Biomass allocation to different organs is a fundamental plant ecophysiological process to better respond to changing environments; yet, it remains poorly understood how patterns of biomass allocation respond to nitrogen (N) additions across terrestrial ecosystems worldwide.

We conducted a meta-analysis using 5474 pairwise observations from 333 articles to assess how N addition affected plant biomass and biomass allocation among different organs. We also tested the ‘ratio-based optimal partitioning’ vs. the ‘isometric allocation’ hypotheses to explain potential N addition effects on biomass allocation.

We found that (i) N addition significantly increased whole plant biomass and the biomass of different organs, but decreased root:shoot ratio (RS) and root mass fraction (RMF) while no effects of N addition on leaf mass fraction and stem mass fraction at the global scale; (ii) the effects of N addition on ratio-based biomass allocation were mediated by individual or interactive effects of moderator variables such as experimental conditions, plant functional types, latitudes and rates of N addition and (iii) N addition did not affect allometric relationships among different organs, suggesting that decreases in RS and RMF may result from isometric allocation patterns following increases in whole plant biomass. Despite alteration of ratio-based biomass allocation between root and shoot by N addition, the unaffected allometric scaling relationships among different organs (including root vs. shoot) suggest that plant biomass allocation patterns are more appropriately explained by the isometric allocation hypothesis rather than the optimal partitioning hypothesis. Our findings contribute to better understand N-induced effects on allometric relationships of terrestrial plants, and suggest that these ecophysiological responses should be incorporated into models that aim to predict how terrestrial ecosystems may respond to enhanced N deposition under future global change scenarios.

The Nihewan Basin of North China, considered the cradle of Eastern civilization, contains a set of late Cenozoic strata and artifacts used by Homo erectus in the early Pleistocene (~1.66 Ma to 780 ka) and the cranial bones and teeth of early H. sapiens from the late middle Pleistocene (~370 to 260 ka). Palynological studies provide an opportunity to explore the living environment of early humans.

Palynological samples from the Hutouliang Section (~603–587 ka) of the Xiaodukou Formation of the Nihewan Basin were treated by heavy liquid flotation. Based on the palynological assemblages from the section, vegetation and climate in the Nihewan Basin were reconstructed.

The dynamic vegetation changed from temperate needle- and broad-leaved mixed forest-steppe (mainly Picea, Abies, Betula, Juglans, Artemisia and Chenopodiaceae) to conifer forest (mainly Pinus, Picea and Abies), which saw the replacement of H. erectus by early H. sapiens. The comparison of the Nihewan Basin with other human sites around the world during the same period reveals that early humans preferred to live in caves, accompanied by relatively open steppe or forest-steppe environments, inhabited by numerous mammals. Therefore, it is inferred that the emergence of dense conifer forest and the disappearance of open steppe environments in the Nihewan Basin at approximately 603–587 ka provide new evidence that early humans followed most mammals to steppe or forest-steppe environments and thus left the Nihewan Basin. These new findings not only enrich our knowledge of early human behavior, such as their diet, migration and settlement, but also fill in gaps in paleovegetation and paleoenvironmental research in the Nihewan Basin during the middle Pleistocene (780–400 ka).

Grassland degradation represents a major challenge in the maintenance of grassland productivity. This process has dramatic impacts on energy flows and soil nutrient dynamics, thus directly or indirectly influencing soil microbes. Here, we aim to (i) examine changes in soil microbial composition, diversity and functionality in response to different levels of grassland degradation (i.e. non-degraded, moderately and severely degraded) in a temperate grassland in Inner Mongolia, and (ii) elucidate biotic and abiotic factors that are responsible for these changes.

The composition structure of soil microbial community was determined by high-throughput sequencing. The functionality of bacterial communities was examined using the tool of FAPROTAX, and functional guilds of fungal communities were quantified using the FUNGuild pipeline.

Grassland degradation significantly decreased soil bacterial diversity but it did not affect fungal diversity. Belowground biomass, soil organic carbon and total nitrogen were positively related to changes in diversity of bacterial community. Grassland degradation significantly increased the relative abundance of Chloroflexi (from 2.48% to 8.40%) and decreased Firmicutes (from 3.62% to 1.08%) of bacterial community. Degradation also significantly increased the relative abundance of Glomeromycota (from 0.17% to 1.53%) and decreased Basidiomycota (from 19.30% to 4.83%) of fungal community. The relative abundance of pathogenic fungi (Didymella and Fusarium) was decreased significantly by degradation. In addition, degradation had a significant impact on putative functionality of soil bacteria related to soil carbon and nitrogen cycling. Our results suggest that soil bacterial community is more sensitive than fungal community in response to degradation in the temperate grassland.

Intraspecific variation in plant traits has important consequences for individual fitness and herbivore foraging. For plants, trait variability across spatial dimensions is well documented. However, temporal dimensions of trait variability are less well known, and may be influenced by seasonal differences in growing degree days (GDD), temperature and precipitation. Here, we aim to quantify intraspecific temporal variation in traits and the underlying drivers for four commonly occurring boreal plant species.

We sampled the elemental and stoichiometric traits (%C, %N, %P, C:N, C:P, N:P) of four common browse species’ foliage across 2 years. Using a two-step approach, we first fitted generalized linear models (GzLM, n = 24) to the species’ elemental and stoichiometric traits, and tested if they varied across years. When we observed evidence for temporal variability, we fitted a second set of GzLMs (n = 8) with temperature, productivity and moisture as explanatory variables.

We found no evidence of temporal variation for most of the elemental and stoichiometric traits of our four boreal plants, with two exceptions. Year was an important predictor for percent carbon across all four species (R2 = 0.47–0.67) and for multiple elemental and stoichiometric traits in balsam fir (5/8, R2 = 0.29–0.67). Thus, variation in percent carbon was related to interannual differences, more so than nitrogen and phosphorus, which are limiting nutrients in the boreal forest. These results also indicate that year may explain more variation in conifers’ stoichiometry than for deciduous plants due to life history differences. GDD was the most frequently occurring variable in the second round of models (8/8 times, R2 = 0.21–0.41), suggesting that temperature is an important driver of temporal variation in these traits.

Field and common garden comparisons are commonly performed to test the rapid evolution of increased vigor in introduced plant populations. Latitudinal clines in phenotypic traits can obscure such evolutionary inferences, particularly when native or introduced populations are distributed across large geographic ranges. We tested whether the latitudinal clines influence comparisons between introduced and native populations of Senecio vulgaris.

Senecio vulgaris is native to Europe but has been introduced in northeastern and southwestern China. To evaluate the performance in terms of growth and reproduction between native European populations and introduced Chinese populations, we compared plant height, number of branches and number of capitula in field populations in native and introduced ranges and in a common garden in Switzerland.

The introduced Chinese populations performed better than the native European populations in the field in terms of plant height and number of capitula, which was consistent with the prediction of the evolution of the increased competitive ability (EICA) hypothesis. The Chinese populations produced more capitula than the European populations when the latitudinal cline was considered in the common garden comparison. When we compared the traits of the northeastern Chinese, southwestern Chinese and European populations in both the field and common garden, the northeastern Chinese populations, at latitudes similar to those of the European populations, exhibited greater plant size and more capitula than the European populations in both the field and common garden. However, the southwestern Chinese populations, at latitudes much lower than those of the European populations, did not perform better than the native populations in terms of reproduction. In conclusion, our results suggest that latitudinal clines in phenotypic traits should be considered in field and common garden comparisons when introduced populations are geographically structured.

The evolution and expression of dispersal-related traits are intertwined with those of other life-history functions and are manifested within various physiological constraints. Such a relationship is predicted between inbreeding levels and dispersability, which may be anatomically and ontogenetically linked so that the selection pressures on one may affect the other. While both the effect of inbreeding on reproductive success and on dispersal strategies received much attention, only a few studies considered both simultaneously. Furthermore, such studies often rely on two dichotomic representations of breeding and dispersal: using selfing versus outcrossing as a representation of breeding level, and dispersal ratio as the sole representation of dispersal strategy.

Here, we used pollination experiments in the heterocarpic Crepis sancta (Asteraceae) to expand in two different manners on the common practice of using dichotomic representations of breeding and dispersal. First, we used pollination treatments that represent a continuum from selfing through pollination by kin to pollination by a distant neighbor. Second, we measured a whole set of continuous morphological and dispersal-related traits, in addition to measurements of reproductive success and dispersal ratio.

The proportion of developed capitula and the number of both dispersed and non-dispersed achenes were significantly lower in the self-pollination treatment in comparison to the outcrossed treatments. The effect of pollen sources on dispersal ratio was not statistically significant, though self-pollinated plants rarely produced non-dispersing seeds. Achene’s biomass increased with distance between parent plants, but pappus width did not, leading to a nonsignificant effect of pollination on falling velocity. Overall, pollen source affected mainly traits that were associated with reproductive output, but it had no clear effect on predominately dispersal-related traits. Such differences in the response of reproduction and dispersal traits to variation in pollen source suggest that dispersal-related selection is probably weak and/or masked by other forces.

Plant populations in managed grasslands are subject to strong selection exerted by grazing, mowing and fertilization. Many previous studies showed that this can cause evolutionary changes in mean trait values, but little is known about the evolution of phenotypic plasticity in response to land use. In this study, we aimed to elucidate the relationships between phenotypic plasticity—specifically, regrowth ability after biomass removal—and the intensity of grassland management and levels of temporal variation therein.

We conducted an outdoor common garden experiment to test if plants from more intensively mown and grazed sites showed an increased ability to regrow after biomass removal. We used three common plant species from temperate European grasslands, with seed material from 58 to 68 populations along gradients of land-use intensity, ranging from extensive (only light grazing) to very intensive management (up to four cuts per year).

In two out of three species, we found significant population differentiation in regrowth ability after clipping. While variation in regrowth ability was unrelated to the mean land-use intensity of populations of origin, we found a relationship with its temporal variation in Plantago lanceolata, where plants experiencing less variable environmental conditions over the last 11 years showed stronger regrowth in reproductive biomass after clipping. Therefore, while mean grazing and mowing intensity may not select for regrowth ability, the temporal stability of the environmental heterogeneity created by land use may have caused its evolution in some species.

In multiflowered species, the architecture of inflorescences is of primary importance in shaping plant attractiveness. The aim of this study was to disentangle the role of inflorescence traits in plant female reproductive success and pollination patterns along the inflorescence in the lax-flowered orchid Anacamptis laxiflora, a terrestrial species exploiting a deceptive pollination strategy. We also evaluated whether the relationship between inflorescence traits and female reproductive success was modified by the height of surrounding vegetation and/or by population density.

We delimited experimental plots in a natural population of A. laxiflora. We tallied the individuals within each plot and categorized low-density plots and high-density plots; then, in part of the plots we manually removed surrounding grass thus producing an equal number of plots with high grass and low grass. Within these plots, we recorded inflorescence traits and female reproductive success (i.e. the number of fruit and their position along the inflorescence). We analyzed these data using generalized linear mixed-effects models (GLMMs) and calculated selection gradients.

We found that all the investigated inflorescence traits influenced female reproductive success. In particular, our GLMMs showed that ‘average flower distance’ was the best predictor for shaping reproductive success patterns. We detected significant positive selection on the investigated inflorescence traits, but these selective trends were strictly linked to both the height of the surrounding vegetation and the population density, suggesting a significant influence of local environmental context in shaping selective patterns. Female reproductive success was not linked to the position of flowers along the inflorescence, suggesting that pollinators visit flowers randomly along the inflorescence without a detectable preference for a specific part. This study highlights the importance of inflorescence traits in shaping female reproductive success of multiflowered deceptive orchids, and confirms a primary role for the environmental context in modifying pollinator-mediated selection patterns.

Positive biodiversity–ecosystem functioning relationships have been widely reported, predominately from grassland ecosystems. However, this does not necessarily have to apply accordingly in more complex situations such as in forests across different vertical strata. For instance, overstorey tree species richness has been shown to be associated with a lower understorey productivity. Whether or not tree species richness effects add to understorey productivity by increasing (i.e. due to habitat heterogeneity) or reducing resource availability (i.e. through increasing competition) and whether understorey productivity is indeed being governed more strongly by tree species identity are likely to change over time. Moreover, studies also suggested that richness–productivity relationships change with the environmental context. Using an experimental forest plantation with manipulated tree species richness, this study examined these temporal and environmental dynamics across strata.

In the context of the Biodiversity-Ecosystem Functioning project in subtropical China (BEF-China), we made use of understorey biomass samples repeatedly collected over a time period of 3 years along a tree species richness gradient. The effects of tree species richness, tree species identities and time were studied across different environmental treatments for their impact on understorey biomass.

While we found significant and consistent tree layer identity effects on understorey biomass, no such effect was encountered for tree species richness. Our results also indicate that among structural layers in forests, there might not be a single, generalizable overstorey species richness–understorey productivity relationship, and that the extent as to which overstorey-related environmental factors such as light transmittance contribute to understorey productivity change with time. Overall, we demonstrate that temporal dynamics should be considered when studying relationship among structural layers in forests.

Given the key functional role of understorey plant communities and the substantial extent of forest cover at the global scale, investigating understorey community responses to elevated CO₂ (eCO₂) concentrations, and the role of soil resources in these responses, is important for understanding the ecosystem-level consequences of rising CO₂ concentrations for forest ecosystems. Here, we evaluated how experimentally manipulating the availabilities of the two most limiting resources in an extremely phosphorus-limited eucalypt woodland in eastern Australia (i.e. water and phosphorus) can modulate the response of the understorey community to eCO₂ in terms of germination, phenology, cover, community composition and leaf traits.

We collected soil containing native soil seed bank to grow experimental understorey plant communities under glasshouse conditions.

Phosphorus addition increased total plant cover, particularly during the first 4 weeks of growth and under high water conditions, a response driven by the graminoid component of the plant community. However, the treatment differences diminished as the experiment progressed, with all treatments converging at ~80% plant cover after ~11 weeks. In contrast, plant cover was not affected by eCO₂. Multivariate analyses reflected temporal changes in the composition of plant communities, from pots where bare soil was dominant to high-cover pots dominated by a diverse community. However, both phosphorus addition and the interaction between water availability and CO₂ affected the temporal trajectory of the plant community during the experiment. eCO₂ also increased community-level specific leaf area, suggesting that functional adaptation of plant communities to eCO₂ may precede the onset of compositional responses. Given that the response of our seed bank-derived understorey community to eCO₂ developed over time and was mediated by interactions with phosphorus and water availability, our results suggest a limited role of eCO₂ in shaping plant communities in water-limited systems, particularly where low soil nutrient availability constrains productivity responses.

The factors affecting species abundance are a subject of ongoing debates in community ecology. Empirical studies have demonstrated that tree abundance is affected by plant functional traits and negative density dependence (NDD). However, few studies have focused on the combined effects of NDD and plant functional traits on species abundance.

In this study, we used tree functional traits and two census data from a 50-ha forest dynamic plot in the Heishiding (HSD) Nature Reserve to explore the combined effects of functional traits and NDD on species abundance. Using hierarchical Bayesian models, we analyzed how neighbor densities affected the survival of saplings from 130 species and extracted posterior means of the coefficients to represent NDD. The structural equation modeling (SEM) analysis was then applied to investigate the causal relationships among species functional traits, NDD and species abundance.

SEM showed that tree functional traits, including specific leaf area (SLA), leaf area (LA), leaf dry matter content (LDMC), leaf N content (LNC), maximum electron transport rate (ETR_max) and conspecific adult negative density dependence (CNDD_adult), together explained 20% of the total variation in tree abundance. Specifically, SLA affected tree abundance both directly and indirectly via CNDD_adult, with a totally negative influence on abundance. LDMC and LNC had only indirect effects mediated by CNDD_adult on tree abundance. ETR_max and LA had directly negative effects on abundance, but their direct connections with CNDD_adult were not observed. In addition, CNDD_adult was negatively correlated with species abundance, indicating that abundant species are under stronger NDD. Among these investigated traits, SLA contributed the most to the variation in CNDD_adult and abundance. We argued that our findings of trait–CNDD_adult–abundance relationships can improve our understanding of the determinants of species commonness and rarity in forests.

Foliar pH of terrestrial plants, a trait tightly associated with plant physiology and nutrient utilization, varies with plant functional types (PFTs) and environmental changes. However, it is yet unclear about the variation in foliar pH of aquatic plants, and the difference between aquatic and terrestrial plants.

Foliar pH, leaf carbon, nitrogen content of plants along the lakeshore zones and the environmental conditions (water or soil pH, water status) of the corresponding vegetation of three small plateau lakes were investigated, to determine the variation and potential influence factors of foliar pH at both PFT and community levels.

Foliar pH varied largely among aquatic plants, and across aquatic, helophytic and terrestrial plants. Floating-leaved macrophytes had more acidic foliage (pH = 4.21 ± 0.05) than emergent (5.71 ± 0.07) and submerged macrophytes (5.82 ± 0.06). Foliar pH of aquatic herbs (5.43 ± 0.10) was lower than that of helophytic (6.12 ± 0.07) and terrestrial herbs (5.74 ± 0.05). Terrestrial herbs had significantly higher foliar pH than woody plants. The variation in foliar pH across PFTs may be mainly ascribed to leaf structure, light utilization and nutrient characteristics. Consistent with the pattern on PFT level, aquatic communities had more acidic foliage than terrestrial communities, which was mainly shaped by species composition, water status and environmental pH. This study documented the first-time foliar pH of aquatic plants, and comparison of foliar pH among various plant types at a landscape scale. Our results provide bases for further exploration of the underlying mechanism and its ecological significance for wetland ecosystems.

Competition among plants in a community usually depends on their nitrogen (N)-use efficiency (NUE) and water-use efficiency (WUE) in arid and semi-arid regions. Artemisia frigida is an indicator species in heavily degraded grassland, however, how its NUE and WUE respond to N addition in different successional stages is still unclear, especially with mowing, a common management practice in semi-arid grasslands.

Based on a long-term controlled experiment with N addition and mowing in an abandoned cropland from 2006 to 2013, we investigated the NUE and WUE of A. frigida in two patches (i.e. grass and herb patches) in 2013 which represented two potential successional stages from herb to grass communities. The coverage of A. frigida was higher (about 50%) in the herb patch than in the grass patch (about 10%). Stable isotopic C (δ 13C) and N (δ 15N) as well as C and N pools were measured in plants and soils. NUE was calculated as leaf C/N, and leaf δ 13C values were used as a proxy for WUE.

N addition did not affect WUE of A. frigida, but significantly decreased NUE by 42.9% and 26.6% in grass and herb patches, respectively. The response of NUE to N addition was related to altering utilization of different N sources (NH₄+vs. NO₃−) by A. frigida according to the changed relationship between leaf δ 15N/soil δ 15N and NUE. Mowing had no effect on NUE regardless of N addition, but significantly increased WUE by 2.3% for A. frigida without N addition in the grass patch. The addition of N reduced the positive effect of mowing on its WUE in grass patch. Our results suggested that decreased NUE and/or WUE of A. frigida under mowing and N addition could reduce its competition, and further accelerate restoration succession from the abandoned cropland to natural grassland in the semi-arid region.

The survival and ecological distribution of plants in arid habitats are mainly conditioned by water availability and physiological adaptations to withstand drought. In the present study, we have compared the physiological responses to drought of two Retama raetam (retama) subspecies from Tunisia, one of them living under the desert climate (subsp. raetam) and the other one growing on the coast (subsp. bovei).

To physiologically characterize the two R. raetam subspecies, and to elucidate their main mechanisms underlying their tolerance to drought stress, parameters related to seed germination, growth, photosynthesis (net photosynthetic rate, intracellular CO₂ concentration, transpiration rate, stomatal conductance and water-use efficiency) and accumulation of osmolytes (proline, glycine betaine [GB] and soluble sugars) were determined in 4-month-old plants subjected to stress for up to 1 month.

Drought significantly inhibited germination, growth and all the evaluated photosynthetic parameters. Plants of R. raetam subsp. bovei were severely affected by drought after 3 weeks of treatment when photosynthesis rates were up to 7-fold lower than in the controls. At the same time, proline and GB significantly accumulated compared with the irrigated controls, but much less than in R. raetam subsp. raetam; in the latter subspecies, proline and GB increased to levels 24- and 6-fold higher, respectively, than in the corresponding controls. In summary, the population living in the desert region exhibited stronger tolerance to drought stress than that adapted to the semiarid littoral climate, suggesting that tolerance in R. raetam is dependent on accumulation of osmolytes.

Terrestrial ecosystem carbon (C) uptake is remarkably regulated by nitrogen (N) availability in the soil. However, the coupling of C and N cycles, as reflected by C:N ratios in different components, has not been well explored in response to climate change.

Here, we applied a data assimilation approach to assimilate 14 datasets collected from a warming experiment in an alpine meadow in China into a grassland ecosystem model. We attempted to evaluate how experimental warming affects C and N coupling as indicated by constrained parameters under ambient and warming treatments separately.

The results showed that warming increased soil N availability with decreased C:N ratio in soil labile C pool, leading to an increase in N uptake by plants. Nonetheless, C input to leaf increased more than N, leading to an increase and a decrease in the C:N ratio in leaf and root, respectively. Litter C:N ratio was decreased due to the increased N immobilization under high soil N availability or warming-accelerated decomposition of litter mass. Warming also increased C:N ratio of slow soil organic matter pool, suggesting a greater soil C sequestration potential. As most models usually use a fixed C:N ratio across different environments, the divergent shifts of C:N ratios under climate warming detected in this study could provide a useful benchmark for model parameterization and benefit models to predict C–N coupled responses to future climate change.

Ozone (O₃) pollution and nitrogen (N) deposition/fertilization often simultaneously affect plant growth. However, research of their interactive effects on leaf N metabolism is still scarce. We investigated their interactive effects, aiming to better understand plant N metabolism processes and biogeochemical cycles under high O₃ pollution and N deposition/fertilization.

Poplar saplings were exposed to two O₃ levels (NF, non-filtered ambient air; NF60, NF + 60 ppb O₃) and four N treatments (N0, no N added; N50, N0 + 50 kg N ha−1 yr−1; N100, N0 + 100 kg N ha−1 yr−1; N200, N0 + 200 kg N ha−1 yr−1) in open-top chambers for 95 days. The indicators related to leaf N metabolism were analyzed, including the activities of N-metabolizing enzymes and the contents of total N, NO₃−-N, NH₄+-N, total amino acid (TAA) and total soluble protein (TSP) in the leaves.

NF60 stimulated the activities of nitrate reductase (NR) by 47.2% at August relative to NF, and stimulated glutamine synthetase (GS) by 57.3% when averaged across all N treatments and sampling times. In contrast, O₃ did not significantly affect TSP and even reduced TAA content in August. Relative to N0, N200 significantly increased light-saturated rate of CO₂ assimilation (A_sat) by 24%, and increased total N content by 70.3% and 43.3% in August and September, respectively, while it reduced photosynthetic N-use efficiency by 26.1% in August. These results suggest that the increase in A_sat and total N content are uncoupled, and that the surplus N is not used to optimize the capacity for carbon assimilation under high N treatment. Simultaneously, high N treatment significantly promoted leaf N metabolism by increasing NO₃−-N contents, NH₄+-N contents, TAA contents and the activities of NR and GS. There was no significant interaction between O₃ and N for all variables.