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Fine roots acquire essential soil resources and mediate biogeochemical cycling in terrestrial ecosystems. Estimates of carbon and nutrient allocation to build and maintain these structures remain uncertain because of the challenges of consistently measuring and interpreting fine-root systems. Traditionally, fine roots have been defined as all roots ≤ 2 mm in diameter, yet it is now recognized that this approach fails to capture the diversity of form and function observed among fine-root orders. Here, we demonstrate how order-based and functional classification frameworks improve our understanding of dynamic root processes in ecosystems dominated by perennial plants. In these frameworks, fine roots are either separated into individual root orders or functionally defined into a shorter-lived absorptive pool and a longer-lived transport fine-root pool. Using these frameworks, we estimate that fine-root production and turnover represent 22% of terrestrial net primary production globally – a c. 30% reduction from previous estimates assuming a single fine-root pool. Future work developing tools to rapidly differentiate functional fine-root classes, explicit incorporation of mycorrhizal fungi into fine-root studies, and wider adoption of a two-pool approach to model fine roots provide opportunities to better understand below-ground processes in the terrestrial biosphere.

Little is known about the response of arbuscular mycorrhizal fungal communities to ecosystem development. We use a long‐term soil chronosequence that includes ecosystem progression and retrogression to quantify the importance of host plant identity as a factor driving fungal community composition during ecosystem development. We identified arbuscular mycorrhizal fungi and plant species from 50 individual roots from each of 10 sites spanning 5–120 000 yr of ecosystem age using terminal restriction fragment length polymorphism (T‐RFLP), Sanger sequencing and pyrosequencing. Arbuscular mycorrhizal fungal communities were highly structured by ecosystem age. There was strong niche differentiation, with different groups of operational taxonomic units (OTUs) being characteristic of early succession, ecosystem progression and ecosystem retrogression. Fungal alpha diversity decreased with ecosystem age, whereas beta diversity was high at early stages and lower in subsequent stages. A total of 39% of the variance in fungal communities was explained by host plant and site age, 29% of which was attributed to host and the interaction between host and site (24% and 5%, respectively). The strong response of arbuscular mycorrhizal fungi to ecosystem development appears to be largely driven by plant host identity, supporting the concept that plant and fungal communities are tightly coupled rather than independently responding to habitat.

Invasions of alien plants are typically studied as invasions of individual species, yet interactions between plants and symbiotic fungi (mutualists and potential pathogens) affect plant survival, physiological traits, and reproduction and hence invasion success. Studies show that plant–fungal associations are frequently key drivers of plant invasion success and impact, but clear conceptual frameworks and integration across studies are needed to move beyond a series of case studies towards a more predictive understanding. Here, we consider linked plant–fungal invasions from the perspective of plant and fungal origin, simplified to the least complex representations or ‘motifs’. By characterizing these interaction motifs, parallels in invasion processes between pathogen and mutualist fungi become clear, although the outcomes are often opposite in effect. These interaction motifs provide hypotheses for fungal-driven dynamics behind observed plant invasion trajectories. In some situations, the effects of plant–fungal interactions are inconsistent or negligible. Variability in when and where different interaction motifs matter may be driven by specificity in the plant–fungal interaction, the size of the effect of the symbiosis (negative to positive) on plants and the dependence (obligate to facultative) of the plant–fungal interaction. Linked plant–fungal invasions can transform communities and ecosystem function, with potential for persistent legacies preventing ecosystem restoration.

The absence of co-evolved mutualists of plants invading a novel habitat is the logical corollary of the more widely recognized 'enemy escape'. To avoid or overcome the loss of mutualists, plants may co-invade with nonnative mutualists, form novel associations with native mutualists or form associations with native cosmopolitan mutualists, which are native but not novel to the invading plant. We tested these hypotheses by contrasting the ectomycorrhizal fungal communities associated with invasive Pinus contorta in New Zealand with co-occurring endemic Nothofagus solandri var. cliffortioides. Fungal communities on Pinus were species poor (14 ectomycorrhizal species) and dominated by nonnative (93%) and cosmopolitan fungi (7%). Nothofagus had a species-rich (98 species) fungal community dominated by native Cortinarius and two cosmopolitan fungi. These results support co-invasion by mutualists rather than novel associations as an important mechanism by which plants avoid or overcome the loss of mutualists, consistent with invasional meltdown.

Herbivores may facilitate or impede exotic plant invasion, depending on their direct and indirect interactions with exotic plants relative to co-occurring natives. However, previous studies investigating direct effects have mostly used pairwise native-exotic comparisons with few enemies, reached conflicting conclusions, and largely overlooked indirect interactions such as apparent competition. Here, we ask whether native and exotic plants differ in their interactions with invertebrate herbivores. We manipulate and measure plant-herbivore and plant-soil biota interactions in 160 experimental mesocosm communities to test several invasion hypotheses. We find that compared with natives, exotic plants support higher herbivore diversity and biomass, and experience larger proportional biomass reductions from herbivory, regardless of whether specialist soil biota are present. Yet, exotics consistently dominate community biomass, likely due to their fast growth rates rather than strong potential to exert apparent competition on neighbors. We conclude that polyphagous invertebrate herbivores are unlikely to play significant direct or indirect roles in mediating plant invasions, especially for fast-growing exotic plants. It is unclear whether plant-herbivore interactions systematically favour exotic plant species. Here the authors investigate plant-herbivore and plant-soil biota interactions in experimental mesocosm communities, finding that exotic plants dominate community biomass despite accumulating more invertebrate herbivores.

1. Below-ground plant functional traits regulate plant-soil interactions and may therefore strongly influence ecosystem responses to global change. Despite this, knowledge of how fine-root functional traits vary among plant species and along environmental gradients has lagged far behind our understanding of above-ground traits. 2. We measured species- and community-level root and leaf trait responses for 50 temperate rain forest species from 28 families of ferns, woody and herbaceous angiosperms and conifers, along a soil chronosequence in New Zealand that exhibits a strong gradient in soil nutrient availability. Relationships among species traits (both above- and below-ground) and their distribution along the chronosequence were tested using phylogenetic generalized least-squares regression to account for plant relatedness. 3. Distinctive root trait syndromes were observed; they were closely linked to species' distribution along the chronosequence. Species growing in the strongly P-limited late stages of the chronosequence had relatively high specific root length (SRL), thin root diameter, high root tissue density, high levels of root branching and low root nutrient concentrations compared to intermediate stages. Species on the youngest site also had high SRL, but had low root tissue density, thick root diameter and high root nutrient concentrations. 4. Species root and leaf nutrient concentrations were positively correlated, reflecting the strong underlying gradient in soil fertility. In contrast, the relationship between SRL and SLA was more complex; there was a weak positive correlation between SRL and SLA, but this conflicted with stronger patterns of increasing SRL and declining SLA with increasing site age. 5. Community-averaged trait values calculated using presence/absence data showed similar trends to the species-level patterns. In contrast, community averages calculated using species abundance-weighted data showed weaker relationships with site age, particularly for morphological traits. This suggests that much of the variation in morphological traits between sites was driven by shifts in the presence of subordinate or ‘rare' species rather than by changes in the dominant species. 6. Synthesis. Our study demonstrates co-ordinated species- and community-level changes in root traits along a soil chronosequence. These results highlight the influence of soil nutrition on plant functional traits and contribute to our understanding of the drivers of community assembly in a changing environment.

Background and scope Plant communities and underlying soils undergo substantial, coordinated shifts throughout ecosystem development. However, shifts in the composition and function of mycorrhizal fungi remain poorly understood, despite their role as a major interface between plants and soil. We synthesise evidence for shifts among mycorrhizal types (i.e., ectomycorrhizas, arbuscular and ericoid mycorrhizas) and in fungal communities within mycorrhizal types along long-term chronosequences that include retrogressive stages. These systems represent strong, predictable patterns of increasing, then declining soil fertility during ecosystem development, and are associated with coordinated changes in plant and fungal functional traits and ecological processes. Conclusions Mycorrhizal types do not demonstrate consistent shifts through ecosystem development. Rather, most mycorrhizal types can dominate at any stage of ecosystem development, driven by biogeography (i.e., availability of mycorrhizal host species), plant community assembly, climate and other factors. In contrast to coordinated shifts in soil fertility, plant traits and ecological processes throughout ecosystem development, shifts in fungal communities within and among mycorrhizal types are weak or idiosyncratic. The consequences of these changes in mycorrhizal communities and their function for plant-soil feedbacks or control over longterm nutrient depletion remain poorly understood, but could be resolved through empirical analyses of longterm soil chronosequences.

Biological invasions are a rapidly increasing driver of global change, yet fundamental gaps remain in our understanding of the factors determining the success or extent of invasions. For example, although most woody plant species depend on belowground mutualists such as mycorrhizal fungi and nitrogen-fixing bacteria, the relative importance of these mutualisms in conferring invasion success is unresolved. Here, we describe how neighborhood context (identity of nearby tree species) affects the formation of belowground ectomycorrhizal partnerships between fungi and seedlings of a widespread invasive tree species, Pseudotsuga menziesii (Douglas-fir), in New Zealand. We found that the formation of mycorrhizal partnerships, the composition of the fungal species involved in these partnerships, and the origin of the fungi (co-invading or native to New Zealand) all depend on neighborhood context. Our data suggest that nearby ectomycorrhizal host trees act as both a reservoir of fungal inoculum and a carbon source for late-successional and native fungi. By facilitating mycorrhization of P. menziesii seedlings, adult trees may alleviate mycorrhizal limitation at the P. menziesii invasion front. These results highlight the importance of studying biological invasions across multiple ecological settings to understand establishment success and invasion speed.