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Interactions between air pollution and terrestrial ecosystems play an important role in the Earth system. However, process-based knowledge of air pollution–terrestrial ecosystem interactions is limited, hindering accurate quantification of how changes in tropospheric chemistry, biogeochemical cycling, and climate affect air quality and its impact on humans and ecosystems. Here we summarize current challenges and future directions for advancing the understanding of air pollution–ecosystem interactions by synthesizing discussions from a multidisciplinary group of scientists at a recent Integrated Land Ecosystem–Atmosphere Processes Study (iLEAPS) early-career workshop. Specifically, we discuss the important elements of air pollution–terrestrial ecosystem interactions, including vegetation and soil uptake and emissions of air pollutants and precursors, in-canopy chemistry, and the roles of human activities, fires, and meteorology. We highlight the need for a coordinated network of measurements of long-term chemical fluxes and related meteorological and ecological quantities with expanded geographic and ecosystem representation, data standardization and curation to reduce uncertainty and enhance observational syntheses, integrated multiscale observational and modeling capabilities, collaboration across scientific disciplines and geographic regions, and active involvement by stakeholders and policymakers. Such an enhanced network will continue to facilitate the process-level understanding and thus predictive ability of interactions between air pollution and terrestrial ecosystems and impacts on local-to-global climate and human health.

Herbivores exert a strong influence on the species composition and richness of plant communities, but the magnitude of their effect on belowground communities remains poorly understood. While an increasing number of studies acknowledge the importance of documenting belowground effects of herbivores, very few of these evaluate variation in the strength of the response from aboveground to belowground communities. Our study documents the long-term consequences of sustained deer herbivory for plant and arthropod communities adjacent to 15 exclosures that have been in place since 1996. We hypothesized that herbivory would alter the composition and diversity of communities, but the strength of the effects of herbivory would weaken from plants, to leaf-litter invertebrates, and to belowground microarthropod communities. First, we found that herbivory negatively impacted plant seedling and sapling abundance and performance, reduced the abundance of ants and the taxonomic richness of arthropods in the litter layer and reduced the richness of soil microarthropod communities. Second, in contrast to our hypothesis, the magnitude of effect size did not vary among trophic levels, indicating that effects of deer herbivory cascade from plants to the leaf-litter and soil arthropod communities with equal strength. While much recent research has focused on how specific traits of plants may mediate the effects of herbivory on associated species, our results suggest that indirect effects of herbivory might influence many components of belowground communities.

Invasive plant species alter soils in ways that may affect the success of subsequent generations, creating plant-soil feedbacks. Ailanthus altissima is an invasive tree introduced two centuries ago to North America. We hypothesized that geographically distinct populations of A. altissima have established feedbacks specific to their local environment, due to soil communities cultivated by A. altissima. We collected seeds and soils from three populations in the eastern United States, and in the greenhouse reciprocally planted all families in all collected soils as well as in a control mixed soil, and in soils that had been irradiated for sterilization. There were positive plant-soil feedbacks for two populations in the live field-collected soils, but strong negative feedbacks for the third population. There were no population-level performance differences or feedbacks in the sterilized population locale soils, supporting a soil biotic basis for feedbacks and for the expression of genetic differentiation in A. altissima. If populations of Ailanthus altissima vary in the extent to which they benefit from and promote these plant-soil biota feedbacks, the interaction between invader and invaded community may be more important in determining the course of invasion than are the characteristics of either alone.

Summary 1. There is relatively little information on how evolutionary processes that alter genetic variation in a focal species may affect interactions with other species, impact the structure or function of the community and ecosystem, and affect evolutionary feedbacks among interacting species through time. 2. Because evolution can occur at ecological time-scales, it is important to understand how major selective events, such as climatic changes, can impact the community of interacting species and ecosystem processes by changing intraspecific genetic variation. 3. The evidence linking genetic variation and evolution to community change and feedbacks has arisen from several different approaches whose results have not been synthesized into one conceptual framework, and whose commonalities may not be fully understood. 4. This review synthesizes several different experimental approaches on how evolution may impact communities and ecosystems and focuses on five main issues: (i) the genetic basis to communities and ecosystems; (ii) the community and ecosystem consequences of among-population genetic differentiation; (iii) the role of local adaptation and co-evolution; (iv) the effects of trans-generational feedbacks and the eco-evo dynamic and; (v) the integration of community and ecosystem genetics and multi-level selection. 5. Evolution can alter intraspecific genetic variation to affect indirect genetic effects and feedbacks. Future studies should investigate how communities and ecosystems are affected when evolution causes the strength of feedbacks to change.

Ecological explanations for the success and persistence of invasive species vastly outnumber evolutionary hypotheses, yet evolution is a fundamental process in the success of any species. The Evolution of Increased Competitive Ability (EICA) hypothesis (Blossey and Nötzold 1995) proposes that evolutionary change in response to release from coevolved herbivores is responsible for the success of many invasive plant species. Studies that evaluate this hypothesis have used different approaches to test whether invasive populations allocate fewer resources to defense and more to growth and competitive ability than do source populations, with mixed results. We conducted a meta‐analysis of experimental tests of evolutionary change in the context of EICA. In contrast to previous reviews, there was no support across invasive species for EICA's predictions regarding defense or competitive ability, although invasive populations were more productive than conspecific native populations under noncompetitive conditions. We found broad support for genetically based changes in defense and competitive plant traits after introduction into new ranges, but not in the manner suggested by EICA. This review suggests that evolution occurs as a result of plant introduction and population expansion in invasive plant species, and may contribute to the invasiveness and persistence of some introduced species.

Background In the emerging field of community and ecosystem genetics, genetic variation and diversity in dominant plant species have been shown to play fundamental roles in maintaining biodiversity and ecosystem function. However, the importance of intraspecific genetic variation and diversity to floral abundance and pollinator visitation has received little attention. Methodology/Principal Findings Using an experimental common garden that manipulated genotypic diversity (the number of distinct genotypes per plot) of Solidago altissima, we document that genotypic diversity of a dominant plant can indirectly influence flower visitor abundance. Across two years, we found that 1) plant genotype explained 45% and 92% of the variation in flower visitor abundance in 2007 and 2008, respectively; and 2) plant genotypic diversity had a positive and non-additive effect on floral abundance and the abundance of flower visitors, as plots established with multiple genotypes produced 25% more flowers and received 45% more flower visits than would be expected under an additive model. Conclusions/Significance These results provide evidence that declines in genotypic diversity may be an important but little considered factor for understanding plant-pollinator dynamics, with implications for the global decline in pollinators due to reduced plant diversity in both agricultural and natural ecosystems.