Shared mycorrhizae but distinct communities of other root-associated microbes on co-occurring native and invasive maples

The temperate forest phyllosphere and rhizosphere microbiome: a case study of sugar maple

The interactions between sugar maple (Acer saccharum, Marshall) and its microbial communities are important for tree fitness, growth, and establishment. Despite recent progress in our understanding of the rhizosphere and phyllosphere microbial communities of sugar maple, many outstanding knowledge gaps remain. This review delves into the relationships between sugar maple and its microbes, as climate change alters plant species distributions. It highlights the multifaceted roles of key microbes, such as arbuscular mycorrhizal (AM) fungi and pathogens, in affecting the distribution and establishment of sugar maple in novel habitats. Furthermore, this review examines how microbial communities in different compartments contribute to tree fitness. Finally, it explores how microbial dispersal and altered species interactions under changing environmental conditions can affect sugar maple's ability to migrate beyond its current range, emphasizing the different scenarios associated with such shifts. In the rhizosphere, AM fungi are known for their roles in nutrient acquisition and improving stress tolerance. Yet, key questions remain about how these fungi interact with other microbes, how soil chemistry and climate change alter these interactions, and how the presence of beneficial microbes influences sugar maple's establishment. Additionally, the role of dark septate endophytes (DSE) in sugar maple's fitness remains underexplored, emphasizing the need for more research on their diversity and functions. In the phyllosphere, microbial communities are subject to shifts due to rising global change, with potential impacts on sugar maple's fitness. These changes may influence the tree's resistance to pathogens, tolerance to environmental stress, and overall health. Yet, our understanding of these interactions relies mostly on short-read sequencing methods targeting marker genes (e.g., 16S, ITS, 18S), which often fail to identify microbes at the species level. Limitations in molecular techniques and poor microbial reference databases hinder our ability to fully characterize tree-associated microbial diversity and functions. Future research should thus prioritize advanced molecular tools such as shotgun, hybrid, or long-read sequencing. Controlled experiments are also needed to establish causal links between sugar maple fitness and microbial communities, and to study whether microbial communities change throughout the tree's lifespan.

Occurrence of Arbuscular Mycorrhizal Herbs Decreases Selectively in Communities Dominated by Invasive Tree Acer negundo

We tested whether one of the consequences predicted for alien plant invasion by the mutualism disruption hypothesis was true in the case of the ash-leaved maple Acer negundo L. The study aimed to determine whether the occurrences of mycorrhizal and nonmycorrhizal herbs varied similarly or differently in communities with varying degrees of A. negundo dominance. The analysis included the results of 78 vegetation descriptions carried out in Belarusian Polesia, the Middle Volga region, and the Middle Urals. Communities with or without A. negundo dominance were described in each region. The mycorrhizal status of plant species was determined using the FungalRoot Database. Species that are more likely to form arbuscular mycorrhiza were found to occur less frequently in A. negundo thickets. On the contrary, a higher probability of the nonmycorrhizal status was associated with a lower frequency of detection in A. negundo thickets. Therefore, the occurrence of arbuscular mycorrhizal herbs was found to selectively decrease in communities dominated by A. negundo.

Interplay of biotic and abiotic factors shapes tree seedling growth and root-associated microbial communities

Root-associated microbes can alleviate plant abiotic stresses, thus potentially supporting adaptation to a changing climate or to novel environments during range expansion. While climate change is extending plant species fundamental niches northward, the distribution and colonization of mutualists (e.g., arbuscular mycorrhizal fungi) and pathogens may constrain plant growth and regeneration. Yet, the degree to which biotic and abiotic factors impact plant performance and associated microbial communities at the edge of their distribution remains unclear. Here, we use root microscopy, coupled with amplicon sequencing, to study bacterial, fungal, and mycorrhizal root-associated microbial communities from sugar maple seedlings distributed across two temperate-to-boreal elevational gradients in southern Québec, Canada. Our findings demonstrate that soil pH, soil Ca, and distance to sugar maple trees are key drivers of root-associated microbial communities, overshadowing the influence of elevation. Interestingly, changes in root fungal community composition mediate an indirect effect of soil pH on seedling growth, a pattern consistent at both sites. Overall, our findings highlight a complex role of biotic and abiotic factors in shaping tree-microbe interactions, which are in turn correlated with seedling growth. These findings have important ramifications for tree range expansion in response to shifting climatic niches.

Regional variation drives differences in microbial communities associated with sugar maple across a latitudinal range

Climate change is prompting plants to migrate and establish novel interactions in new habitats. Because of the pivotal roles 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 which can negatively impact maple establishment; (2) and revealed reductions in fungal leaf pathogens which 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.

Soil properties and microbial community in the rhizosphere of Populus alba var. pyramidalis along a chronosequence

By maintaining soil structure and quality, soil microbial communities usually play important role in many forest ecosystem processes, including ecological succession. Understanding changes in the microbial communities of areas afforested with stands of different ages is of interest in ecology. Populus alba var. pyramidalis Bunge has been widely planted in Northwest China for ecological restoration. Rhizospheric soil samples were collected from 4-, 9-, 15-, 25- and 30-year-old plantations of P. alba to measure soil characteristics and soil microbial community diversity using Illumina MiSeq sequencing. The soil nutrition concentration and enzymatic activities decreased with depth of soil layer increased. In terms of stand age, alkaline phosphatase and dehydrogenase and the contents of nitrate N, available P and soil organic content (SOC) increased gradually. According to Illumina MiSeq sequencing results, the fungal and bacterial community structure varied with stand age, and diversity of fungi was less than bacteria. With increasing stand age, fungal community diversity indexes first increased and then decreased, peaked at 25y or 30y. RDA results suggested that soil available P and nitrate N were the most important factors governing fungal community structure, while available P contributed significantly to the variance of the bacterial community. Structural equation modelling (SEM) results indicated soil available P, nitrate N and SOC contents largely explained the shift in the microbial community structure along the cultivation chronosequence, and soil enzyme activities were related with changes in microbial community. Our results illustrated that the successional changes in microbial communities in the P. alba plantations can largely be attributed to changes in soil nutrition level along the chronosequence.