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Climate change is prompting plants to migrate and establish novel interactions in new habitats. Because of the pivotal roles that microbes have on plant health and function, it is important to understand the ecological consequences of these shifts in host-microbe interactions with range expansion. Here we examine how the diversity of plant-associated microbes varies along the host's current range and extended range according with climate change predictions, and assess the relative influence of host genotype (seed provenance) and environment in structuring the host microbiome. We collected sugar maple seeds from across the species current range, then planted them in temperate and mixedwood/transitional forests (current range) and in the boreal region (beyond range but predicted future range in response to climate change). We used amplicon sequencing to quantify bacterial, fungal, and mycorrhizal communities from seedling leaves and roots. Variation among sites and regions were the main drivers of the differences in host microbial communities, whereas seed provenance did not play a large role. No unifying pattern was observed for microbial community richness, diversity, or specialization, demonstrating the complexity of responses of different taxa on above- and belowground plant compartments. Along the latitudinal gradient, we (1) observed reductions in mycorrhizal diversity that can negatively impact maple establishment; (2) and revealed reductions in fungal leaf pathogens that can have opposite effects. Our results highlight the need for an integrated approach including the examination of various microbial taxa on different plant compartments to improve our understanding of plant range shifts and plant-microbe interactions.

The functional trait approach proposes that relating traits of organisms within a community to variation in abiotic and biotic characteristics of their environment will provide insight on the mechanisms of community assembly. As traits at a given trophic level might act as filters for the selection of traits at another trophic level, we hypothesized that traits of consumers and of their resources covary in space. We evaluated complementary predictions about top‐down (negative) and bottom‐up (positive) trait covariation in a detrital food web. Additionally, we tested whether positive trait covariation was better explained by the Resource Concentration Hypothesis (i.e., most commonly represented trait values attract abundant consumers) or the Resource Specialization Hypothesis (i.e., resource diversity increases niche availability for the consumers). Macroarthopods were collected with pitfall traps over two summers in three forested sites of southern Quebec in 110 plots that varied in tree species composition. Six feeding traits of consumers (detritivores and predators) and six palatability traits of their resources (leaf litter and prey) were matched to assess spatial covariation. Trait matches included consumer biting force/resource toughness, detritivore mandibular gape/leaf thickness, predator/prey body size ratio, etc. Our results demonstrate for the first time a covariation between feeding traits of detritivores and palatability traits of leaf litter (31–34%), and between feeding traits of litter‐dwelling predators and palatability traits of potential prey (38–44%). The observed positive covariation supports both the Resource Concentration Hypothesis and Resource Specialization Hypothesis. Spatial covariation of consumer and resource traits provides a new tool to partially predict the structure of the detrital food web. Nonetheless, top‐down regulation remains difficult to confirm. Further research on top‐down processes will be undoubtedly necessary to refine our capacity to interpret the effect of biotic interactions on co‐distribution.

Intraguild predation (IGP), that is, feeding interaction between two consumers that share the same resource species, is commonly observed in natural food webs. IGP expands vertical niche space and slows down energy flows from lower to higher trophic levels, which potentially affects the diversity and dynamics of food webs. Here, we use food-web models to investigate the effects of IGP on species diversity and ecosystem functioning. We first simulate a five-species food-web module with different strengths of IGP at the herbivore and/or carnivore level. Results show that as the strength of IGP within a trophic level increases, the biomass of its resource level increases because of predation release; this increased biomass in turn alters the energy fluxes and biomass of other trophic levels. These results are then extended by subsequent simulations of more diverse food webs. As the strength of IGP increases, simulated food webs maintain (1) higher species diversity at different trophic levels, (2) higher total biomasses at different trophic levels, and (3) larger energy fluxes across trophic levels. Our results challenge the intuitive hypothesis that food-web structure should maximize the efficiency of energy transfer across trophic levels; instead, they suggest that the assembly of food webs should be governed by a balance between efficiency (of energy transfer) and persistence (i.e., the maintenance of species and biomasses). Our simulations also show that the relationship between biodiversity and ecosystem functioning (e.g., total biomass or primary production) is much stronger in the presence of IGP, reconciling the contrast from recent studies based on food-chain and food-web models. Our findings shed new light on the functional role of IGP and contribute to resolving the debate on structure, diversity and functioning in complex food webs.

Understanding the impacts of species extinctions on the functioning of food webs is a challenging task because of the complexity of ecological interactions. We report the impacts of experimental species extinctions on the functioning of two food webs of freshwater and marine systems. We used a linear model to partition the variance among the multiple components of the diversity effect (linear group richness, nonlinear group richness, and identity). The identity of each functional group was the best explaining variable of ecosystem functioning for both systems. We assessed the contribution of each functional group in multifunctional space and found that, although the effect of functional group varied across ecosystem functions, some functional groups shared common effects on functions. This study is the first experimental demonstration that functional identity dominates the effects of extinctions on ecosystem functioning, suggesting that generalizations are possible despite the inherent complexity of interactions.

We present a theory extending the source-sink concept with an ecosystem perspective. We analyze a model for meta-ecosystem dynamics in a heterogeneous environment to study how the spatial flows of materials such as inorganic nutrients and nutrients sequestered into producers, herbivores, and detritus affect the community dynamics. We show that spatial flows of an inorganic nutrient (direct nutrient flow) and organic matter (indirect nutrient flow) through detritus, producer, or herbivore compartments can reverse the source-sink dynamics of a local ecosystem. More precisely, the balance between such direct and indirect nutrient flows determines the net direction of nutrient flows between two ecosystems of contrasted productivities. It allows a sink to turn into a source and vice versa. This effect of nutrient flows on source and sink dynamics is robust to the ecosystem structure (with and without herbivores) and to specific ecosystem compartments contributing to nutrient flows (primary producers, herbivores, or detritus). Ecosystems in distinct localities thus interact together with the structure at one place influencing that of the other. In meta-ecosystems, the source-sink dynamics of an organism is not only constrained by its dispersal from the source to the sink, but also by the fertility and community composition in the neighborhood responsible for spatial flows of nutrients and energy. The meta-ecosystem perspective provides a powerful theoretical framework to address novel questions in spatial ecosystem ecology.

Understanding coexistence of highly shade-tolerant tree species is a longstanding challenge for forest ecologists. A conceptual model for the coexistence of sugar maple (Acer saccharum) and American beech (Fagus grandibfolia) has been proposed, based on a low-light survival/high-light growth trade-off, which interacts with soil fertility and small-scale spatiotemporal variation in the environment. In this study, we first tested whether the spatial distribution of seedlings and saplings can be predicted by the spatiotemporal variability of light availability and soil fertility, and second, the manner in which the process of environmental filtering changes with regeneration size. We evaluate the support for this hypothesis relative to the one for a neutral model, i.e., for seed rain density predicted from the distribution of adult trees. To do so, we performed intensive sampling over 86 quadrats (5 x 5 m) in a 0.24-ha plot in a mature maple-beech community in Quebec, Canada. Maple and beech abundance, soil characteristics, light availability, and growth history (used as a proxy for spatiotemporal variation in light availability) were finely measured to model variation in sapling composition across different size classes. Results indicate that the variables selected to model species distribution do effectively change with size, but not as predicted by the conceptual model. Our results show that variability in the environment is not sufficient to differentiate these species' distributions in space. Although species differ in their spatial distribution in the small size classes, they tend to correlate at the larger size class in which recruitment occurs. Overall, the results are not supportive of a model of coexistence based on small-scale variations in the environment. We propose that, at the scale of a local stand, the lack of fit of the model could result from the high similarity of species in the range of environmental conditions encountered, and we suggest that coexistence would be stable only at larger spatial scales at which variability in the environment is greater.