Plant range expansion is occurring at a rapid pace, largely in response to human-induced climate warming. While the movement of plants along latitudinal and altitudinal gradients is well documented, effects on the belowground microbial communities remains largely unknown. Further, in range expansion not all plant species are equal: in a new range the relatedness between range-expanding plant species and native flora can influence plant-microbe interactions. Here we used a latitudinal gradient across Europe to examine bacterial and fungal communities in the rhizosphere and surrounding soils of range-expanding plant species. We selected range expanders with and without congeneric natives in the new range, and as a control, the congeneric natives, totaling 382 plant individuals collected across Europe. In general, a plant’s status as range expander was a weak predictor of bacterial and fungal community composition. However, microbial communities of range-expanding plant species became more similar to each other farther from their original range. Range expanders unrelated to the native community also experienced a decrease in the ratio of plant pathogens to symbionts, giving weak support to the enemy release hypothesis. Even at a continental scale the effects of plant range expansion on the belowground microbiome are detectable, though changes to specific taxa remain difficult to decipher.

Current climate change has led to latitudinal and altitudinal range expansions of numerous species. During such range expansions, plant species are expected to experience changes in interactions with other organisms, especially with belowground biota that have a limited dispersal capacity. Nematodes form a key component of the belowground food web as they include bacterivores, fungivores, omnivores and root herbivores. However, their community composition under climate change‐driven intracontinental range‐expanding plants has been studied almost exclusively under controlled conditions, whereas little is known about actual patterns in the field. Here, we use novel molecular sequencing techniques combined with morphological quantification in order to examine nematode communities in the rhizospheres of four range‐expanding and four congeneric native species along a 2,000 km latitudinal transect from South‐Eastern to North‐Western Europe. We tested the hypotheses that latitudinal shifts in nematode community composition are stronger in range‐expanding plant species than in congeneric natives and that in their new range, range‐expanding plant species accumulate fewest root‐feeding nematodes. Our results show latitudinal variation in nematode community composition of both range expanders and native plant species, while operational taxonomic unit richness remained the same across ranges. Therefore, range‐expanding plant species face different nematode communities at higher latitudes, but this is also the case for widespread native plant species. Only one of the four range‐expanding plant species showed a stronger shift in nematode community composition than its congeneric native and accumulated fewer root‐feeding nematodes in its new range. We conclude that variation in nematode community composition with increasing latitude occurs for both range‐expanding and native plant species and that some range‐expanding plant species may become released from root‐feeding nematodes in the new range.

Climate change is accelerating the spread of plants and their associated species to new ranges. The differences in range shift capacity of the various types of species may disrupt long-term co-evolved relationships especially those belowground, however, this may be less so for seed-borne endophytic microbes. We collected seeds and soil of the range-expanding Centaurea stoebe and the congeneric Centaurea jacea from three populations growing in Slovenia (native range of both Centaurea species) and the Netherlands (expanded range of C. stoebe, native range of C. jacea). We isolated and identified endophytic fungi directly from seeds, as well as from roots of the plants grown in Slovenian, Dutch or sterilized soil to compare fungal endophyte composition. Furthermore, we investigated whether C. stoebe hosts a reduced community composition of endophytes in the expanded range due to release from plant-species specific fungi while endophyte communities in C. jacea in both ranges are similar. We cultivated 46 unique and phylogenetically diverse endophytes. A majority of the seed endophytes resembled potential pathogens, while most root endophytes were not likely to be pathogenic. Only one endophyte was found in both roots and seeds, but was isolated from different plant species. Unexpectedly, seed endophyte diversity of southern C. stoebe populations was lower than of populations from the north, while the seed endophyte community composition of northern C. stoebe populations was significantly different southern C. stoebe as well as northern and southern C. jacea populations. Root endophyte diversity was considerably lower in C. stoebe than in C. jacea independent of plant and soil origin, but this difference disappeared when plants were grown in sterile soils. We conclude that the community composition of fungal endophytes not only differs between related plant species but also between populations of plants that expand their range compared to their native habitat. Our results suggest that fungal endophytes of two Centaurea species are not able to systemically infect plants. We highlight that endophytes remain poorly studied and further work should investigate the functional importance of endophytes.

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The knowledge on species' habitat preferences at local scales across its range is an essential condition for 18 defining the most appropriate habitat management for the conservation of any species. In this study, we 19 combined field observations from three European countries with breeding experiments under field conditions to 20 identify oviposition and larval preferences of Coenonympha oedippus on micro-scale level across contrasting 21 habitat types (wet vs. dry). Despite the wide geographical range and the different habitats we found some 22 common features: (i) vegetation structure of the herb layer is an essential factor for oviposition site electivity and 23 successful development of premature stages; (ii) high cover of litter and/or dwarf shrubs in the microhabitat 24 (larval: 45-70% , oviposition: 40-50%) creates a herb layer rich in gaps; at their edges eggs are deposited and 25 the caterpillars are adequately sun-exposed; (iii) egg-laying females are not selective regarding oviposition 26 substratum; (iv) oviposition height is adjusted to positions with direct sunlight or warm substratum; (v) the host-27 plants coverage in oviposition sites was high: 45 and 50% (wet), 18% or 41% (including potential host-plants) 28 (dry); (vi) the most important host-plant is Carex panicea (wet) and Carex humilis (dry), but Molinia caerulea 29 (wet) and Festuca rupicola (dry) are also used regularly; (vii) the availability of winter-green host-plants in the 30 vicinity of hibernated larvae plays a substantial role in their survival. As regular mowing or grazing would 31 remove the litter and destroy the gaps, the management should be restricted to selective reed cutting or manual 32 shrub removal. Only selective mowing during winter (December-February) can be recommended for keeping 33 the habitat open where the reduction of bushes is not sufficient. 34 35