Urbanisation has led to increasing homogenization of plant communities across cities. However, it is unclear whether these patterns extend to cosmopolitan plant species at the genetic level. We examined genome-wide genetic patterns in six widespread plant species (three Poaceae and three Asteraceae) across five cities in the USA (Boston, Baltimore, Minneapolis-St. Paul, Phoenix, and Los Angeles) using reduced-representation sequencing. We assessed genetic structure, differentiation, and patterns of isolation by distance (IBD) and environment (IBE) to determine if species were genetically homogeneous or differentiated by city, percentage of impervious surface, or both. Most species exhibited limited population structure overall, with Poa annua (annual bluegrass), Taraxacum officinale (dandelion), and Cynodon dactylon (Bermuda grass) showing no significant genetic differentiation among cities, a pattern consistent with high gene flow mediated by human activity. Notable exceptions included city-level differences in Erigeron canadensis (horseweed) and Lactuca serriola (prickly lettuce), especially in Phoenix. We also observed low genetic diversity in Digitaria sanguinalis (crabgrass) from Phoenix, suggesting recent founder effects or selection via environmental filtering. Erigeron canadensis, the only native species studied, displayed stronger differentiation by city, along with significant isolation by temperature and distance. Among all species, we found no evidence for population structure by impervious surface. Our findings indicate that widespread population genetic structure patterns of cosmopolitan plants are likely to depend more on species attributes (e.g., self-compatibility) and human-mediated dispersal than on urbanisation per se.

Previous work has found that anthropogenic inputs of nitrogen (N) and phosphorus (P) impact heterotrophic respiration during soil organic matter decomposition in grasslands, a critical pathway through which carbon (C) is lost from soil to the atmosphere. While N addition typically reduces heterotrophic respiration, why the strength and direction of this N effect varies among sites is unclear. To address this, we conducted a 339-day laboratory incubation to measure heterotrophic respiration from nine grasslands across North America that have received 10 years of factorial N and P fertilization. N addition reduced cumulative respiration most at sites with low pH, low microbial allocation towards N acquisition, and high soil C concentration and availability. However, N addition had neutral rather than positive effects on heterotrophic respiration in sites with high pH and decomposer allocation towards N acquisition. Across sites, a decade of N addition reduced heterotrophic respiration by similar to 24 %, driven by reductions in microbial biomass. Heterotrophic respiration was less sensitive to P addition, despite its increasing microbial biomass. However, simultaneous N and P addition did ameliorate negative N effects. These results show that previously observed variation in the response of heterotrophic respiration to N addition can be explained by soil C availability and pH status, widely measured factors which can be used to predict how grassland C fluxes may change under continuing nutrient deposition.

Stability can be desirable for many natural and social systems. Temporal stability, the invariability of a system over time, can be enhanced by resisting displacement during perturbations, accelerating recovery after them, or both1-4. Likewise, resilience (sensu proximity to unperturbed levels after a perturbation5-10) also has components of withstanding (resistance) and recovering after perturbations11,12. Here we develop and test new predictions for how temporal stability and resilience depend on their resistance and recovery components. We find that temporal stability could often be predicted from resistance, even without information about how quickly the system recovers. By contrast, resilience is predicted to depend at least as much on recovery as on resistance, as in earlier theory11,12. Using plant productivity data from the world's longest-running biodiversity experiment, we find that long-term temporal stability, quantified over a quarter century at the ecosystem or species level, is predicted with moderate accuracy from single-year estimates of resistance alone, with only slight improvement by also considering recovery. Resilience was predicted with moderate accuracy by a combination of resistance and recovery at the ecosystem level. We also find that ecosystem drought resistance can be forecasted by monitoring temporal stability before the drought. Our results reveal that long-term temporal stability and short-term resistance may often be predicted from one another and clarify how resistance and recovery can be leveraged to enhance the stability of both natural and managed systems.

Despite growing investments in nature-based solutions for urban resilience, their design often overlooks a fundamental biological process: evolution. Populations of organisms that sustain nature-based solutions are dynamic and can evolve over time. Rapid evolutionary changes, driven by urban environmental stressors, such as pollution, climate extremes, and habitat fragmentation, can reshape species' traits, alter interactions, and shift ecosystem functions. We synthesize evidence of evolutionary change across systems that serve as nature-based solutions in urban contexts and show how evolutionary processes can enhance or undermine their performance. We propose four testable hypotheses linking evolutionary dynamics to nature-based solutions and outline design strategies to maintain adaptive potential. Integrating evolution into nature-based solutions is essential to ensure long-term and efficient functionality under accelerating environmental change.

Intensification of agriculture and industry over the past two centuries has increased the amount of biologically limiting nutrients entering ecosystems. In areas such as grasslands, where growth is primarily constrained by one or more nutrients, increasing anthropogenic nutrient input has the potential to restructure plant communities and alter ecosystem carbon pools and cycling. Using a 13-year nutrient-manipulation experiment, we show that plant biomass increased with addition of nitrogen alone but even more strongly when nitrogen was combined with phosphorus or potassium and micronutrients, providing evidence for serial co-limitation of biomass. In contrast, plant functional group composition responded only when all nutrients were added together. The effects of nutrients on soil organic matter were mediated through changes in soil pH and base cation concentrations, and there was no direct effect of nitrogen alone, or in combination with other nutrients, on carbon fluxes or soil organic matter. Our work demonstrates how co-limitation may shift through time and may depend the effect of fertilization on soil pH and base cation concentration. Our experiment reveals that nitrogen added alone for more than 13 years does not affect plant community composition at our study site, overturning a long-standing interpretation of past work.

Ectomycorrhizal (ECM) fungi have long been thought to reduce litter decomposition in nitrogen (N)‐limited ecosystems by outcompeting saprotrophs for litter N (a phenomenon known as the ‘Gadgil effect’). However, recent research has called the generality of this effect into question, by demonstrating that ECM fungi can increase or decrease organic matter decomposition in different forests. The ecological factors driving such variation in the size and direction of ECM fungal effects on decomposition remain unclear. Here, we tested the hypothesis that ECM fungi would suppress decomposition of N‐poor, recalcitrant litter more in forests with lower N‐availability by exacerbating saprotrophic N limitation. We conducted an in situ ECM fungal and root reduction experiment (via soil trenching) in nine pine forests across three US states, which varied in soil and litter N content, climate and pine host ( Pinus muricata in California, P. elliottii in Florida and P. resinosa in Minnesota). In each site, we decomposed needle litter from (1) a pine species native to that site and (2) a common litter, P. strobus , for 1 year. Contrary to our expectations, ECM fungi either stimulated (California) or had no effect on (Florida and Minnesota) pine needle decomposition. Across sites, ECM fungal stimulation of decomposition did increase with total soil N content, but was unrelated to inorganic N availability. Furthermore, despite previous work suggesting that competition for N structures ECM fungal–saprotroph interactions, trenching effects on decomposition did not differ between pine litter types, despite large differences in initial litter C:N ratios, recalcitrance and net litter N immobilization. Synthesis . Taken together, our results add to a growing body of evidence that the ‘Gadgil effect’ is not universal, even in the N‐poor litter of temperate pine forests where it was first described and is often invoked. Furthermore, the inconsistency of relationships between trenching effects with different metrics of decomposer N supply and demand calls into question the central role of N in structuring fungal interguild interactions.

Litter decomposition is one of the largest carbon (C) fluxes in terrestrial ecosystems and links aboveground biomass to soil C pools. In grasslands, decomposition drivers have received substantial attention but the role of grassland herbivores in influencing decay rates is often ignored despite their potentially large effects on standing biomass and nutrient cycling. Recent work has demonstrated that nutrient addition increases early-stage decay and suppresses late-stage decay. Mammalian herbivores can mediate the effects of nutrient supply on biomass, suggesting herbivores may alter the effects of nutrients on decomposition, though this is largely unknown. We examined how herbivory mediates the effects of nutrient supply on long-term decomposition across 19 grassland sites of the Nutrient Network distributed experiment. At each site, a full-factorial experiment of combined nitrogen (N), phosphorus (P), and micronutrient (K) enrichment (‘control’ or ‘ + NPK’) and mammalian herbivore (> 50 g) exclusion (‘unfenced’ or ‘fenced’) was carried out in a randomized block design. We hypothesized that nutrient effects on litter decomposition would be strongest where herbivores caused the greatest reductions in aboveground plant biomass (i.e., at sites with more intense herbivory). After accounting for wide variation in decomposition rates across sites, we found that, within sites, elevated nutrients increased early-stage decay and suppressed late-stage decay. In contrast, neither herbivore exclusion (i.e., fencing) nor site level changes in aboveground biomass due to herbivory altered the nutrient effects on decomposition rates. Across grasslands, our results indicate that elevated nutrient supply modifies litter decomposition rates independent of herbivore impacts.

Understanding the determinants of urban forest diversity and structure is important for preserving biodiversity and sustaining ecosystem services in cities. However, comprehensive field assessments are resource-intensive, and landscape-level approaches may overlook heterogeneity within urban regions. To address this challenge, we combined remote sensing with field inventories to comprehensively map and analyze urban forest attributes in forest patches across the Minneapolis-St. Paul Metropolitan Area (MSPMA) in a multistep process. First, we developed predictive machine learning models of forest attributes by integrating data from forest inventories (from 40 12.5-m-radius plots) with Global Ecosystem Dynamics Investigation (GEDI) observations and Sentinel-2-derived land surface phenology (LSP). These models enabled accurate predictions of forest attributes, specifically nine metrics of plant diversity (tree species richness, tree abundance, and understory plant abundance), structure (average canopy height, dbh, and canopy density), and structural complexity (variability in canopy height, dbh, and canopy density) with relative errors ranging between 11% and 21%. Second, we applied these machine learning models to predict diversity metrics for 804 additional plots from GEDI and Sentinel-2. Finally, we applied Bayesian multilevel models to the predicted diversity metrics to assess the influence of multiple factors-patch dimensions, landscape attributes, plot position, and jurisdictional agency-on these forest attributes across the 804 predicted plots. The models showed all predictors have some degree of effect on forest attributes, presenting varying explanatory power with R2 values ranging from 0.071 to 0.405. Overall, plot characteristics (e.g., distance to nearest trail, proximity to forest edge) and jurisdictional agency explained a large portion of the variability across patches, whereas patch and landscape characteristics did not. The relative effect of plot versus management sets of predictors on the marginal ΔR2 was heterogeneous across metrics and ecological subsections (an ecological classification designation). The multiplicity of determinants influencing urban forests emphasizes the intricate nature of urban ecosystems and highlights nuanced, heterogeneous relationships between urban ecological and anthropogenic factors that determine forest properties. Effectively enhancing biodiversity in urban forests requires assessments, management, and conservation strategies tailored for context-specific characteristics.

Fungal necromass is increasingly recognized as a key component of soil carbon (C) and nitrogen (N) cycling. However, how C and N loss from fungal necromass during decomposition is impacted by global change factors such as anthropogenic N addition and changes to soil C supply (e.g. via changing root exudation and rhizosphere priming) remains unclear and understudied relative to plant tissues. To address these gaps, we conducted a year‐long decomposition experiment with four species of fungal necromass incubated across four forested sites in plots that had received inorganic N and/or labile C fertilization for two decades in Minnesota, USA. We found that necromass chemistry was the primary driver of C and N loss from fungal necromass as well as the response to fertilization. Specifically, N addition suppressed late‐stage decomposition, but this effect was weaker in melanin‐rich necromass, contrary to the hypothesis based on plant litter dynamics that N addition should suppress the decomposition of more complex organic molecules. Labile C addition had no effect on either the early or late stages of necromass decomposition. Nitrogen release from necromass also varied among species, with N‐poor necromass having lower N release after controlling for differences in mass loss via regression. The relatively minor effects of N fertilization on the proportion of initial necromass N released suggest that N demand by decomposers is the primary control on N loss during fungal necromass decomposition. Synthesis . Together, our results stress the importance of the afterlife effects of fungal chemical composition to forest soil C and N cycles. Further, they demonstrate that C and N release from this critical pool can be reduced by ongoing anthropogenic N addition.

Anthropogenic activities add more reactive nitrogen (N) to the environment than all natural sources combined, and the fate of this N is of environmental concern. If N that is deposited on terrestrial ecosystems through atmospheric deposition is retained in plant tissues or soil organic matter, it could stimulate carbon (C) storage in plant biomass or soils. However, added N also could increase soil inorganic N concentrations and leaching, potentially polluting watersheds, particularly in areas with low-N soils and/or a high propensity for leaching, such as sandy or arid areas. Here, we assessed N allocation and retention across a 13-year experimental N addition gradient in a temperate grassland. We found that N accumulation decreased significantly at mid- to high levels of N addition compared to the Control, such that ecosystem N pools were equivalent across a 10 g m−2 year−1 range of annual N addition rates (0–10 g N m−2 year−1), which spans most of the global range of N deposition. Nitrogen addition increased plant tissue percent N, but the total pool of N did not increase because of reduced plant biomass, particularly in roots. Nitrogen addition also increased soil inorganic N concentrations. Our results indicate that N addition is unlikely to increase grassland N pools, particularly in sandy or low-fertility ecosystems with a high potential for leaching because high application rates lead to N saturation, and additional inputs are lost.

Biodiversity promotes ecosystem productivity and stability, positive impacts that often strengthen over time. But ongoing global changes such as rising atmospheric carbon dioxide (CO2) levels and anthropogenic nitrogen (N) deposition may modulate the impact of biodiversity on ecosystem productivity and stability over time. Using a quarter-century grassland biodiversity-global change experiment we show that diversity increasingly enhanced productivity over time irrespective of global change treatments. In contrast, the positive influence of diversity on ecosystem stability strengthened over time under ambient conditions but weakened to varying degrees under global change treatments, largely driven by a greater reduction in species asynchrony under global changes. Thus, over 25 years, CO2 and N enrichment gradually eroded some of the positive effects of biodiversity on ecosystem stability. As elevated CO2, N eutrophication, and biodiversity loss increasingly co-occur in grasslands globally, our results raise concerns about their potential joint detrimental effects on long-term grassland stability.

Humans have increased atmospheric carbon dioxide (CO2), causing major changes in global climate while concurrently increasing the supply of biologically limiting nutrients especially nitrogen (N). Despite myriad negative effects on ecosystems and human health, nutrient pollution can increase the storage of soil carbon (C) in grasslands and retired farmland, potentially reducing atmospheric CO2. However, the persistence of nutrient-induced soil carbon gains remains a knowledge gap at the heart of a potential policy dilemma: whether reducing nutrient pollution could lead to the release of soil carbon that accumulated under high nutrient supply. Here we use a four-decade experiment conducted on retired, marginal cropland to demonstrate that nutrient addition increased soil C storage after intensive tilling, and that these soil C gains persisted for at least three decades following fertilizer and tilling cessation. This occurred despite plant biomass rapidly returning to pre-fertilization levels and plant community composition recovering from the effects of fertilization. These results demonstrate that nutrient-induced increases in soil C can persist for decades following reduced nutrient pollution as long as the soil remains untilled. Nutrient-induced soil carbon gains in grasslands can persist for decades after fertilization stops as long as the soil remains untilled, according to a multidecadal field experiment on retired agricultural fields with sandy soils.

Urban tree canopy cover is often unequally distributed across cities such that more socially vulnerable neighborhoods often have lower tree canopy cover than less socially vulnerable neighborhoods. However, how the diversity and composition of the urban canopy affect the nature of social-ecological benefits (and burdens), including the urban forest's vulnerability to climate change, remains underexamined. Here, we synthesize tree inventories developed by multiple organizations and present a species-specific, geolocated database of more than 600,000 urban trees across the 7-county Minneapolis-St. Paul (MSP) metropolitan area in the Upper Midwest of the United States. We find that tree diversity across the MSP is variable yet dominated by a few species (e.g., Fraxinus pennsylvanica, Acer platanoides, and Gleditsia triacanthos), contributing to the vulnerability of the MSP urban forest to future climate change and disturbances. In contrast to tree canopy cover, tree diversity was not well predicted by socioeconomic or demographic factors. However, our analysis identified areas where both climate and social vulnerability are high. Our results add to a growing body of literature emphasizing the importance of considering how complex and interacting social and ecological factors drive urban forest diversity and composition when pursuing management objectives.

Plant disease often increases with N, decreases with CO2, and increases as biodiversity is lost (i.e., the dilution effect). Additionally, all these factors can indirectly alter disease by changing host biomass and hence density-dependent disease transmission. Yet over long periods of time as communities undergo compositional changes, these biomass-mediated pathways might fade, intensify, or even reverse in direction. Using a field experiment that has manipulated N, CO2, and species richness for over 20 years, we compared severity of a specialist rust fungus (Puccinia andropogonis) on its grass host (Andropogon gerardii) shortly after the experiment began (1999) and twenty years later (2019). Between these two sampling periods, two decades apart, we found that disease severity consistently increased with N and decreased with CO2. However, the relationship between diversity and disease reversed from a dilution effect in 1999 (more severe disease in monocultures) to an amplification effect in 2019 (more severe disease in mixtures). The best explanation for this reversal centered on host density (i.e., aboveground biomass), which was initially highest in monoculture, but became highest in mixtures two decades later. Thus, the diversity-disease pattern reversed, but disease consistently increased with host biomass. These results highlight the consistency of N and CO2 as drivers of plant disease in the Anthropocene and emphasize the critical role of host biomass—despite potentially variable effects of diversity—for relationships between biodiversity and disease.

Measuring the chemical traits of leaf litter is important for understanding plants’ roles in nutrient cycles, including through nutrient resorption and litter decomposition, but conventional leaf trait measurements are often destructive and labor-intensive. Here, we develop and evaluate the performance of partial least-squares regression (PLSR) models that use reflectance spectra of intact or ground leaves to estimate leaf litter traits, including carbon and nitrogen concentration, carbon fractions, and leaf mass per area (LMA). Our analyses included more than 300 samples of senesced foliage from 11 species of temperate trees, including needleleaf and broadleaf species. Across all samples, we could predict each trait with moderate-to-high accuracy from both intact-leaf litter spectra (validation R 2 = 0.543-0.941; %RMSE = 7.49-18.5) and ground-leaf litter spectra (validation R 2 = 0.491-0.946; %RMSE = 7.00-19.5). Notably intact-leaf spectra yielded better predictions of LMA. Our results support the feasibility of building models to estimate multiple chemical traits from leaf litter of a range of species. In particular, the success of intact-leaf spectral models allows non-destructive trait estimation in a matter of seconds, which could enable researchers to measure the same leaves over time in studies of nutrient resorption.

Anthropogenic nitrogen (N) deposition is unequally distributed across space and time, with inputs to terrestrial ecosystems impacted by industry regulations and variations in human activity. Soil carbon (C) content normally controls the fraction of mineralized N that is nitrified ( ƒ nitrified ), affecting N bioavailability for plants and microbes. However, it is unknown whether N deposition has modified the relationships among soil C, net N mineralization, and net nitrification. To test whether N deposition alters the relationship between soil C and net N transformations, we collected soils from coniferous and deciduous forests, grasslands, and residential yards in 14 regions across the contiguous United States that vary in N deposition rates. We quantified rates of net nitrification and N mineralization, soil chemistry (soil C, N, and pH), and microbial biomass and function (as beta-glucosidase (BG) and N -acetylglucosaminidase (NAG) activity) across these regions. Following expectations, soil C was a driver of ƒ nitrified across regions, whereby increasing soil C resulted in a decline in net nitrification and ƒ nitrified . The ƒ nitrified value increased with lower microbial enzymatic investment in N acquisition (increasing BG:NAG ratio) and lower active microbial biomass, providing some evidence that heterotrophic microbial N demand controls the ammonium pool for nitrifiers. However, higher total N deposition increased ƒ nitrified , including for high soil C sites predicted to have low ƒ nitrified , which decreased the role of soil C as a predictor of ƒ nitrified . Notably, the drop in contemporary atmospheric N deposition rates during the 2020 COVID-19 pandemic did not weaken the effect of N deposition on relationships between soil C and ƒ nitrified . Our results suggest that N deposition can disrupt the relationship between soil C and net N transformations, with this change potentially explained by weaker microbial competition for N. Therefore, past N inputs and soil C should be used together to predict N dynamics across terrestrial ecosystems.

Urban forests provide ecosystem services important for regulating climate, conserving biodiversity, and maintaining human well-being. However, these forests vary in composition and physiological traits due to their unique biophysical and social contexts. This variation complicates assessing the functions and services of different urban forests. To compare the characteristics of the urban forest, we sampled the species composition and two externally sourced traits (drought tolerance and water-use capacity) of tree and shrub species in residential yards, unmanaged areas, and natural reference ecosystems within six cities across the contiguous US. As compared to natural and unmanaged forests, residential yards had markedly higher tree and shrub species richness, were composed primarily of introduced species, and had more species with low drought tolerance. The divergence between natural and human-managed areas was most dramatic in arid climates. Our findings suggest that the answer to the question of "what is an urban forest" strongly depends on where you look within and between cities.