water absorption from soil in responses to the changes in precipitation and soil moisture. This self-adaptive characteristic is conducive for trees to survive the extreme drought.

Aims The purpose of this study is to investigate the relationship between net photosynthetic rate (P_n) and nitrogen allocation in Gossypium spp. leaves, to analyze the limiting factors that affect the improvement of photosynthetic nitrogen use efficiency (PNUE), and to reveal the photosynthetic physiological regulation mechanism of nitrogen use efficiency.

Methods In this study, the gas exchange parameters, nitrogen content and proportion of different components, cell wall content per unit area (CW_area) of 16 genotypes of Gossypium spp., including G. hirsutum, G. barbadense, G. arboreum and G. herbaceum, were measured at flowering and bolling stages. The relationship between P_n,CW_area, PNUE and nitrogen content of different components was analyzed.

Important findings The results showed that when nitrogen content per unit area (N_area) was less than 3.75 g·m^-2, the P_n was very significant positively correlated with N_area, When N_area exceeds this value, the P_n was not correlated with N_area, while P_n had a significant positive correlation with Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) nitrogen content (N_r) and bioenergetics nitrogen content (N_b), a marginal positive correlation with light harvesting system nitrogen content (N_L) in photosynthetic apparatus, indicating the nitrogen content of photosynthetic apparatus (N_p) plays more important role than N_area in the photosynthetic capacity. In addition, P_n was also positively correlated with stomatal conductance (G_s). The low P_n of genotype ‘CHANG ZI 1 HAO’ was mainly related to low N_p, while low P_n of genotype ‘MOC-620’ was mainly related to low G_s. The proportion of nitrogen components in photosynthetic apparatus was also significantly and inversely correlated with PNUE. However, the fractions of nitrogen allocated to photosynthetic apparatus components were affected by N_area and CW_area. The correlation analysis showed that the fraction of nitrogen allocation to bioenergetics (P_b) and light harvesting system (P_L) was inversely correlated with N_area; the fraction of nitrogen allocation to Rubisco (P_r), P_L and P_b were significantly, extremely significant and marginally negatively correlated with CW_area, respectively. The results showed that with the increase of N_area and/or CW_area, leaves tended to allocate nitrogen to non-photosynthetic apparatus, including stored nitrogen component and cell wall component. In addition, cell wall may reduce PNUE by influencing mesophyll conductance (g_m). It’s likely to improve the photosynthetic nitrogen use efficiency in genotype ‘CHANG ZI 1 HAO’ and ‘MOC-620’ by increasing the fraction of N allocation to photosynthetic apparatus and decreasing allocation to non-photosynthetic apparatus.