Warwick J. Allen

Introduction: Applied nucleation aims to restore degraded ecosystems efficiently by establishing vegetation patches that increase in biodiversity and extent through natural dispersal and facilitation. Although widely tested in terrestrial ecosystems, its long‐term outcomes in wetlands remain poorly understood, particularly how restored communities develop over time and how post‐restoration trajectories compare with reference ecosystems. Objectives: We evaluated outcomes after 21 years of an applied nucleation experiment to restore a mined restiad raised bog in northern New Zealand. Methods: Nuclei were established as raised peat islands (approximately 10 m diameter) and seeded with the early‐successional shrub mānuka ( Leptospermum repo ) in 2002. Vegetation composition and peat properties were assessed within nuclei and the surrounding matrix at three distances from an adjacent natural reference peatland. Nuclei plots were surveyed after 2, 9, and 21 years, while matrix and natural peatland plots were measured after 21 years. Results: Mānuka successfully established, followed by the establishment of peat‐forming restiad species bamboo rush ( Sporadanthus ferrugineus ) and wire rush ( Empodisma robustum ), the latter one of three species facilitated by mānuka. Beta diversity among nuclei increased over time, reflecting continued recruitment and increasing community heterogeneity. Plant community composition within nuclei became progressively more similar to the natural peatland, and after 21 years, nuclei supported higher diversity and greater compositional similarity to the natural peatland than did surrounding matrix areas, which also showed recovery toward the reference community. However, persistent differences in peat bulk density, water content, and nutrient concentrations revealed enduring legacies of mining disturbance. Conclusions: Applied nucleation can initiate ecological recovery in degraded peatlands. Establishment of early‐successional nurse species facilitates the return of peat‐forming plants and promotes recovery toward a reference community, even where disturbance legacies persist.

Nitrogen deposition often promotes plant invasions and can arise from multiple sources. However, it remains unclear whether native legumes facilitate invasive plants directly through biotic nitrogen transfer or indirectly via soil microbial mediation. It is also unknown how exogenous nitrogen addition influences these facilitative effects. To address these questions, we combined evidence from a greenhouse experiment and global meta‐analysis. The greenhouse experiment tested the effects of neighboring plants (a native nitrogen‐fixing legume Albizia julibrissin ; a native non‐nitrogen‐fixing legume Styphnolobium japonicum ; or a conspecific) on the growth of invasive Rhus typhina and its native congener R. chinensis . Within‐pot membrane treatments were used to obstruct microbial and nitrogen movement between focal and neighboring plants, with and without added nitrogen. The total biomass of invasive R. typhina increased when grown with the nitrogen‐fixing legume compared with conspecific neighbors, but not when grown with the non‐nitrogen‐fixing legume. Positive effects of the nitrogen‐fixing legume on R. typhina were suppressed when nitrogen transfer or soil microbial movement was obstructed by membranes. Exogenous nitrogen addition did not alter the growth of invasive R. typhina or its interactions with legumes. The biomass of native R. chinensis did not differ among neighboring plant treatments, indicating that facilitation by legumes favored the invasive species. The global meta‐analysis of 268 effect sizes from 46 studies of the effects of legumes on neighboring plants also revealed that native legumes enhanced invasive plants' performance. Together, our findings recognize nitrogen‐fixing legumes as overlooked facilitators of plant invasion, operating through both nutrient and microbial mediated pathways. These findings highlight a paradox in which native species can act as “turncoats,” accelerating invasion and reshaping plant communities under global change.

Global environmental changes generate novel communities. Some species adapt to novel community contexts by demonstrating interaction fidelity, or consistently interacting with the same species, while others demonstrate interaction flexibility, or interacting with whichever partners are available (i.e., rewiring). However, the drivers and benefits of fidelity are unclear. Here, we use data from a large-scale mesocosm experiment to determine whether plant characteristics (e.g., provenance, functional group) and community contexts impact plant interaction fidelity to belowground mutualist and antagonist partners (i.e., root fungi and bacteria), and how this fidelity relates to plant performance. We found that the plant-antagonist relationships exhibited higher fidelity than plant-mutualist relationships with the strength of fidelity depending on a combination of plant characteristics and community context. Conversely, plant-mutualist fidelity impacted plant performance with competing effects depending on plant provenance and aboveground herbivore presence. Our study advances understanding of how species interactions influence plant performance in a changing world. Statement of authorship: AMM, JDT, JMT and DBS designed the concept. All authors contributed to refining the ideas. AM coded the analysis to produce the results with contributions from JDT, DBS, and BBM. HRL and DBS helped AMM with the statistical analysis. LPW, WJA, IAD, and JMT designed the experiment while LPW and WJA conducted the experiment. AMM wrote the first draft of the manuscript with assistance from all authors. IAD, JMT, and JDT obtained funding. All authors contributed to further editing of the manuscript.

Summary: Integrating plant physiological traits (ideal weed hypothesis) and interactions with enemies (enemy release hypothesis) may be key to understanding plant invasions. Contrary to enemy release, recent evidence suggests that invasive plants often accumulate generalist enemies due to the same r‐selected physiological traits that often drive invasive success. Despite high enemy loads, successful invasive plants can remain dominant due to high growth rates rather than due to lack of damage. Consequently, generalist herbivore and pathogen populations may be amplified by invaders and can spillover onto native plants, with these indirect interactions disproportionately affecting native rather than invasive plant species via apparent competition. Where this occurs, the generalist enemies of invasive plants may instead be hidden allies that amplify invader success and impacts.

Changes in biodiversity reflect processes acting at multiple spatial scales, from local to global, among habitats and within communities. This complexity makes it difficult to measure mechanisms that have traditionally interested ecologists, such as environmental filters. To resolve this, we propose an approach to partition temporal changes in biodiversity into contributions from selection at multiple scales. We applied this approach to study changes in the biodiversity of invertebrate herbivores from a large‐scale, plant community experiment. Though the experiment was designed to foster distinct insect communities due to differences in host plants, our approach showed that selection among these treatments was a negligible facet of diversity change. These effects were swamped by rapid changes in relative abundances of aphids due to both immigration and selection across the metacommunity. More broadly, our work highlights how total change in biodiversity across a biogeographic region can be partitioned into logically distinct mechanisms.

Fungal diversity influences both plant diversity and ecosystem functioning, but how fungi mediate the relationship between plant diversity and ecosystem multifunctionality is not as well understood. To address this knowledge gap, we manipulate plant species richness and soil fungal diversity (via fungicide addition) in 190 experimental plant communities and measure ten ecosystem functions to assess ecosystem multifunctionality. We find that reduced fungal diversity (via fungicide addition) decreases ecosystem multifunctionality, but only in plant communities with low species richness, indicating that soil fungal diversity can buffer the effects of plant diversity loss on ecosystem multifunctionality. Selection effects (i.e., superiority of dominant plant species) and phylogenetic clustering of the fungal community (i.e., functional redundancy) increase with plant species richness when fungicide is added, revealing potential mechanisms through which species-rich plant communities can mitigate the negative effects of reduced fungal diversity and maintain ecosystem multifunctionality. Our study emphasizes the importance of interactions between plant and fungal diversity for ecosystem multifunctionality, and highlights the need to conserve fungal diversity, especially in ecosystems with low plant diversity that are threatened by global change.

Summary: Invasion by exotic plants is a major threat to ecosystem biodiversity globally. Although mutualistic belowground associations can play a significant role in successful invasion, studies have shown mixed results regarding the effects of plant invasion on arbuscular mycorrhizal fungi (AMF). Here, we tested how exotic dominance (i.e. invasion extent) in plant communities impacts arbuscular mycorrhizal fungal community diversity, composition, and generalism at the scale of individual plants and plant communities, and whether these impacts are explained or moderated by plant root traits (root diameter). We characterized root traits and arbuscular mycorrhizal fungal communities in the roots of native and exotic plants growing together in experimental plant communities that ranged in invasion extent (exotic plant dominance from 0% to 100%). Increases in exotic dominance decreased arbuscular mycorrhizal fungal diversity in both individual plant roots and plant communities. Greater relative abundance of generalist AMF was also observed in plant communities with increasing exotic dominance. Although root diameter affected fungal composition, it did not moderate or drive the effect of exotics. Our results highlight the role of invasion extent in understanding how arbuscular mycorrhizal fungal communities respond to exotic plant invasion and the importance of preserving belowground biodiversity.

Changes in biodiversity reflect processes acting at multiple spatial scales, including globally, among habitats and within communities. This complexity makes it difficult to analyse the mechanisms that change biodiversity over time. To resolve this, we propose a novel approach to partition temporal changes in biodiversity into contributions from selection at multiple scales. We apply this approach to study changes in the biodiversity of invertebrate herbivores from a large-scale, plant community experiment. Though the experiment was designed to foster distinct insect communities due to differences in host plants, our approach shows that selection among these treatments was a negligible facet of diversity change. Instead, the dominant source of community dynamics was rapid changes in the relative abundances of individual species. These shifts produced surprisingly small changes in biodiversity. More broadly our work highlights how total change in biodiversity across a biogeographic region can be partitioned into logically distinct mechanisms.