The impacts of atmospheric nitrogen (N) deposition amount on plant communities have been extensively explored. However, the responses of plant communities to the ratio of reduced (NH₄⁺–N) and oxidized (NO₃⁻–N) forms remain unclear in natural ecosystems. A field N enrichment experiment using different NH₄⁺–N/NO₃⁻–N ratios was conducted in a natural semi-arid grassland in northern China from 2014 to 2019. Nitrogen addition tended to reduce plant species richness and significantly enhanced plant community aboveground net primary productivity (ANPP). Neither plant species richness nor plant ANPP at species and community levels was significantly affected by NH₄⁺–N/NO₃⁻–N ratios. At the plant functional group level, ANPP of grasses was not significantly affected by the NH₄⁺–N/NO₃⁻–N ratios examined, whereas ANPP of forbs was significantly increased at 1:1 NH₄⁺–N/NO₃⁻–N. Regardless of N supplied using the different ratios of NH₄⁺–N/NO₃⁻–N examined, plant community ANPP was positively associated with growing season precipitation. Unexpectedly, 1:1 NH₄⁺–N/NO₃⁻–N (NH₄NO₃) significantly improved the positive response of plant community ANPP to precipitation (it had the biggest slope value). Our results suggest that precipitation was the main determinant of the influence of NH₄⁺–N/NO₃⁻–N ratios on plant community ANPP. Therefore, the results of our study showed that without referring to NH₄⁺–N/NO₃⁻–N ratios and precipitation, models using NH₄NO₃ enrichment may overestimate the positive effect of atmospheric N deposition on ecosystem ANPP in semi-arid ecozones.

Heavy metal pollution is widespread, and has an increasing trend in some countries and regions. It can be easily accumulated in plants, leading to plant species loss and affecting plant community composition. Artificial restoration can conserve plant diversity in contaminated soils and accelerate the recovery of polluted ecosystems. The application of nitrogen (N) and phosphorus (P) is inexpensive and convenient, which can increase the resistance of plants to adversity and promote the growth of plants in heavy metal polluted soils. In order to examine the effect of N and P nutrition on the conservation of plant community, we conducted a comparison experiment in greenhouse using soil with low N and P concentration, and set five treatments: C (soil with no heavy metals and fertilizer addition), H (soil with heavy metals addition but with no fertilizer), HN (soil with heavy metals and N addition), HP treatment(soil with heavy metals and P addition), HNP treatment (soil with heavy metals, N and P addition). Our results showed that heavy metal pollution reduced plant species by 300%, and significantly decreased plant diversity (P < 0.05). N addition increased the richness of plant species and increased the dominance of Euphorbia peplus, but had no significant effect on plant diversity and community structure, while reduced the evenness of plant species. P addition of HP and HNP treatments restored plant species richness and increased plant diversity under heavy metal pollution. The plant community structures of these two treatments were more similar to that of group C. Compared with N addition, P addition had a better performance to restoring the species composition and relative dominance of plant communities. Our results provided a guidance for the restoration of plant communities and the conservation of plant species in low N and P concentration soils with the context of heavy metal pollution.

Changes in soil nutrients variability could significantly interact with other global change processes (such as community dynamics, biological invasion). Global exchange and accumulation of alien species caused environmental and economic threats in the introduced ranges. Their invasion success or not in local plant communities is largely depended on the interactions and competitive outcomes with other species and environmental conditions. Here, we tested whether the interactions of nutrient variability and competitor identity influence plant performance, potential invasion success of invasive species and their co-existence with native species. In both greenhouse and field experiment, we subjected three congeneric and naturally co-occurring pairs of invasive alien and native clonal plants in China to different nutrient variability (constant high, multiple pulses and/or single pulse) and competitor identity (intra-specific competitors, native competitors, invasive competitors and both native & invasive competitors). Our results showed that total biomass or the increase of cover of invasive species was significantly larger than those of the native species regardless of competitor identity. Native competitors significantly decreased biomass proportion of native species, but did not affect that of invasive species. The whole community with invasive target species accumulated more total biomass than with native species under multiple pulses nutrient when with the native competitors. Invasive species produced significantly higher biomass proportion than natives under all competitor identity treatments except for native & invasive competitors. Multiple mixed competitors (i.e. native & invasive competitors) decreased the plant performance and dominance of invasive target species, to some extent, thus construction of multi-species competition might facilitate coexistence of native and invasive species in communities. Interactions between native competitors or native & invasive competitors, and nutrient variability play important roles in plant performance and potential invasion success in communities. Multiple invasional interference may have significant implications for the co-existence of invasive and native species, and for management of invasive species.

Understorey communities can dominate forest plant diversity and strongly affect forest ecosystem structure and function. Understoreys often respond sensitively but inconsistently to drivers of ecological change, including nitrogen (N) deposition. Nitrogen deposition effects, reflected in the concept of critical loads, vary greatly not only among species and guilds, but also among forest types. Here, we characterize such context dependency as driven by differences in the amounts and forms of deposited N, cumulative deposition, the filtering of N by overstoreys, and available plant species pools. Nitrogen effects on understorey trajectories can also vary due to differences in surrounding landscape conditions; ambient browsing pressure; soils and geology; other environmental factors controlling plant growth; and, historical and current disturbance/management regimes. The number of these factors and their potentially complex interactions complicate our efforts to make simple predictions about how N deposition affects forest understoreys. We review the literature to examine evidence for context dependency in N deposition effects on forest understoreys. We also use data from 1814 European temperate forest plots to test the ability of multi-level models to characterize context-dependent understorey responses across sites that differ in levels of N deposition, community composition, local conditions and management history. This analysis demonstrated that historical management, and plot location on light and pH-fertility gradients, significantly affect how understorey communities respond to N deposition. We conclude that species' and communities' responses to N deposition, and thus the determination of critical loads, vary greatly depending on environmental contexts. This complicates our efforts to predict how N deposition will affect forest understoreys and thus how best to conserve and restore understorey biodiversity. To reduce uncertainty and incorporate context dependency in critical load setting, we should assemble data on underlying environmental conditions, conduct globally distributed field experiments, and analyse a wider range of habitat types.

Particulate matter (PM) is one of the most serious environmental problems, exacerbating respiratory and vascular illnesses. Plants have the ability to reduce non-point source PM pollution through retention on leaves and branches. Studies of the dynamic processes of PM retention by plants and the mechanisms influencing this process will help to improve the efficiency of urban greening for PM reduction. We examined dynamic processes of PM retention and the major factors influencing PM retention by six trees with different branch structure characteristics in wind tunnel experiments at three different wind speeds. The results showed that the changes of PM numbers retained by plant leaves over time were complex dynamic processes for which maximum values could exceed minimum values by over 10 times. The average value of PM measured in multiple periods and situations can be considered a reliable indicator of the ability of the plant to retain PM. The dynamic processes were similar for PM₁₀ and PM₂.₅. They could be clustered into three groups simulated by continually-rising, inverse U-shaped, and U-shaped polynomial functions, respectively. The processes were the synthetic effect of characteristics such as species, wind speed, period of exposure and their interactions. Continually-rising functions always explained PM retention in species with extremely complex branch structure. Inverse U-shaped processes explained PM retention in species with relatively simple branch structure and gentle wind. The U-shaped processes mainly explained PM retention at high wind speeds and in species with a relatively simple crown. These results indicate that using plants with complex crowns in urban greening and decreasing wind speed in plant communities increases the chance of continually-rising or inverse U-shaped relationships, which have a positive effect in reducing PM pollution.

Ozone (O3) critical levels (CLe) are still poorly developed for herbaceous vegetation. They are currently based on single species responses which do not reflect the multi-species nature of semi-natural vegetation communities. Also, the potential effects of other factors like the nitrogen (N) input are not considered in their derivation, making their use uncertain under natural conditions.Exposure- and dose-response relationships were derived from two open-top chamber experiments exposing a mixture of 6 representative annual Mediterranean pasture species growing in natural soil to 4 O3 fumigation levels and 3 N inputs. The Deposition of O3 and Stomatal Exchange model (DO3SE) was modified to account for the multi-species nature of the canopy following a big-leaf approach. This new approach was used for estimating a multi-species phytotoxic O3 dose (PODy-MS). Response relationships were derived based on O3 exposure (AOT40) and flux (PODy-MS) indices.The treatment effects were similar in the two seasons: O3 reduced the aboveground biomass growth and N modulated this response. Gas exchange rates presented a high inter-specific variability and important inter-annual fluctuations as a result of varying growing conditions during the two years. The AOT40-based relationships were not statistically significant except when the highest N input was considered alone. In contrast, PODy-MS relationships were all significant but for the lowest N input level. The influence of the N input on the exposure- and dose-response relationships implies that N can modify the O3 CLe. However, this is an aspect that has not been considered so far in the methodologies for establishing O3 CLe. Averaging across N input levels, a multi-species O3 CLe (CLef-MS) is proposed POD1-MS = 7.9 mmol m⁻², accumulated over 1.5 month with a 95% confidence interval of (5.9, 9.8). Further efforts will be needed for comparing the CLef-MS with current O3 CLef based on single species responses.

Perennial plant communities in the proximity of metal smelters and refineries may receive substantial inputs of base metal particulate as well as sulphate from the co-emission of sulphur dioxide. The Ni refinery at Port Colborne (Canada) operated by Inco (now Vale Canada Ltd.) emitted Ni, Co and Cu, along with sulphur dioxide, between 1918 and 1984. The objectives were to determine if vascular plant community composition, including standing litter, in twenty-one woodlots on clay or organic soil, were related to soil Ni concentration which decreased in concentration with distance from the Ni refinery. The soil Ni concentration in the clay woodlots ranged from 16 to 4130 mg Ni/kg, and in the organic woodlots, ranged from 98 to 22,700 mg Ni/kg. The concentrations of Co and Cu in the soils were also elevated, and highly correlated with soil Ni concentration. In consequence, each series of woodlots constituted a ‘fixed ratio ray’ of metal mixture exposure. For each of the woodlots, there were 16 independent measurements of ‘woodlot status’ which were correlated with elevated soil Ni concentration. Of the 32 combinations, there were eight linear correlations with soil Ni concentration, considerably more than would be expected by chance alone at a p-value of 0.05. With the exception of mean crown rating for shrubs at the clay sites, the correlations were consistent with the hypothesis that increased soil metal concentrations would be correlated with decreased diversity, plant community health or fitness, and increased accumulation of litter. Only five of the eight linear correlations were from the organic woodlots, suggesting that the observations were not confounded with soil type nor range in soil metal concentrations.

Nitrogen deposition has been shown to have significant impacts on a range of vegetation types resulting in eutrophication and species compositional change. Data from a re-survey of 89 coastal sites in Scotland, UK, c. 34 years after the initial survey were examined to assess the degree of change in species composition that could be accounted for by nitrogen deposition. There was an overall increase in the Ellenberg Indicator Value for nitrogen (EIV-N) of 0.15 between the surveys, with a clear shift to species characteristic of more eutrophic situations. This was most evident for Acid grassland, Fixed dune, Heath, Slack and Tall grass mire communities and despite falls in EIV-N for Improved grass, Strand and Wet grassland. The increase in EIV-N was highly correlated to the cumulative deposition between the surveys, and for sites in south-east Scotland, eutrophication impacts appear severe. Unlike other studies, there appears to have been no decline in species richness associated with nitrogen deposition, though losses of species were observed on sites with the very highest levels of SOx deposition. It appears that dune vegetation (specifically Fixed dune) shows evidence of eutrophication above 4.1 kg N ha−1 yr−1, or 5.92 kg N ha−1 yr−1 if the lower 95% confidence interval is used. Coastal vegetation appears highly sensitive to nitrogen deposition, and it is suggested that major changes could have occurred prior to the first survey in 1976.

The use of accumulation bioindicator to assess metal bioavailability has mainly concerned individual species. This work addresses this issue at the plant community level. Metal content within different species from plant communities found at three contaminated and one uncontaminated site was compared. Results showed that for two contaminated sites, leaf metals concentrations were comparable to those in plants from control site, i.e. approx (mg/kg) 0.1 Cd, 0.2 Cr, 9.2 Cu, 1.8 Ni, 0.5 Pb and 42 Zn. Only plants from the third site showed higher metal contents, ranging from 1.5- to 8-fold those of the control community. This contrasted with ammonium acetate–EDTA extractions, which indicated a very high “availability” of metals at the three sites, as compared to the control site. Thus, metal content in plant communities provided accurate information on actual transfer toward the ensemble of vegetation, which could be used to establish site-specific “fingerprints” of metal bioavailability.

Human activity is changing air quality and climate in the US Pacific Northwest. In a first application of non-metric multidimensional scaling to a large-scale, framework dataset, we modeled lichen community response to air quality and climate gradients at 1416 forested 0.4 ha plots. Model development balanced polluted plots across elevation, forest type and precipitation ranges to isolate pollution response. Air and climate scores were fitted for remaining plots, classed by lichen bioeffects, and mapped. Projected 2040 temperatures would create climate zones with no current analogue. Worst air scores occurred in urban-industrial and agricultural valleys and represented 24% of the landscape. They were correlated with: absence of sensitive lichens, enhancement of nitrophilous lichens, mean wet deposition of ammonium >0.06 mg l-1, lichen nitrogen and sulfur concentrations >0.6% and 0.07%, and SO2 levels harmful to sensitive lichens. The model can detect changes in air quality and climate by scoring re-measurements. Lichen-based air quality and climate gradients in western Oregon and Washington are responsive to regionally increasing nitrogen availability and to temperature changes predicted by climate models.