Functional biogeography studies the spatio-temporal variations in patterns of traits and functional diversity, their ecological determinants and effects on ecosystem functioning. With the exponential growth in trait data, this field has developed rapidly in the recent decades and made major progress in exploring the response of species distribution, community structure and composition, and ecosystem properties on environmental changes based on traits. In this paper, we reviewed core objectives, historical developments, main research advance and future directions in the field of plant functional biogeography. Traits are the focus of research in functional biogeography. Here, we first described major findings on the spatial patterns of key traits in plant organs (i.e. leaves, stems, roots, and flowers, along with fruits and seeds) to the whole plants, and their relationships with environment, showing that traits variations are the results of plant adaptive evolution and environmental filtering. Secondly, we summarized the indicators of functional diversity, assessed the spatial distributions of functional diversity, and identified their determinants. We also summarized the main data sources of traits and related gap-filling approaches. Next, we reviewed trait associations and trade-offs among and within organs as well as in the entire plants, focusing on the global leaf economics spectrums and wood economics spectrum, and pointing out the strategies of plants to obtain and allocate important resource (i.e. carbon, nutrients and water). We summarized how trait-based approaches help to predict species distribution, and the link between trait diversity with ecosystem functions. We highlighted the challenges in current research and emphasized the importance to focus on the coordination and trade-offs among multiple traits along with both inter- and intra-specific trait variation in future research, transferring species-based models to individual-based ones, and to adopt approaches like trait networks to quantify the links among traits and their response to environmental changes, further to explore adaptation of plants across scales. Meanwhile, we suggested potential improvement in application of current research advances, which may be useful in constructing next-generation vegetation models and guiding the function-based conservation of plant diversity in future research.

Plant species change soil abiotic and biotic properties which in turn influence the performance of plants, leading to so-called “plant-soil feedbacks” (PSF). It is the prerequisite of plant-soil feedbacks that plant species can cause specific changes in soil microbial communities which are characterized by specialized soil pathogens and mutualists. Specialized microbes can have substantial effects on host plants, but likely do not influence the performance of non-host plants. PSF have been used to interpret ecological processes of different scales since the concept was proposed in the 1990s, such as succession, interspecific competition, biological invasion and effects of global changes on terrestrial ecosystems. In recent years, community ecologists and theoretical ecologists have started to integrate the research of PSF and community ecology, resulting in fundamental progress. In this review paper, we introduce soil microbe-mediated PSF and its implications for plant species coexistence, community structure and ecosystem functions. Classical PSF theory assumes that soil microbes can generate stabilizing process which promotes plant coexistence. However, recent studies show that soil microbes can also cause fitness difference between plant species which can influence species coexistence through equalizing process. Community ecologists predict that rare species have less negative or more positive PSF than abundant species, thereby leading to negative correlations between plant landscape abundance and PSF. However, empirical evidence demonstrates inconsistent patterns such as negative, positive and neutral correlations, and coevolution of plants and soil pathogens is key to reconcile these patterns. Soil microbes are also considered as a fundamental factor regulating succession. Dilution of soil microbial effects is a mechanism of positive plant diversity-productivity relationships. Specialist pathogens and mutualists accumulate in the soil of monocultures, but their negative and positive effects are diluted in multi-species mixtures, thereby increasing and decreasing biodiversity effects on productivity, respectively. We suggest three directions for future studies: empirical testing for specialization of plants and soil microbes, multi-dimensional species coexistence and eco-evolutionary dynamics in plant-soil feedbacks.

Aims The response of plant phenology to climate warming is an important element of global change research. At present, studies on plant phenology response to climate warming are in severe shortage for high-altitude ecosystems, especially regarding responses to multiple-level warming.

Methods We conducted a multiple-level warming experiment in an alpine meadow on Qingzang Plateau, and monitored plant phenology of two dominant species, including the timing of green up, budding and flowering in 2015, 2017, 2018 and 2021.

Important findings The results showed that plant phenology of different species exhibited various trends under warming. For Kobresia pygmaea, delay in phenological development, including green up, budding and flowering, was positively correlated with temperature increases. However, the timing of phenological stages of Potentilla saundersiana showed advancing first, and then delay with increasing temperature. These results suggest that plant phenology of alpine meadow asynchronously responds to increased temperature. In addition, temperature increase exerts delayed effects on plant phenology over long-term. The structural equation modeling showed that temperature increase consistently delayed the green up of K. pygmaea, and low-level warming advanced phenological development of P. saundersiana, but this advancing trend reversed under high-level warming. Importantly, soil moisture plays a key role in determining the magnitude and direction of phenological response to climate warming in our study. Our findings indicate the asynchronous characteristics of plant phenology response to climate warming in alpine meadow ecosystems, and provide basis to predict responses of high-altitude ecosystems to climate change in the future.

Aims Many factors can influence the invasiveness of alien plants, and among them endophytes may play a key role. Thus, the aim of this study is to investigate the effects of endophytic nitrogen-fixing bacteria on the growth strategy of invasive plants.

Methods We grew the invasive plant Sphagneticola trilobata and its native congener S. calendulacea infected by endophytic nitrogen-fixing bacteria or not under two nitrogen levels (low and high) and compared their growth and total nitrogen content.

Important findings The endophytic bacteria Kosakonia sp. WTB-JS007, isolated from S. trilobata, had different effects on the growth strategy of the two species (S. trilobata and S. calendulacea) and such an effect did not depend on the nitrogen levels. Under the low nitrogen level, inoculation with WTB-JS007 showed no significant effect on the growth or total nitrogen content of S. calendulacea, but significantly increased aboveground biomass (by 30.48%), promoted stolon length, decreased belowground biomass (by 56.58%), and enhanced total nitrogen content (by 47.51%) of S. trilobata. Similarly, under the high nitrogen level, endophytic bacteria stimulated the aboveground growth of S. trilobata, but had no effect on that of S. calendulacea. These results suggest that endophytic nitrogen-fixing bacteria can differently affect the growth, biomass allocation and nitrogen uptake of invasive and its co-occurring native plant species. Such a difference in the growth strategy can facilitate the rapid growth and expansion of the aboveground part of invasive plants and thus promote their invasiveness.

Aims Exotic plants invasions impact both function and stability of ecosystems. Compared with native plants, invasive plants are generally characterized by stronger stress resistance and resource utilization abilities. Nitrogen deposition, as an important issue of global change, can directly increase soil nitrogen availability and promote plant invasions. Soil microorganisms, including both fungi and bacteria, play an important role in regulating soil nutrient availability and nutrient uptake, both of which are highly associated with successful invasion of plants. However, the effects of soil fungi and bacteria on the growth of invasive plants from different origins under nitrogen deposition background remain unclear.

Methods To understand the effects of soil fungi and bacteria on the growth of invasive plants from different provenances with nitrogen addition, both original and invasive Triadica sebifera were chosen as model plant populations. Soil bacterial inhibitor (streptomycin) and fungal inhibitor (iprodione) were applied to regulate the activity of soil bacteria and fungi communities. Nitrogen deposition was simulated by nitrogen addition treatment to understand the growth response of T. sebifera with different population origins as affected by different soil microbial communities under the context of nitrogen deposition.

Important findings The results showed that, invasive provenance of T. sebifera presented substantial growth advantage in terms of plant height, leaf number and biomass compare with that of native provenance. Soil bacterial and fungal inhibitor applications significantly reduced aboveground biomass of T. sebifera. Moreover, the growth of T. sebifera is more dependent on soil bacteria. Nitrogen addition and its interaction with soil bacteria and fungi significantly affected both the growth and resource allocation of T. sebifera, which could have enhanced the competitive ability of T. sebifera for resources during range invasion process, and should be focused in future studies.

Aims Leaf traits are closely related to plant light use efficiency and photosynthesis. They can indicate plant adaptation strategies to the environment. Spartina alterniflora is a major alien invasive plant in many coastal wetlands, and it seriously threatens coastal wetland ecosystems in China. Tidal flooding is one of the main limiting factors for the growth and distribution of S. alterniflora in coastal wetlands. However, there has been very little research directly examining the pattern and adaptation mechanism of leaf traits of S. alterniflora along a tidal gradient.

Methods In this study, a tidal elevation control platform was established in Zhangjiang Estuary, Fujian. We studied the response pattern and driving factors of leaf functional traits (length, width, length width ratio, area, dry mass, and specific leaf area) of S. alterniflora to the tidal gradient (relative elevation).

Important findings The results showed that: (1) The leaf length, leaf width, leaf area, and leaf dry mass of S. alterniflora decreased with increasing elevation, whereas the leaf length width ratio increased with increasing elevation. (2) The specific leaf area of S. alterniflora and elevation showed a hump-shaped relationship. (3) The effects of inundation frequency, soil porewater salinity, and soil water content on leaf traits were different. The leaf length, leaf width, leaf area, and leaf dry mass of S. alterniflora increased with increasing inundation frequency and soil water content, but decreased with increasing soil porewater salinity; the leaf length width ratio of S. alterniflora decreased with increasing inundation frequency and soil water content, but increased with increasing soil porewater salinity; the specific leaf area of S. alterniflora increased first and then decreased with increasing inundation frequency, and increased with increasing soil water content. In summary, the patterns and main driving factors of leaf traits of S. alterniflora differed along a tidal gradient, and this finding may be due to differences in the effects of leaf traits on plant physiological processes. Thus, S. alterniflora can adapt to changes in tidal elevation by adjusting leaf traits and their trade-offs. This study provides a new perspective for understanding and predicting the ecological adaptation of S. alterniflora to sea level rise in coastal wetlands.

Aims The dynamics of radial growth and micro-variation in tree stems are jointly controlled by species-specific properties and environmental factors, and reflect the characteristics of trees in response to change of environments. Exploring the dynamic characteristics of diameter changes in various temporal scales and their relationships with environmental factors is an important way for revealing the species ecophysiological properties, particularly in semiarid areas where the trees are subjected to high risk of water stress. Our objectives were to clarify the year-round radial growth patterns and dynamics of micro-variations in stems of two semiarid afforestation species, Quercus mongolicavar. liaotungensis and Robinia pseudoacacia,and their response to major environmental factors.

Methods We used DC3 dendrometers to monitor stem diameters in order to investigate year-round dynamics of radial growth and stem micro-variations of the two species, Q. mongolicavar. liaotungensis and R. pseudoacacia, in the loess hilly region. Soil moisture dynamics and the main meteorological factors driving transpiration were simultaneously monitored for analyses on the relationship between stem micro-variation and environmental factors.

Important findings The year-round change of stem diameter in the two species could be divided into a contraction phase during the non-growing season, a transition phase and a growing phase during the growing season. The radial growth of Q. mongolicavar. liaotungensis and R. pseudoacacia started around April 7 and May 4, respectively, and ceased in late September. The diameter changes for the growing phase in Q. mongolicavar. liaotungensis and R. pseudoacacia could be fitted by the exponential saturation growth function and the linear growth function, respectively. The diurnal courses of stem micro-variation for both species were grouped monthly, which showed a typical non-growing (November to March of next year) and a typical growing season (June to September) patterns, while it differed between the two species in April and May, probably due to their species-specific phenological rhythm. The daily maximum diameter shrinkage for both tree species was negatively correlated with air temperature and air vapor pressure deficit during the non-growing season, but positively with those during the growing season. The diameter shrinkage and expansion during non-growing season were strongly influenced by air temperature, while those during the growing season were mainly caused by changes in water within the stem due to transpiration and replenishment. The daily variation of diameter could be used to characterize the water loss through transpiration. The water loss per unit of air vapor pressure deficit in Q. mongolicavar. liaotungensis showed significant difference under two periods differing in soil moisture condition, whereas that in R. pseudoacacia did not reach a significant level, indicating that Q. mongolicavar. liaotungensis adjusted its response level of transpiration to the driving factor. Those results may contribute to clarifying the mechanism of stem micro-variation in the two species and the species-specific strategies for regulation of leaf transpiration in response to changes in soil water condition.

Aims Accurately quantifying the contribution of shallow, middle and deep soil water sources to the root water uptake is the prerequisite for understanding water uptake strategy of plants. This paper evaluates the effects of different water isotopes input methods on plant water sources analysis results in Bayesian mixing model MixSIAR.

Methods Soil and plant xylem samples were taken five times from May to September in 2019 in two apple (Malus pumila) orchards at 7- and 18-year age in Changwu Tableland of Shaanxi Province. Soil water contents and isotope ratios (δ²H and δ¹⁸O) were measured, and the different input methods of single isotope (²H and ¹⁸O), dual isotopes (²H & ¹⁸O) and xylem hydrogen corrected dual isotopes (²H(+8.1) & ¹⁸O) coupled with MixSIAR model were used to estimate the contribution ratio of different soil layers (0-0.4, 0.4-2, >2 m root depth) to orchards root water uptake.

Important findings The results showed that compared to the ²H isotope method, the contribution from soil layer below 2 m was lower and that from the surface 0-0.4 m was higher using the ¹⁸O isotope method, which was close to the ²H(+8.1) & ¹⁸O isotope method. Compared with ²H & ¹⁸O dual isotopes method, the contribution ratio from surface 0-0.4 m soil layer was higher using the ²H(+8.1) & ¹⁸O method when the surface soil water isotope was enriched, and that was lower when the surface soil water isotope was depleted. The corrected apple xylem hydrogen isotopes were closer to the evaporation line of soil water isotopes, thus the analysis methods of ¹⁸O and ²H(+8.1) & ¹⁸O accorded more with the isotope mass balance during root water uptake than ²H and ²H & ¹⁸O methods. Soil water contents in 0-2 m in 18 years old apple orchard showed greater seasonal variation than that in 7 years old apple orchard, and are more dependent on 0-0.4 m surface soil water. For the root water uptakes in 7- and 18-year apple orchard, the yearly-averaged contributions of deep soil water were 19% and 23%, respectively, showing no significant difference. We suggest that more attention should be drawn on the influence of different isotope input methods when using water stable isotopes to estimate plant water sources contribution in future studies.

Aims The storage and regulation mechanisms of non-structural carbohydrates (NSC) in plants reflect the response of plant growth and metabolism to environmental changes. In the scenario of global warming, the increasing frequency of extreme climate events such as heat wave, which is bound to affect the carbon budget and carbon distribution of plants. However, the effects of complex heat wave patterns (different frequency and interval time) on the distribution and regulation mechanism of NSC among different plant organs are still unclear. The objective of this study is to elucidate the mechanism of carbon budget at the level of plant organs under heat waves.

Methods We conducted the simulated heat wave events through the combined action of open top chamber (OTC) and electric heater to examine the changes and distributions of NSC content and biomass among organs (stems, leaves and roots) of Phoebe bournei and Schima superba.Five different heat wave frequency and interval treatments were set, including no heat wave (CK), one heat wave (HW), two heat wave interval of 7 days (2HW₇), two heat wave interval of 30 days (2HW₃₀) and two heat wave interval of 45 days (2HW₄₅).

Important findings (1) The repeated heat wave (2HW₇)significantly increased soluble sugar content in stems of P. bournei, but had no significant effect on soluble sugar and NSC contents in roots and leaves. 2HW₇ significantly increased starch content in the stem and root of S. superba, but had no significant effect on soluble sugar and NSC contents. These results indicated that the NSC allocation and regulation in different broad-leaved tree species and organs response to heat waves were divergent. (2) NSC content in P. bournei stem under 2HW₃₀ and 2HW₄₅ were significantly lower than 2HW₇, and starch content in S. superba stem and root were also significantly lower than 2HW₇, which was no significant difference with CK. These results suggested that multiple heat waves exist a cumulative effect. The magnitude of the cumulative effect was closely related to the heat wave interval time. (3) The biomass of all P. bournei organs in 2HW₇ treatment group was significantly increased, while the biomass of stem and root of S. superba showed no significant differences under different heat wave patterns, suggesting that P. bournei increased the storage of NSC and distributed to all organs to resist the heat wave stress, while S. superba stored photosynthetic products as starch only in leaf to resist the heat wave stress. Our results revealed that heat waves with different frequencies and intervals had an accumulative effect on plants, and the ability of plant to cope with heat wave stress via regulating NSC content in different organs was related to the accumulative effect.

Aims In order to better understand the mechanisms of nocturnal water activity in tree stems, this paper explores the spatiotemporal dynamics of nocturnal sap flow and stem water filling in typical poplar plantations in the North China Plain and environmental influencing factors.

Methods Taking rainfed Populus tomentosa as the research object, the thermal diffusion method was used to continuously monitor the sap flow rate at different heights of stem in different growing periods. Soil water content and meteorological factors were measured simultaneously. Dynamics of nocturnal sap flow at different heights and its correlations with environmental factors were compared.

Important findings Before the rainy season, the ratio of nocturnal sap flow to daily sap flow at 0.35 and 1.30 m of the stem was significantly higher than that after the rainy season, while the ratio at 7.00 m increased by 49% after the rainy season. Nocturnal sap flow rate at different heights of the stem showed a high synchronization before the rainy season, and decreased with the increase of stem height. After the rainy season, the main water resource of nocturnal water use changed from root water uptake to stored water in stem base, which leading to 66% decrease of nocturnal sap flow at stem base. Meanwhile, the spatial characteristics of nocturnal sap flow at different heights also varied in different periods. After the rainy season, stem water refilling mainly occurred in 4.00-7.00 m stem segment with mean daily water refilling amount of 4.16 L, while the refilling amount in 1.30-4.00 m stem segment was much lower than others, possibly due to its primary function of water transport. Before the rainy season, vapor pressure deficit, air temperature and soil water content at 3 m depth were significantly correlated with nocturnal sap flow rate at different heights, but this relation was obviously weakened after the rainy season. And the correlations between the environmental impact factors and nocturnal sap flow were different at each height. This study finds the spatiotemporal variations of nocturnal sap flow and stem water refilling, which provide scientific support for optimizing diurnal water cycle and motion process simulation of poplar plantation in North China Plain.

Aims Under the background of changing carbon cycle process in forest ecosystems caused by global environmental change, the microbial carbon use efficiency (CUE) in forest rhizosphere soil is critical to determine the strength of microbial anabolism and catabolism in forest ecosystems. However, the variation and influencing factors of microbial CUE in rhizosphere soils at different altitudes remain undetermined.

Methods Rhizosphere soil at six different altitudes spanning four forest belts in Taibai Mountain was sampled to determine the physical and chemical properties, extracellular enzyme activity, and characteristics of microbial community and vegetation. Based on the stoichiometric ratio, the soil microbial CUE was estimated. Furthermore, the variation in microbial CUE of rhizosphere soil along the altitude gradient was analyzed to quantify the influencing factors of microbial CUE.

Important findings The results showed that the microbial CUE of rhizosphere soil exhibited an overall upward trend with the increase in altitude. The microbial CUE increased by 4.36% from 0.505 at the lowest altitude to 0.527 at the highest altitude, but decreased at 1 603 and 2 405 m. Based on the Mantel analysis, we identified four categories of factors (i.e., altitude, soil matrix, vegetation and microbe) that related to microbial CUE in rhizosphere soil. The variations of microbial CUE in rhizosphere soil are affected by multiple environmental factors, with the dominant factor being soil matrix (such as dissolved organic carbon (DOC) content, ammonium nitrogen (NH⁺₄-N) content), followed by vegetation. Furthermore, the altitude factor and the microbial factor explained 2.6% and 3.1% of the CUE change, respectively. Although the microbial factors exerted no significant impact on microbial CUE, soil matrix, vegetation and microbe jointly explained 47.0% of the microbial CUE change. The variance partitioning analysis (VPA) quantitatively revealed the contribution of environmental factors to the change of microbial CUE, where soil matrix and vegetation explained 17.0% and 5.7% of the variation, respectively. While the interaction between soil matrix and vegetation accounted for 31.9% of the changes in microbial CUE. The above results indicated that the high-altitude rhizosphere soil in Taibai Mountain has a high carbon sequestration potential, and the carbon sequestration of forest rhizosphere soil may decrease with the intensification of global warming. The vertical temperature difference and the vertical differentiation of the vegetation belt induced by altitude gradient will alter the growth and metabolism environment of microorganisms in the rhizosphere soil. The comprehensive effect of multiple environmental factors dominated by soil matrix impacts the CUE of soil microorganisms, and ultimately changes the assimilation and catabolism processes of soil carbon. The results of this study can provide a scientific basis for the carbon assimilation capacity and carbon sequestration potential of forest soil microorganisms in Qinling Mountains, as well as the forest soil carbon cycle under the background of global change.