Tim Seipel

Questions: There is growing evidence of plant invasions in high‐elevation mountain ecosystems, but how these invasion patterns relate to native species diversity remains understudied. While the biotic resistance hypothesis states that highly diverse communities are less likely to be invaded than less diverse ones, the opposite pattern might arise from facilitative interactions in harsh environments, if these prevail at high elevations, especially in the alpine zone. Here, we investigate whether the relationship between native and non‐native plant species diversity changes with elevation, and whether it is modified by disturbance resulting from mountain roads. Location: Global. Methods: By using standardized vegetation surveys and plant trait measurements, we modeled the relationship between non‐native species richness and native species richness and functional diversity across elevation gradients, and whether they differ between (semi‐)natural habitats and adjacent disturbed roadside habitats. Results: At high elevations, we recorded more non‐native species in functionally diverse natural habitats and in species‐rich roadside habitats. At lower elevations, non‐native and native species richness were positively associated, but fewer non‐native species established in functionally diverse natural habitats. Lateral spread of non‐native species from disturbed roadsides into natural habitats occurred with the same frequency at high and low elevations. Conclusions: Our results suggest that once non‐native species reach high elevations along dispersal corridors such as roads, native plant diversity may help promote, rather than inhibit, their spread into natural plant communities, contradicting the biotic resistance hypothesis. While the positive correlation between non‐native and native species richness likely relies on beneficial environmental heterogeneity promoting both species groups, the seemingly inhibiting effect of native functional diversity at low elevations and its assumed supporting role at high elevations might reflect biotic processes such as limiting similarity and facilitation. This suggests that a complex interplay between native diversity and human disturbances shapes the invasibility of mountain ecosystems.

Motivation: The database initiated by the Mountain Invasion Research Network (MIREN) is one of the few globally implemented standardized vegetation surveys in mountains. The focus is on collecting plant species occurrences at 5‐year intervals along and adjacent to mountain roads spanning elevational gradients, with the goal of providing critical information about the ongoing spread of non‐native plants and the potential impacts of climate and human‐caused disturbance in the form of road infrastructure on mountain vegetation. Main Types of Variables Contained: To date, the database contains almost 170,000 georeferenced records of plant species. Overall, 6854 vascular plant species were observed in 3364 plots across 25 regions, with many of these plots being revisited every 5 years. Each plot is identified by its coordinates, elevation and distance to the road. The status of each individual species (native, non‐native or unknown) is determined in accordance with regional databases. Spatial Location and Grain: Plant species were surveyed at 20 sample sites, placed at regular elevational intervals along each mountain road, with typically three roads selected per region. Each sample site is subdivided into three 2 m × 50 m plots, one parallel to the road and two perpendicular to it, extending into adjacent vegetation. The 25 studied regions cover all continents except Antarctica. Time Period and Grain: Data were collected between 2007 and 2023, with surveys being repeated at 5‐year intervals in a subset of regions. Three regions include four time‐steps, while seven and three regions were sampled three times or twice, respectively. In most regions, long‐term monitoring is ongoing. Major Taxa and Level of Measurement: All vascular plant species. Software Format: Data are available through a .csv file on Zenodo ( https://doi.org/10.5281/zenodo.15749536 ).

Aim: Mountain roads host plant communities that differ strongly from the adjacent natural vegetation. Besides the effect of propagule pressure, altered disturbance and soil processes, one of the reasons given for the strong influence of mountain roads on species distributions is a significantly altered soil thermal microclimate in the roadside compared to the adjacent vegetation, as a direct consequence of road disturbance. However, the thermal microclimatic differences between roadside and natural vegetation have rarely been quantified, particularly across large spatial extents. This study provides the first global quantification of roadside soil temperature patterns along elevational gradients. Location: Mountain roads in eight mountain regions from the Mountain Invasion Research Network (MIREN): Argentina, Chile, Czech Republic, Norway, Spain (Tenerife and La Palma), Switzerland, and the USA. Methods: In this study, we analysed in situ measured topsoil temperatures (< 10 cm) and forest cover of roadsides and adjacent natural vegetation plots, in a systematically paired design. Results: Across most regions, roadside soils exhibited consistently warmer maxima (3.62°C ± 2.61°C) and colder minima (1.39°C ± 1.40°C) than soils in adjacent vegetation. Although temperature distributions between roadside and natural habitats largely overlapped, these systematic shifts indicate increased thermal variability and higher frost risk in roadside environments. Main Conclusions: Roadsides create distinct thermal microhabitats even within heterogeneous mountain landscapes. These altered temperature regimes, particularly when combined with vegetation structure and other environmental factors such as soil moisture, may influence plant performance and distribution. Our findings highlight the ecological relevance of thermal microclimates in understanding biodiversity patterns along mountain roads.

Aim: Mountain ecosystems are experiencing increased invasion of non‐native plants. These increases in non‐native species put mountains at risk of biotic homogenisation and a reduction of biodiversity. Our study aims to test if non‐native plant species are contributing to biotic homogenisation along roadways in mountain regions and how this changes along elevation gradients and across spatial scales. Location: 18 globally distributed mountain regions. Time Period: 2012–2023. Major Taxa Studied: Vascular plants. Methods: We used standardised vegetation surveys including species cover from 18 mountain regions worldwide to analyse whether the addition of non‐native species to the native flora increased or decreased Bray–Curtis dissimilarity (i.e., beta‐diversity) among roadside plant communities along elevation gradients ranging from 15 to 3919 m a.s.l. We tested this at the local, regional, continental and global scales using mixed‐effects models and confirmed it using null models. Results: In the New World, we mainly observed homogenisation across regions and scales, as beta‐diversity was mostly lower with the addition of non‐native species. This was particularly true for low elevations. In contrast, we predominantly found community differentiation in the Old World, specifically at smaller (i.e., local and regional) scales. At the global scale, communities became more similar through the addition of non‐native species at all elevations. Main Conclusions: Large‐scale homogenisation might be interpreted as a signal that high‐elevation plant communities along roadways may become more similar as non‐native species continue to spread upwards. Future studies should investigate the mechanisms driving the observed patterns of both homogenisation and differentiation by non‐native species, and explore the potential consequences of these patterns for ecosystem function and resilience.

Mountain roads have repeatedly shown to host significantly different plant species communities compared to the adjacent natural vegetation. Besides the effect of propagule pressure, altered disturbance regime and soil processes, one of the reasons given for the strong influence of mountain roads on species distributions is a significantly altered microclimate in the roadside compared to the adjacent vegetation, a direct consequence of the altered disturbance regime. However, the microclimatic differences between roadside and natural vegetation have rarely been quantified, particularly lacking global analyses, hampering a better understanding of their importance for mountain biodiversity. In this study, we analysed in-situ measured soil temperatures along mountain roads in seven mountain regions across the globe, in order to assess the impact of mountain roads on a range of bioclimatic variables across the elevational gradient. Our results undeniably show the importance of roadsides as unique microhabitats, even in heterogeneous mountain environments. In most regions, roadside soils had warmer maxima (3.95 ± 2.35°C warmer) and colder minima (0.85 ± 1.11 °C colder) than the soil in the adjacent vegetation, with higher frost risks in winter. Therefore, we recommend future research to incorporate the notion that the local microclimates created by mountain roads could play a critical role in species redistributions in space and time.

Darwin’s Naturalization Conundrum (DNC) states that non-native species closely related to the native community are either more likely to succeed because shared adaptations help them overcome environmental filtering, or less likely to succeed because of strong competition with their native relatives. Despite extensive research, no general patterns have so far emerged. One reason may be that the relative importance of competition and environmental filtering depends on environmental conditions. To test this hypothesis, we conducted a global assessment of DNC examining patterns of phylogenetic relatedness of non-native plant species to the native community along gradients of elevation and anthropogenic disturbance in mountains. Phylogenetic distance of non-native to native species decreased with increasing elevation and in disturbed plant communities. Our results help resolve DNC by showing that the environmental context sets expectations for patterns of relatedness between non-native and native species and helps illuminate the ecological and evolutionary processes generating these patterns.

Canada thistle is a pervasive perennial weed, causing challenges to agricultural and natural ecosystems globally. Although research has focused on the phenology, genetics, and control of Canada thistle, little is known about the rhizosphere microbiome and the role plant–microbe interactions play in invasion success. This study investigated the rhizosphere microbiome of Canada thistle across diverse climates, soils, and crops in the U.S. northern Great Plains. Soil and rhizosphere samples were collected and bacterial 16S and fungal ITS2 sequencing were performed to characterize the core microbiome and identify potential factors contributing to invasion success. Amplicon sequencing revealed a stable core microbiome that was detected in the Canada thistle rhizosphere across all locations. The core microbiome was dominated by the bacterial phyla Actinobacteriota and Proteobacteria and fungal phyla Ascomycota and Basidiomycota. Differential abundance analysis showed rhizosphere fungal communities were enriched in pathogen-containing genera with a 1.7-fold greater abundance of Fusaria and a 2.6-fold greater abundance of Gibberella compared to bulk soil. Predictive functional profiling showed rhizosphere communities were enriched (p < 0.05, FDR corrected) in plant pathogen fungal guilds which represented 19% of the fungal community. The rhizosphere microbiome was similar in composition across environments, highlighting the stable association between Canada thistle and specific microbial taxa. This study characterized the core microbiome of Canada thistle, and the findings highlight plant–microbe interactions shaping invasive behavior. These findings are important for understanding the ecological impacts of plant invasion and soil-microbe ecological processes.

We assessed the impact of road disturbances on the dominant mycorrhizal types in ecosystems at the global level and how this mechanism can potentially lead to lasting plant community changes. We used a database of coordinated plant community surveys following mountain roads from 894 plots in 11 mountain regions across the globe in combination with an existing database of mycorrhizal–plant associations in order to approximate the relative abundance of mycorrhizal types in natural and disturbed environments. Our findings show that roadside disturbance promotes the cover of plants associated with arbuscular mycorrhizal (AM) fungi. This effect is especially strong in colder mountain environments and in mountain regions where plant communities are dominated by ectomycorrhizal (EcM) or ericoid‐mycorrhizal (ErM) associations. Furthermore, non‐native plant species, which we confirmed to be mostly AM plants, are more successful in environments dominated by AM associations. These biogeographical patterns suggest that changes in mycorrhizal types could be a crucial factor in the worldwide impact of anthropogenic disturbances on mountain ecosystems. Indeed, roadsides foster AM‐dominated systems, where AM‐fungi might aid AM‐associated plant species while potentially reducing the biotic resistance against invasive non‐native species, often also associated with AM networks. Restoration efforts in mountain ecosystems will have to contend with changes in the fundamental make‐up of EcM‐ and ErM plant communities induced by roadside disturbance.

High-elevation ecosystems are among the few ecosystems worldwide that are not yet heavily invaded by non-native plants. This is expected to change as species expand their range limits upwards to fill their climatic niches and respond to ongoing anthropogenic disturbances. Yet, whether and how quickly these changes are happening has only been assessed in a few isolated cases. Starting in 2007, we conducted repeated surveys of non-native plant distributions along mountain roads in 11 regions from 5 continents. We show that over a 5- to 10-year period, the number of non-native species increased on average by approximately 16% per decade across regions. The direction and magnitude of upper range limit shifts depended on elevation across all regions. Supported by a null-model approach accounting for range changes expected by chance alone, we found greater than expected upward shifts at lower/mid elevations in at least seven regions. After accounting for elevation dependence, significant average upward shifts were detected in a further three regions (revealing evidence for upward shifts in 10 of 11 regions). Together, our results show that mountain environments are becoming increasingly exposed to biological invasions, emphasizing the need to monitor and prevent potential biosecurity issues emerging in high-elevation ecosystems.

Crops emit a variety of volatile organic compounds (VOCs) that serve as attractants or repellents for pests and their natural enemies. Crop rotations, off-farm chemical inputs, and mechanical and cultural tactics – collectively called cropping systems – alter soil nutrients, moisture content, and microbial communities, all of which have the potential to alter crop VOC emissions. Soil legacy effects of diversified cropping systems have been shown to enhance crop VOC emissions in greenhouse studies, but how they influence emissions under field conditions remains virtually unknown. To determine the effect of cropping systems on plant VOC emissions in the field, air samples were collected from the headspace of wheat (Triticum aestivum L. Judee) grown in simplified wheat-fallow rotations or diversified wheat-cover crop rotations where cover crops were terminated by grazing cattle. Across two growing seasons, wheat grown in rotation with fallow emitted greater amounts of Z-3-hexenyl acetate and β-ocimene, key attractants for wheat stem sawfly (Cephus cinctus Norton), a major pest of wheat. While overall VOC blends were relatively similar among cropping system during the first growing season, emissions varied substantially in the second year of this study where wheat grown in rotation with cover crops emitted substantially greater quantities of volatile compounds characteristic of abiotic stress. Below-average precipitation in the second growing season, in addition to reduced soil water content in cover crop rotations, suggests that cropping system effects on wheat VOCs may have been driven primarily by water availability, a major factor limiting crop growth in dryland agriculture. While the specific mechanisms driving changes in VOC emissions were not explicitly tested, this work shows that agricultural practices applied in one growing season can differentially influence crop VOC emissions in the next through soil legacy effects, illustrating additional avenues through which cropping systems may be leveraged to enhance pest management.